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| author | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
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| committer | Thomas Voss <mail@thomasvoss.com> | 2024-11-27 20:54:24 +0100 |
| commit | 4bfd864f10b68b71482b35c818559068ef8d5797 (patch) | |
| tree | e3989f47a7994642eb325063d46e8f08ffa681dc /doc/rfc/rfc5863.txt | |
| parent | ea76e11061bda059ae9f9ad130a9895cc85607db (diff) | |
doc: Add RFC documents
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diff --git a/doc/rfc/rfc5863.txt b/doc/rfc/rfc5863.txt new file mode 100644 index 0000000..fbe2586 --- /dev/null +++ b/doc/rfc/rfc5863.txt @@ -0,0 +1,2859 @@ + + + + + + +Internet Engineering Task Force (IETF) T. Hansen +Request for Comments: 5863 AT&T Laboratories +Category: Informational E. Siegel +ISSN: 2070-1721 Consultant + P. Hallam-Baker + Default Deny Security, Inc. + D. Crocker + Brandenburg InternetWorking + May 2010 + + + DomainKeys Identified Mail (DKIM) + Development, Deployment, and Operations + +Abstract + + DomainKeys Identified Mail (DKIM) allows an organization to claim + responsibility for transmitting a message, in a way that can be + validated by a recipient. The organization can be the author's, the + originating sending site, an intermediary, or one of their agents. A + message can contain multiple signatures, from the same or different + organizations involved with the message. DKIM defines a domain-level + digital signature authentication framework for email, using public + key cryptography and using the domain name service as its key server + technology. This permits verification of a responsible organization, + as well as the integrity of the message content. DKIM will also + provide a mechanism that permits potential email signers to publish + information about their email signing practices; this will permit + email receivers to make additional assessments about messages. + DKIM's authentication of email identity can assist in the global + control of "spam" and "phishing". This document provides + implementation, deployment, operational, and migration considerations + for DKIM. + +Status of This Memo + + This document is not an Internet Standards Track specification; it is + published for informational purposes. + + This document is a product of the Internet Engineering Task Force + (IETF). It represents the consensus of the IETF community. It has + received public review and has been approved for publication by the + Internet Engineering Steering Group (IESG). Not all documents + approved by the IESG are a candidate for any level of Internet + Standard; see Section 2 of RFC 5741. + + + + + + +Hansen, et al. Informational [Page 1] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + http://www.rfc-editor.org/info/rfc5863. + +Copyright Notice + + Copyright (c) 2010 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (http://trustee.ietf.org/license-info) in effect on the date of + publication of this document. Please review these documents + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + the Trust Legal Provisions and are provided without warranty as + described in the Simplified BSD License. + + This document may contain material from IETF Documents or IETF + Contributions published or made publicly available before November + 10, 2008. The person(s) controlling the copyright in some of this + material may not have granted the IETF Trust the right to allow + modifications of such material outside the IETF Standards Process. + Without obtaining an adequate license from the person(s) controlling + the copyright in such materials, this document may not be modified + outside the IETF Standards Process, and derivative works of it may + not be created outside the IETF Standards Process, except to format + it for publication as an RFC or to translate it into languages other + than English. + +Table of Contents + + 1. Introduction ....................................................4 + 2. Using DKIM as Part of Trust Assessment ..........................4 + 2.1. A Systems View of Email Trust Assessment ...................4 + 2.2. Choosing a DKIM Tag for the Assessment Identifier ..........6 + 2.3. Choosing the Signing Domain Name ...........................8 + 2.4. Recipient-Based Assessments ...............................10 + 2.5. Filtering .................................................12 + 3. DKIM Key Generation, Storage, and Management ...................15 + 3.1. Private Key Management: Deployment and Ongoing + Operations ................................................16 + 3.2. Storing Public Keys: DNS Server Software Considerations ...17 + 3.3. Per-User Signing Key Management Issues ....................18 + 3.4. Third-Party Signer Key Management and Selector + Administration ............................................19 + 3.5. Key Pair / Selector Life Cycle Management .................19 + + + +Hansen, et al. Informational [Page 2] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + 4. Signing ........................................................21 + 4.1. DNS Records ...............................................21 + 4.2. Signing Module ............................................21 + 4.3. Signing Policies and Practices ............................22 + 5. Verifying ......................................................23 + 5.1. Intended Scope of Use .....................................23 + 5.2. Signature Scope ...........................................23 + 5.3. Design Scope of Use .......................................24 + 5.4. Inbound Mail Filtering ....................................24 + 5.5. Messages Sent through Mailing Lists and Other + Intermediaries ............................................25 + 5.6. Generation, Transmission, and Use of Results Headers ......25 + 6. Taxonomy of Signatures .........................................26 + 6.1. Single Domain Signature ...................................26 + 6.2. Parent Domain Signature ...................................27 + 6.3. Third-Party Signature .....................................27 + 6.4. Using Trusted Third-Party Senders .........................29 + 6.5. Multiple Signatures .......................................30 + 7. Example Usage Scenarios ........................................31 + 7.1. Author's Organization - Simple ............................32 + 7.2. Author's Organization - Differentiated Types of Mail ......32 + 7.3. Author Domain Signing Practices ...........................32 + 7.4. Delegated Signing .........................................34 + 7.5. Independent Third-Party Service Providers .................35 + 7.6. Mail Streams Based on Behavioral Assessment ...............35 + 7.7. Agent or Mediator Signatures ..............................36 + 8. Usage Considerations ...........................................36 + 8.1. Non-Standard Submission and Delivery Scenarios ............36 + 8.2. Protection of Internal Mail ...............................37 + 8.3. Signature Granularity .....................................38 + 8.4. Email Infrastructure Agents ...............................39 + 8.5. Mail User Agent ...........................................40 + 9. Security Considerations ........................................41 + 10. Acknowledgements ..............................................41 + 11. References ....................................................42 + 11.1. Normative References .....................................42 + 11.2. Informative References ...................................42 + Appendix A. Migration Strategies .................................43 + A.1. Migrating from DomainKeys .................................43 + A.2. Migrating Hash Algorithms .................................48 + A.3. Migrating Signing Algorithms ..............................49 + Appendix B. General Coding Criteria for Cryptographic + Applications .........................................50 + + + + + + + + +Hansen, et al. Informational [Page 3] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +1. Introduction + + DomainKeys Identified Mail (DKIM) allows an organization to claim + responsibility for transmitting a message, in a way that can be + validated by a recipient. This document provides practical tips for + those who are developing DKIM software, mailing list managers, + filtering strategies based on the output from DKIM verification, and + DNS servers; those who are deploying DKIM software, keys, mailing + list software, and migrating from DomainKeys [RFC4870]; and those who + are responsible for the ongoing operations of an email infrastructure + that has deployed DKIM. + + The reader is encouraged to read the DKIM Service Overview document + [RFC5585] before this document. More detailed guidance about DKIM + and Author Domain Signing Practices (ADSP) can also be found in the + protocol specifications [RFC4871], [RFC5617], and [RFC5672]. + + The document is organized around the key concepts related to DKIM. + Within each section, additional considerations specific to + development, deployment, or ongoing operations are highlighted where + appropriate. The possibility of the use of DKIM results as input to + a local reputation database is also discussed. + +2. Using DKIM as Part of Trust Assessment + +2.1. A Systems View of Email Trust Assessment + + DKIM participates in a trust-oriented enhancement to the Internet's + email service, to facilitate message handling decisions, such as for + delivery and for content display. Trust-oriented message handling + has substantial differences from the more established approaches that + consider messages in terms of risk and abuse. With trust, there is a + collaborative exchange between a willing participant along the + sending path and a willing participant at a recipient site. In + contrast, the risk model entails independent, unilateral action by + the recipient site, in the face of a potentially unknown, hostile, + and deceptive sender. This translates into a very basic technical + difference: in the face of unilateral action by the recipient and + even antagonistic efforts by the sender, risk-oriented mechanisms are + based on heuristics, that is, on guessing. Guessing produces + statistical results with some false negatives and some false + positives. For trust-based exchanges, the goal is the deterministic + exchange of information. For DKIM, that information is the one + identifier that represents a stream of mail for which an independent + assessment is sought (by the signer). + + + + + + +Hansen, et al. Informational [Page 4] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + A trust-based service is built upon a validated Responsible + Identifier that labels a stream of mail and is controlled by an + identity (role, person, or organization). The identity is + acknowledging some degree of responsibility for the message stream. + Given a basis for believing that an identifier is being used in an + authorized manner, the recipient site can make and use an assessment + of the associated identity. An identity can use different + identifiers, on the assumption that the different streams might + produce different assessments. For example, even the best-run + marketing campaigns will tend to produce some complaints that can + affect the reputation of the associated identifier, whereas a stream + of transactional messages is likely to have a more pristine + reputation. + + Determining that the identifier's use is valid is quite different + from determining that the content of a message is valid. The former + means only that the identifier for the responsible role, person, or + organization has been legitimately associated with a message. The + latter means that the content of the message can be believed and, + typically, that the claimed author of the content is correct. DKIM + validates only the presence of the identifier used to sign the + message. Even when this identifier is validated, DKIM carries no + implication that any of the message content, including the + RFC5322.From field [RFC5322], is valid. Surprisingly, this limit to + the semantics of a DKIM signature applies even when the validated + signing identifier is the same domain name as is used in the + RFC5322.From field! DKIM's only claim about message content is that + the content cited in the DKIM-Signature: field's h= tag has been + delivered without modification. That is, it asserts message content + integrity -- between signing and verifying -- not message content + validity. + + As shown in Figure 1, this enhancement is a communication between a + responsible role, person, or organization that signs the message and + a recipient organization that assesses its trust in the signer. The + recipient then makes handling decisions based on a collection of + assessments, of which the DKIM mechanism is only a part. In this + model, as shown in Figure 1, validation is an intermediary step, + having the sole task of passing a validated Responsible Identifier to + the Identity Assessor. The communication is of a single Responsible + Identifier that the Responsible Identity wishes to have used by the + Identity Assessor. The Identifier is the sole, formal input and + output value of DKIM signing. The Identity Assessor uses this + single, provided Identifier for consulting whatever assessment + databases are deemed appropriate by the assessing entity. In turn, + output from the Identity Assessor is fed into a Handling Filter + + + + + +Hansen, et al. Informational [Page 5] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + engine that considers a range of factors, along with this single + output value. The range of factors can include ancillary information + from the DKIM validation. + + Identity Assessment covers a range of possible functions. It can be + as simple as determining whether the identifier is a member of some + list, such as authorized operators or participants in a group that + might be of interest for recipient assessment. Equally, it can + indicate a degree of trust (reputation) that is to be afforded the + actor using that identifier. The extent to which the assessment + affects the handling of the message is, of course, determined later, + by the Handling Filter. + + +------+------+ +------+------+ + | Author | | Recipient | + +------+------+ +------+------+ + | ^ + | | + | +------+------+ + | -->| Handling |<-- + | -->| Filter |<-- + | +-------------+ + | ^ + V Responsible | + +-------------+ Identifier +------+------+ + | Responsible |. . . . . . . . . . .>| Identity | + | Identity | . . | Assessor | + +------+------+ . . +-------------+ + | V . ^ ^ + V . . | | + +------------------.-------.--------------------+ | | + | +------+------+ . . . > . +-------------+ | | | +-----------+ + | | Identifier | | Identifier +--|--+ +--+ Assessment| + | | Signer +------------->| Validator | | | Databases | + | +-------------+ +-------------+ | +-----------+ + | DKIM Service | + +-----------------------------------------------+ + + Figure 1: Actors in a Trust Sequence Using DKIM + +2.2. Choosing a DKIM Tag for the Assessment Identifier + + The signer of a message needs to be able to provide precise data and + know what that data will mean upon delivery to the Assessor. If + there is ambiguity in the choice that will be made on the recipient + side, then the sender cannot know what basis for assessment will be + used. DKIM has three values that specify identification information + and it is easy to confuse their use, although only one defines the + + + +Hansen, et al. Informational [Page 6] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + formal input and output of DKIM, with the other two being used for + internal protocol functioning and adjunct purposes, such as auditing + and debugging. + + The salient values include the s=, d= and i= parameters in the DKIM- + Signature: header field. In order to achieve the end-to-end + determinism needed for this collaborative exchange from the signer to + the assessor, the core model needs to specify what the signer is + required to provide to the assessor. The update to RFC 4871 + [RFC5672] specifies: + + DKIM's primary task is to communicate from the Signer to a + recipient-side Identity Assessor a single Signing Domain + Identifier (SDID) that refers to a responsible identity. DKIM MAY + optionally provide a single responsible Agent or User Identifier + (AUID)... A receive-side DKIM verifier MUST communicate the + Signing Domain Identifier (d=) to a consuming Identity Assessor + module and MAY communicate the User Agent Identifier (i=) if + present.... To the extent that a receiver attempts to intuit any + structured semantics for either of the identifiers, this is a + heuristic function that is outside the scope of DKIM's + specification and semantics. + + The single, mandatory value that DKIM supplies as its output is: + + d= This specifies the "domain of the signing entity". It is a + domain name and is combined with the selector to form a DNS + query. A receive-side DKIM verifier needs to communicate the + Signing Domain Identifier (d=) to a consuming Identity Assessor + module and can also communicate the User Agent Identifier (i=) + if present. + + The adjunct values are: + + s= This tag specifies the selector. It is used to discriminate + among different keys that can be used for the same d= domain + name. As discussed in Section 4.3 of [RFC5585], "If verifiers + were to employ the selector as part of an assessment mechanism, + then there would be no remaining mechanism for making a + transition from an old, or compromised, key to a new one". + Consequently, the selector is not appropriate for use as part + or all of the identifier used to make assessments. + + i= This tag is optional and provides the "[t]he Agent or User + Identifier (AUID) on behalf of which the SDID is taking + responsibility" [RFC5672]. The identity can be in the syntax + + + + + +Hansen, et al. Informational [Page 7] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + of an entire email address or only a domain name. The domain + name can be the same as for d= or it can be a sub-name of the + d= name. + + NOTE: Although the i= identity has the syntax of an email + address, it is not required to have those semantics. That is, + "the identity of the user" need not be the same as the user's + mailbox. For example, the signer might wish to use i= to + encode user-related audit information, such as how they were + accessing the service at the time of message posting. + Therefore, it is not possible to conclude anything from the i= + string's (dis)similarity to email addresses elsewhere in the + header. + + So, i= can have any of these properties: + + * Be a valid domain when it is the same as d= + + * Appear to be a subdomain of d= but might not even exist + + * Look like a mailbox address but might have different semantics + and therefore not function as a valid email address + + * Be unique for each message, such as indicating access details + of the user for the specific posting + + This underscores why the tag needs to be treated as being opaque, + since it can represent any semantics, known only to the signer. + + Hence, i= serves well as a token that is usable like a Web cookie, + for return to the signing Administrative Management Domain (ADMD) -- + such as for auditing and debugging. Of course in some scenarios the + i= string might provide a useful adjunct value for additional + (heuristic) processing by the Handling Filter. + +2.3. Choosing the Signing Domain Name + + A DKIM signing entity can serve different roles, such as being the + author of content, the operator of the mail service, or the operator + of a reputation service that also provides signing services on behalf + of its customers. In these different roles, the basis for + distinguishing among portions of email traffic can vary. For an + entity creating DKIM signatures, it is likely that different portions + of its mail will warrant different levels of trust. For example: + + * Mail is sent for different purposes, such as marketing versus + transactional, and recipients demonstrate different patterns of + acceptance between these. + + + +Hansen, et al. Informational [Page 8] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + * For an operator of an email service, there often are distinct + sub-populations of users warranting different levels of trust + or privilege, such as paid versus free users, or users engaged + in direct correspondence versus users sending bulk mail. + + * Mail originating outside an operator's system, such as when it + is redistributed by a mailing-list service run by the operator, + will warrant a different reputation from mail submitted by + users authenticated with the operator. + + It is therefore likely to be useful for a signer to use different d= + subdomain names, for different message traffic streams, so that + receivers can make differential assessments. However, too much + differentiation -- that is, too fine a granularity of signing domains + -- makes it difficult for the receiver to discern a sufficiently + stable pattern of traffic for developing an accurate and reliable + assessment. So the differentiation needs to achieve a balance. + Generally, in a trust system, legitimate signers have an incentive to + pick a small stable set of identities, so that recipients and others + can attribute reputations to them. The set of these identities a + receiver trusts is likely to be quite a bit smaller than the set it + views as risky. + + The challenge in using additional layers of subdomains is whether the + extra granularity will be useful for the Assessor. In fact, + excessive levels invite ambiguity: if the Assessor does not take + advantage of the added granularity in the entire domain name that is + provided, they might unilaterally decide to use only some rightmost + part of the identifier. The signer cannot know what portion will be + used. That ambiguity would move the use of DKIM back to the realm of + heuristics, rather than the deterministic processing that is its + goal. + + Hence, the challenge is to determine a useful scheme for labeling + different traffic streams. The most obvious choices are among + different types of content and/or different types of authors. + Although stability is essential, it is likely that the choices will + change, over time, so the scheme needs to be flexible. + + + + + + + + + + + + + +Hansen, et al. Informational [Page 9] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + For those originating message content, the most likely choice of + subdomain naming scheme will by based upon type of content, which can + use content-oriented labels or service-oriented labels. For example: + + transaction.example.com + newsletter.example.com + bugreport.example.com + support.example.com + sales.example.com + marketing.example.com + + where the choices are best dictated by whether they provide the + Identity Assessor with the ability to discriminate usefully among + streams of mail that demonstrate significantly different degrees of + recipient acceptance or safety. Again, the danger in providing too + fine a granularity is that related message streams that are labeled + separately will not benefit from an aggregate reputation. + + For those operating messaging services on behalf of a variety of + customers, an obvious scheme to use has a different subdomain label + for each customer. For example: + + widgetco.example.net + moviestudio.example.net + bigbank.example.net + + However, it can also be appropriate to label by the class of service + or class of customer, such as: + + premier.example.net + free.example.net + certified.example.net + + Prior to using domain names for distinguishing among sources of data, + IP Addresses have been the basis for distinction. Service operators + typically have done this by dedicating specific outbound IP Addresses + to specific mail streams -- typically to specific customers. For + example, a university might want to distinguish mail from the + administration, versus mail from the student dorms. In order to make + the adoption of a DKIM-based service easier, it can be reasonable to + translate the same partitioning of traffic, using domain names in + place of the different IP Addresses. + +2.4. Recipient-Based Assessments + + DKIM gives the recipient site's Identity Assessor a verifiable + identifier to use for analysis. Although the mechanism does not make + claims that the signer is a Good Actor or a Bad Actor, it does make + + + +Hansen, et al. Informational [Page 10] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + it possible to know that use of the identifier is valid. This is in + marked contrast with schemes that do not have authentication. + Without verification, it is not possible to know whether the + identifier -- whether taken from the RFC5322.From field, the + RFC5321.MailFrom command, or the like -- is being used by an + authorized agent. DKIM solves this problem. Hence, with DKIM, the + Assessor can know that two messages with the same DKIM d= identifier + are, in fact, signed by the same person or organization. This + permits a far more stable and accurate assessment of mail traffic + using that identifier. + + DKIM is distinctive, in that it provides an identifier that is not + necessarily related to any other identifier in the message. Hence, + the signer might be the author's ADMD, one of the operators along the + transit path, or a reputation service being used by one of those + handling services. In fact, a message can have multiple signatures, + possibly by any number of these actors. + + As discussed above, the choice of identifiers needs to be based on + differences that the signer thinks will be useful for the recipient + Assessor. Over time, industry practices establish norms for these + choices. + + Absent such norms, it is best for signers to distinguish among + streams that have significant differences, while consuming the + smallest number of identifiers possible. This will limit the + burden on recipient Assessors. + + A common view about a DKIM signature is that it carries a degree of + assurance about some or all of the message contents, and in + particular, that the RFC5322.From field is likely to be valid. In + fact, DKIM makes assurances only about the integrity of the data and + not about its validity. Still, presumptions of the RFC5322.From + field validity remain a concern. Hence, a signer using a domain name + that is unrelated to the domain name in the RFC5322.From field can + reasonably expect that the disparity will warrant some curiosity, at + least until signing by independent operators has produced some + established practice among recipient Assessors. + + With the identifier(s) supplied by DKIM, the Assessor can consult an + independent assessment service about the entity associated with the + identifier(s). Another possibility is that the Assessor can develop + its own reputation rating for the identifier(s). That is, over time, + the Assessor can observe the stream of messages associated with the + identifier(s) developing a reaction to associated content. For + example, if there is a high percentage of user complaints regarding + + + + + +Hansen, et al. Informational [Page 11] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + signed mail with a d= value of "widgetco.example.net", the Assessor + might include that fact in the vector of data it provides to the + Handling Filter. This is also discussed briefly in Section 5.4. + +2.5. Filtering + + The assessment of the signing identifier is given to a Handling + Filter that is defined by local policies, according to a potentially + wide range of different factors and weightings. This section + discusses some of the kinds of choices and weightings that are + plausible and the differential actions that might be performed. + Because authenticated domain names represent a collaborative sequence + between signer and Assessor, actions can sometimes reasonably include + contacting the signer. + + The discussion focuses on variations in Organizational Trust versus + Message Stream Risk, that is, the degree of positive assessment of a + DKIM-signing organization, and the potential danger present in the + message stream signed by that organization. While it might seem that + higher trust automatically means lower risk, the experience with + real-world operations provides examples of every combination of the + two factors, as shown in Figure 2. For each axis, only three levels + of granularity are listed, in order to keep discussion manageable. + In real-world filtering engines, finer-grained distinctions are + typically needed, and there typically are more axes. For example, + there are different types of risk, so that an engine might + distinguish between spam risk versus virus risk and take different + actions based on which type of problematic content is present. For + spam, the potential damage from a false negative is small, whereas + the damage from a false positive is high. For a virus, the potential + danger from a false negative is extremely high, while the likelihood + of a false positive when using modern detection tools is extremely + low. However, for the discussion here, "risk" is taken as a single + construct. + + The DKIM d= identifier is independent of any other identifier in a + message and can be a subdomain of the name owned by the signer. This + permits the use of fine-grained and stable distinctions between + different types of message streams, such as between transactional + messages and marketing messages from the same organization. Hence, + the use of DKIM might permit a richer filtering model than has + typically been possible for mail-receiving engines. + + Note that the realities of today's public Internet Mail environment + necessitate having a baseline handling model that is quite + suspicious. Hence, "strong" filtering rules really are the starting + point, as indicated for the UNKNOWN cell. + + + + +Hansen, et al. Informational [Page 12] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + The table indicates differential handling for each combination, such + as how aggressive or broad-based the filtering could be. + Aggressiveness affects the types of incorrect assessments that are + likely. So, the table distinguishes various characteristics, + including: 1) whether an organization is unknown, known to be good + actors, or known to be bad actors; and 2) the assessment of messages. + It includes advice about the degree of filtering that might be done, + and other message disposition. Perhaps unexpectedly, it also lists a + case in which the receiving site might wish to deliver problematic + mail, rather than redirecting or deleting it. The site might also + wish to contact the signing organization and seek resolution of the + problem. + + +-------------+-----------------------------------------------+ + | S T R E A M * O R G A N I Z A T I O N A L T R U S T | + | R I S K * Low Medium High | + | +***************+***************+***************+ + | Low * BENIGN: | DILIGENT: | PRISTINE | + | * Moderate | Mild | Accept | + | * filter | filter | | + | +---------------+---------------+---------------+ + | Medium * UNKNOWN: | TYPICAL: | PROTECTED: | + | * Strong | Targeted | Accept & | + | * filter | filter | Contact | + | +---------------+---------------+---------------+ + | High * MALICIOUS: | NEGLIGENT: | COMPROMISED: | + | * Block & | Block | Block & | + | * Counter | | Contact | + +-------------+---------------+---------------+---------------+ + + Figure 2: Trust versus Risk Handling Tradeoffs Example + + [LEGEND] + + AXES + + Stream Risk: This is a measure of the recent history of a message + stream and the severity of problems it has presented. + + Organizational Trust: This combines longer-term history about + possible stream problems from that organization, and its + responsiveness to problem handling. + + CELLS (indicating reasonable responses) + + Labels for the cells are meant as a general assessment of an + organization producing that type of mail stream under that + circumstance. + + + +Hansen, et al. Informational [Page 13] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Benign: There is some history of sending good messages, with very + few harmful messages having been received. This stream + warrants filtering that does not search for problems very + aggressively, in order to reduce the likelihood of false + positives. + + Diligent: The stream has had a limited degree of problems and the + organization is consistently successful at controlling their + abuse issues and in a timely manner. + + Pristine: There is a history of a clean message stream with no + problems, from an organization with an excellent reputation. + So, the filter primarily needs to ensure that messages are + delivered; catching stray problem messages is a lesser concern. + In other words, the paramount concern, here, is false + positives. + + ----- + + Unknown: There is no history with the organization. Apply an + aggressive level of "naive" filtering, given the nature of the + public email environment. + + Typical: The stream suffers significant abuse issues and the + organization has demonstrated a record of having difficulties + resolving them in a timely manner, in spite of legitimate + efforts. Unfortunately, this is the typical case for service + providers with an easy and open subscription policy. + + Protected: An organization with a good history and/or providing + an important message stream for the receiving site is subject + to a local policy that messages are not allowed to be blocked, + but the stream is producing a problematic stream. The receiver + delivers messages, but works quickly with the organization to + resolve the matter. + + ----- + + Malicious: A persistently problematic message stream is coming + from an organization that appears to contribute to the problem. + The stream will be blocked, but the organization's role is + sufficiently troubling to warrant following up with others in + the anti-abuse or legal communities, to constrain or end their + impact. + + + + + + + +Hansen, et al. Informational [Page 14] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Negligent: A persistently problematic message stream is coming + from an organization that does not appear to be contributing to + the problem, but also does not appear to be working to + eliminate it. At the least, the stream needs to be blocked. + + Compromised: An organization with a good history has a stream + that changes and becomes too problematic to be delivered. The + receiver blocks the stream and works quickly with the + organization to resolve the matter. + +3. DKIM Key Generation, Storage, and Management + + By itself, verification of a digital signature only allows the + verifier to conclude with a very high degree of certainty that the + signature was created by a party with access to the corresponding + private signing key. It follows that a verifier requires means to + (1) obtain the public key for the purpose of verification and (2) + infer useful attributes of the key holder. + + In a traditional Public Key Infrastructure (PKI), the functions of + key distribution and key accreditation are separated. In DKIM + [RFC4871], these functions are both performed through the DNS. + + In either case, the ability to infer semantics from a digital + signature depends on the assumption that the corresponding private + key is only accessible to a party with a particular set of + attributes. In a traditional PKI, a Trusted Third Party (TTP) + vouches that the key holder has been validated with respect to a + specified set of attributes. The range of attributes that can be + attested in such a scheme is thus limited only to the type of + attributes that a TTP can establish effective processes for + validating. In DKIM, TTPs are not employed and the functions of key + distribution and accreditation are combined. + + Consequently, there are only two types of inference that a signer can + make from a key published in a DKIM key record: + + 1. That a party with the ability to control DNS records within a DNS + zone intends to claim responsibility for messages signed using + the corresponding private signature key. + + 2. That use of a specific key is restricted to the particular subset + of messages identified by the selector. + + The ability to draw any useful conclusion from verification of a + digital signature relies on the assumption that the corresponding + private key is only accessible to a party with a particular set of + + + + +Hansen, et al. Informational [Page 15] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + attributes. In the case of DKIM, this means that the party that + created the corresponding DKIM key record in the specific zone + intended to claim responsibility for the signed message. + + Ideally, we would like to draw a stronger conclusion, that if we + obtain a DKIM key record from the DNS zone example.com, that the + legitimate holder of the DNS zone example.com claims responsibility + for the signed message. In order for this conclusion to be drawn, it + is necessary for the verifier to assume that the operational security + of the DNS zone and corresponding private key are adequate. + +3.1. Private Key Management: Deployment and Ongoing Operations + + Access to signing keys needs to be carefully managed to prevent use + by unauthorized parties and to minimize the consequences if a + compromise were to occur. + + While a DKIM signing key is used to sign messages on behalf of many + mail users, the signing key itself needs to be under direct control + of as few key holders as possible. If a key holder were to leave the + organization, all signing keys held by that key holder need to be + withdrawn from service and, if appropriate, replaced. + + If key management hardware support is available, it needs to be used. + If keys are stored in software, appropriate file control protections + need to be employed, and any location in which the private key is + stored in plaintext form needs to be excluded from regular backup + processes and is best not accessible through any form of network + including private local area networks. Auditing software needs to be + used periodically to verify that the permissions on the private key + files remain secure. + + Wherever possible, a signature key needs to exist in exactly one + location and be erased when no longer used. Ideally, a signature key + pair needs to be generated as close to the signing point as possible, + and only the public key component transferred to another party. If + this is not possible, the private key needs to be transported in an + encrypted format that protects the confidentiality of the signing + key. A shared directory on a local file system does not provide + adequate security for distribution of signing keys in plaintext form. + + Key escrow schemes are not necessary and are best not used. In the + unlikely event of a signing key becoming lost, a new signature key + pair can be generated as easily as recovery from a key escrow scheme. + + + + + + + +Hansen, et al. Informational [Page 16] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + To enable accountability and auditing: + + o Responsibility for the security of a signing key needs to + ultimately vest in a single named individual. + + o Where multiple parties are authorized to sign messages, each + signer needs to use a different key to enable accountability and + auditing. + + Best practices for management of cryptographic keying material + require keying material to be refreshed at regular intervals, + particularly where key management is achieved through software. + While this practice is highly desirable, it is of considerably less + importance than the requirement to maintain the secrecy of the + corresponding private key. An operational practice in which the + private key is stored in tamper-proof hardware and changed once a + year is considerably more desirable than one in which the signature + key is changed on an hourly basis but maintained in software. + +3.2. Storing Public Keys: DNS Server Software Considerations + + In order to use DKIM, a DNS domain holder requires (1) the ability to + create the necessary DKIM DNS records and (2) sufficient operational + security controls to prevent insertion of spurious DNS records by an + attacker. + + DNS record management is often operated by an administrative staff + that is different from those who operate an organization's email + service. In order to ensure that DKIM DNS records are accurate, this + imposes a requirement for careful coordination between the two + operations groups. If the best practices for private key management + described above are observed, such deployment is not a one-time + event; DNS DKIM selectors will be changed over time as signing keys + are terminated and replaced. + + At a minimum, a DNS server that handles queries for DKIM key records + needs to allow the server administrators to add free-form TXT + records. It would be better if the DKIM records could be entered + using a structured form, supporting the DKIM-specific fields. + + Ideally, DNS Security (DNSSEC) [RFC4034] needs to be employed in a + configuration that provides protection against record insertion + attacks and zone enumeration. In the case that NextSECure version 3 + (NSEC3) [RFC5155] records are employed to prevent insertion attack, + the OPT-OUT flag needs to be clear. (See [RFC5155] section 6 for + details.) + + + + + +Hansen, et al. Informational [Page 17] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +3.2.1. Assignment of Selectors + + Selectors are assigned according to the administrative needs of the + signing domain, such as for rolling over to a new key or for the + delegation of the right to authenticate a portion of the namespace to + a TTP. Examples include: + + jun2005.eng._domainkey.example.com + + widget.promotion._domainkey.example.com + + It is intended that assessments of DKIM identities be based on the + domain name, and not include the selector. While past practice of a + signer can permit a verifier to infer additional properties of + particular messages from the structure DKIM key selector, unannounced + administrative changes such as a change of signing software can cause + such heuristics to fail at any time. + +3.3. Per-User Signing Key Management Issues + + While a signer can establish business rules, such as the issue of + individual signature keys for each end-user, DKIM makes no provision + for communicating these to other parties. Out-of-band distribution + of such business rules is outside the scope of DKIM. Consequently, + there is no means by which external parties can make use of such keys + to attribute messages with any greater granularity than a DNS domain. + + If per-user signing keys are assigned for internal purposes (e.g., + authenticating messages sent to an MTA (Mail Transfer Agent) for + distribution), the following issues need to be considered before + using such signatures as an alternative to traditional edge signing + at the outbound MTA: + + External verifiers will be unable to make use of the additional + signature granularity without access to additional information + passed out of band with respect to [RFC4871]. + + If the number of user keys is large, the efficiency of local + caching of key records by verifiers will be lower. + + A large number of end users is be less likely to do an adequate + job of managing private key data securely on their personal + computers than is an administrator running an edge MTA. + + + + + + + + +Hansen, et al. Informational [Page 18] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +3.4. Third-Party Signer Key Management and Selector Administration + + A DKIM key record only asserts that the holder of the corresponding + domain name makes a claim of responsibility for messages signed under + the corresponding key. In some applications, such as bulk mail + delivery, it is desirable to delegate use of the key. That is, to + allow a third party to sign on behalf of the domain holder. The + trust relationship is still established between the domain holder and + the verifier, but the private signature key is held by a third party. + + Signature keys used by a third-party signer need to be kept entirely + separate from those used by the domain holder and other third-party + signers. To limit potential exposure of the private key, the + signature key pair needs to be generated by the third-party signer + and the public component of the key transmitted to the domain holder, + rather than have the domain holder generate the key pair and transmit + the private component to the third-party signer. + + Domain holders needs to adopt a least-privilege approach and grant + third-party signers the minimum access necessary to perform the + desired function. Limiting the access granted to third-party signers + serves to protect the interests of both parties. The domain holder + minimizes its security risk and the TTP signer avoids unnecessary + liability. + + In the most restrictive case, domain holders maintain full control + over the creation of key records. They can employ appropriate key + record restrictions to enforce limits on the messages for which the + third-party signer is able to sign. If such restrictions are + impractical, the domain holder needs to delegate a DNS subzone for + publishing key records to the third-party signer. It is best that + the domain holder NOT allow a third-party signer unrestricted access + to its DNS service for the purpose of publishing key records. + +3.5. Key Pair / Selector Life Cycle Management + + Deployments need to establish, document, and observe processes for + managing the entire life cycle of an asymmetric key pair. + +3.5.1. Example Key Deployment Process + + When it is determined that a new key pair is required: + + 1. A Key Pair is generated by the signing device. + + 2. A proposed key selector record is generated and transmitted to + the DNS administration infrastructure. + + + + +Hansen, et al. Informational [Page 19] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + 3. The DNS administration infrastructure verifies the authenticity + of the key selector registration request. If accepted: + + 1. A key selector is assigned. + + 2. The corresponding key record is published in the DNS. + + 3. Wait for DNS updates to propagate (if necessary). + + 4. Report assigned key selector to signing device. + + 4. The signer verifies correct registration of the key record. + + 5. The signer begins generating signatures using the new key pair. + + 6. The signer terminates any private keys that are no longer + required due to issue of replacement. + +3.5.2. Example Key Termination Process + + When it is determined that a private signature key is no longer + required: + + 1. The signer stops using the private key for signature operations. + + 2. The signer deletes all records of the private key, including in- + memory copies at the signing device. + + 3. The signer notifies the DNS administration infrastructure that + the signing key is withdrawn from service and that the + corresponding key records can be withdrawn from service at a + specified future date. + + 4. The DNS administration infrastructure verifies the authenticity + of the key selector termination request. If accepted, + + 1. The key selector is scheduled for deletion at a future time + determined by site policy. + + 2. Wait for deletion time to arrive. + + 3. The signer either publishes a revocation key selector with an + empty public-key data (p=) field, or deletes the key selector + record entirely. + + 5. As far as the verifier is concerned, there is no functional + difference between verifying against a key selector with an empty + p= field, and verifying against a missing key selector: both + + + +Hansen, et al. Informational [Page 20] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + result in a failed signature and the signature needs to be + treated as if it had not been there. However, there is a minor + semantic difference: with the empty p= field, the signer is + explicitly stating that the key has been revoked. The empty p= + record provides a gravestone for an old selector, making it less + likely that the selector might be accidentally reused with a + different public key. + +4. Signing + + Creating messages that have one or more DKIM signatures requires + support in only two outbound email service components: + + o A DNS Administrative interface that can create and maintain the + relevant DNS names -- including names with underscores -- and + resource records (RR). + + o A trusted module, called the signing module, which is within the + organization's outbound email handling service and which creates + and adds the DKIM-Signature: header field(s) to the message. + + If the module creates more than one signature, there needs to be the + appropriate means of telling it which one(s) to use. If a large + number of names are used for signing, it will help to have the + administrative tool support a batch-processing mode. + +4.1. DNS Records + + A receiver attempting to verify a DKIM signature obtains the public + key that is associated with the signature for that message. The + DKIM-Signature: header in the message contains the d= tag with the + basic domain name doing the signing and serving as output to the + Identity Assessor and the s= tag with the selector that is added to + the name, for finding the specific public key. Hence, the relevant + <selector>._domainkey.<domain-name> DNS record needs to contain a + DKIM-related RR that provides the public key information. + + The administrator of the zone containing the relevant domain name + adds this information. Initial DKIM DNS information is contained + within TXT RRs. DNS administrative software varies considerably in + its abilities to support DKIM names, such as with underscores, and to + add new types of DNS information. + +4.2. Signing Module + + The module doing signing can be placed anywhere within an + organization's trusted Administrative Management Domain (ADMD); + obvious choices include department-level posting agents, as well as + + + +Hansen, et al. Informational [Page 21] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + outbound boundary MTAs to the open Internet. However, any other + module, including the author's MUA (Mail User Agent), is potentially + acceptable, as long as the signature survives any remaining handling + within the ADMD. Hence, the choice among the modules depends upon + software development, administrative overhead, security exposures, + and transit-handling tradeoffs. One perspective that helps to + resolve this choice is the difference between the increased + flexibility, from placement at (or close to) the MUA, versus the + streamlined administration and operation that is more easily obtained + by implementing the mechanism "deeper" into the organization's email + infrastructure, such as at its boundary MTA. + + Note the discussion in Section 2.2 concerning the use of the i= tag. + + The signing module uses the appropriate private key to create one or + more signatures. (See Section 6.5 for a discussion of multiple + signatures.) The means by which the signing module obtains the + private key(s) is not specified by DKIM. Given that DKIM is intended + for use during email transit, rather than for long-term storage, it + is expected that keys will be changed regularly. For administrative + convenience, it is best not to hard-code key information into + software. + +4.3. Signing Policies and Practices + + Every organization (ADMD) will have its own policies and practices + for deciding when to sign messages (message stream) and with what + domain name, selector, and key. Examples of particular message + streams include all mail sent from the ADMD versus mail from + particular types of user accounts versus mail having particular types + of content. Given this variability, and the likelihood that signing + practices will change over time, it will be useful to have these + decisions represented through run-time configuration information, + rather than being hard-coded into the signing software. + + As noted in Section 2.3, the choice of signing name granularity + requires balancing administrative convenience and utility for + recipients. Too much granularity is higher administrative overhead + and might well attempt to impose more differential analysis on the + recipient than they wish to support. In such cases, they are likely + to use only a super-name -- right-hand substring -- of the signing + name. When this occurs, the signer will not know what portion is + being used; this then moves DKIM back to the non-deterministic world + of heuristics, rather than the mechanistic world of signer/recipient + collaboration that DKIM seeks. + + + + + + +Hansen, et al. Informational [Page 22] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +5. Verifying + + A message recipient can verify a DKIM signature to determine if a + claim of responsibility has been made for the message by a trusted + domain. + + Access control requires two components: authentication and + authorization. By design, verification of a DKIM signature only + provides the authentication component of an access control decision + and needs to be combined with additional sources of information such + as reputation data to arrive at an access control decision. + +5.1. Intended Scope of Use + + DKIM requires that a message with a signature that is found to be + invalid is to be treated as if the message had not been signed at + all. + + If a DKIM signature fails to verify, it is entirely possible that the + message is valid and that either there is a configuration error in + the signer's system (e.g., a missing key record) or that the message + was inadvertently modified in transit. It is thus undesirable for + mail infrastructure to treat messages with invalid signatures less + favorably than those with no signatures whatsoever. Contrariwise, + creation of an invalid signature requires a trivial amount of effort + on the part of an attacker. If messages with invalid signatures were + to be treated preferentially to messages with no signatures + whatsoever, attackers will simply add invalid signature blocks to + gain the preferential treatment. It follows that messages with + invalid signatures need to be treated no better and no worse than + those with no signature at all. + +5.2. Signature Scope + + As with any other digital signature scheme, verifiers need to + consider only the part of the message that is inside the scope of the + message as being authenticated by the signature. + + For example, if the l= option is employed to specify a content length + for the scope of the signature, only the part of the message that is + within the scope of the content signature would be considered + authentic. + + + + + + + + + +Hansen, et al. Informational [Page 23] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +5.3. Design Scope of Use + + Public key cryptography provides an exceptionally high degree of + assurance, bordering on absolute certainty, that the party that + created a valid digital signature had access to the private key + corresponding to the public key indicated in the signature. + + In order to make useful conclusions from the verification of a valid + digital signature, the verifier is obliged to make assumptions that + fall far short of absolute certainty. Consequently, mere validation + of a DKIM signature does not represent proof positive that a valid + claim of responsibility was made for it by the indicated party, that + the message is authentic, or that the message is not abusive. In + particular: + + o The legitimate private key holder might have lost control of its + private key. + + o The legitimate domain holder might have lost control of the DNS + server for the zone from which the key record was retrieved. + + o The key record might not have been delivered from the legitimate + DNS server for the zone from which the key record was retrieved. + + o Ownership of the DNS zone might have changed. + + In practice, these limitations have little or no impact on the field + of use for which DKIM is designed, but they can have a bearing if use + is made of the DKIM message signature format or key retrieval + mechanism in other specifications. + + In particular, the DKIM key retrieval mechanism is designed for ease + of use and deployment rather than to provide a high assurance Public + Key Infrastructure suitable for purposes that require robust non- + repudiation such as establishing legally binding contracts. + Developers seeking to extend DKIM beyond its design application need + to consider replacing or supplementing the DNS key retrieval + mechanism with one that is designed to meet the intended purposes. + +5.4. Inbound Mail Filtering + + DKIM is frequently employed in a mail filtering strategy to avoid + performing content analysis on email originating from trusted + sources. Messages that carry a valid DKIM signature from a trusted + source can be whitelisted, avoiding the need to perform computation + and hence energy-intensive content analysis to determine the + disposition of the message. + + + + +Hansen, et al. Informational [Page 24] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Mail sources can be determined to be trusted by means of previously + observed behavior and/or reference to external reputation or + accreditation services. The precise means by which this is + accomplished is outside the scope of DKIM. + +5.4.1. Non-Verifying Adaptive Spam Filtering Systems + + Adaptive (or learning) spam filtering mechanisms that are not capable + of verifying DKIM signatures need to, at minimum, be configured to + ignore DKIM header data entirely. + +5.5. Messages Sent through Mailing Lists and Other Intermediaries + + Intermediaries, such as mailing lists, pose a particular challenge + for DKIM implementations, as the message processing steps performed + by the intermediary can cause the message content to change in ways + that prevent the signature passing verification. + + Such intermediaries are strongly encouraged to deploy DKIM signing so + that a verifiable claim of responsibility remains available to + parties attempting to verify the modified message. + +5.6. Generation, Transmission, and Use of Results Headers + + In many deployments, it is desirable to separate signature + verification from the application relying on the verification. A + system can choose to relay information indicating the results of its + message authentication efforts using various means; adding a "results + header" to the message is one such mechanism [RFC5451]. For example, + consider the cases where: + + o The application relying on DKIM signature verification is not + capable of performing the verification. + + o The message can be modified after the signature verification is + performed. + + o The signature key cannot be available by the time that the message + is read. + + In such cases, it is important that the communication link between + the signature verifier and the relying application be sufficiently + secure to prevent insertion of a message that carries a bogus results + header. + + An intermediary that generates results headers need to ensure that + relying applications are able to distinguish valid results headers + issued by the intermediary from those introduced by an attacker. For + + + +Hansen, et al. Informational [Page 25] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + example, this can be accomplished by signing the results header. At + a minimum, results headers on incoming messages need to be removed if + they purport to have been issued by the intermediary but cannot be + verified as authentic. + + Further discussion on trusting the results as relayed from a verifier + to something downstream can be found in [RFC5451]. + +6. Taxonomy of Signatures + + As described in Section 2.1, a DKIM signature tells the signature + verifier that the owner of a particular domain name accepts some + responsibility for the message. It does not, in and of itself, + provide any information about the trustworthiness or behavior of that + identity. What it does provide is a verified identity to which such + behavioral information can be associated, so that those who collect + and use such information can be assured that it truly pertains to the + identity in question. + + This section lays out a taxonomy of some of the different identities, + or combinations of identities, that might usefully be represented by + a DKIM signature. + +6.1. Single Domain Signature + + Perhaps the simplest case is when an organization signs its own + outbound email using its own domain in the SDID [RFC5672] of the + signature. For example, Company A would sign the outbound mail from + its employees with d=companyA.example. + + In the most straightforward configuration, the addresses in the + RFC5322.From field would also be in the companyA.example domain, but + that direct correlation is not required. + + A special case of the single domain signature is an author signature + as defined by the Author Domain Signing Practices specification + [RFC5617]. Author signatures are signatures from an author's + organization that have an SDID value that matches that of an + RFC5322.From address of the signed message. + + Although an author signature might, in some cases, be proof against + spoofing the domain name of the RFC5322.From address, it is important + to note that the DKIM and ADSP validation apply only to the exact + address string and not to look-alike addresses or to the human- + friendly "display-name" or names and addresses used within the body + of the message. That is, it only protects against the misuse of a + precise address string within the RFC5322.From field and nothing + else. For example, a message from bob@domain.example with a valid + + + +Hansen, et al. Informational [Page 26] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + signature where d=d0main.example would fail an ADSP check because the + signature domain, however similar, is distinct; however, a message + from bob@d0main.example with a valid signature where d=d0main.example + would pass an ADSP check, even though to a human it might be obvious + that d0main.example is likely a malicious attempt to spoof the domain + domain.example. This example highlights that ADSP, like DKIM, is + only able to validate a signing identifier: it still requires some + external process to attach a meaningful reputation to that + identifier. + +6.2. Parent Domain Signature + + Another approach that might be taken by an organization with multiple + active subdomains is to apply the same (single) signature domain to + mail from all subdomains. In this case, the signature chosen would + usually be the signature of a parent domain common to all subdomains. + For example, mail from marketing.domain.example, + sales.domain.example, and engineering.domain.example might all use a + signature where d=domain.example. + + This approach has the virtue of simplicity, but it is important to + consider the implications of such a choice. As discussed in + Section 2.3, if the type of mail sent from the different subdomains + is significantly different or if there is reason to believe that the + reputation of the subdomains would differ, then it can be a good idea + to acknowledge this and provide distinct signatures for each of the + subdomains (d=marketing.domain.example, sales.domain.example, etc.). + However, if the mail and reputations are likely to be similar, then + the simpler approach of using a single common parent domain in the + signature can work well. + + Another approach to distinguishing the streams using a single DKIM + key would be to leverage the AUID [RFC5672] (i= tag) in the DKIM + signature to differentiate the mail streams. For example, marketing + email would be signed with i=@marketing.domain.example and + d=domain.example. + + It's important to remember, however, that under core DKIM semantics, + the AUID is opaque to receivers. That means that it will only be an + effective differentiator if there is an out-of-band agreement about + the i= semantics. + +6.3. Third-Party Signature + + A signature whose domain does not match the domain of the + RFC5322.From address is sometimes referred to as a third-party + signature. In certain cases, even the parent domain signature + + + + +Hansen, et al. Informational [Page 27] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + described above would be considered a third-party signature because + it would not be an exact match for the domain in the RFC5322.From + address. + + Although there is often heated debate about the value of third party + signatures, it is important to note that the DKIM specification + attaches no particular significance to the identity in a DKIM + signature ([RFC4871], [RFC5672]). The identity specified within the + signature is the identity that is taking responsibility for the + message, and it is only the interpretation of a given receiver that + gives one identity more or less significance than another. In + particular, most independent reputation services assign trust based + on the specific identifier string, not its "role": in general they + make no distinction between, for example, an author signature and a + third-party signature. + + For some, a signature unrelated to the author domain (the domain in + the RFC5322.From address) is less valuable because there is an + assumption that the presence of an author signature guarantees that + the use of the address in the RFC5322.From header is authorized. + + For others, that relevance is tied strictly to the recorded + behavioral data assigned to the identity in question, i.e., its trust + assessment or reputation. The reasoning here is that an identity + with a good reputation is unlikely to maintain that good reputation + if it is in the habit of vouching for messages that are unwanted or + abusive; in fact, doing so will rapidly degrade its reputation so + that future messages will no longer benefit from it. It is therefore + low risk to facilitate the delivery of messages that contain a valid + signature of a domain with a strong positive reputation, independent + of whether or not that domain is associated with the address in the + RFC5322.From header field of the message. + + Third-party signatures encompass a wide range of identities. Some of + the more common are: + + Service Provider: In cases where email is outsourced to an Email + Service Provider (ESP), Internet Service Provider (ISP), or other + type of service provider, that service provider can choose to + DKIM-sign outbound mail with either its own identifier -- relying + on its own, aggregate reputation -- or with a subdomain of the + provider that is unique to the message author but still part of + the provider's aggregate reputation. Such service providers can + also encompass delegated business functions such as benefit + management, although these will more often be treated as trusted + third-party senders (see below). + + + + + +Hansen, et al. Informational [Page 28] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Parent Domain: As discussed above, organizations choosing to apply a + parent-domain signature to mail originating from subdomains can + have their signatures treated as third party by some verifiers, + depending on whether or not the "t=s" tag is used to constrain the + parent signature to apply only to its own specific domain. The + default is to consider a parent-domain signature valid for its + subdomains. + + Reputation Provider: Another possible category of third-party + signature would be the identity of a third-party reputation + provider. Such a signature would indicate to receivers that the + message was being vouched for by that third party. + +6.4. Using Trusted Third-Party Senders + + For most of the cases described so far, there has been an assumption + that the signing agent was responsible for creating and maintaining + its own DKIM signing infrastructure, including its own keys, and + signing with its own identity. + + A different model arises when an organization uses a trusted third- + party sender for certain key business functions, but still wants that + email to benefit from the organization's own identity and reputation. + In other words, the mail would come out of the trusted third party's + mail servers, but the signature applied would be that of the + controlling organization. + + This can be done by having the third party generate a key pair that + is designated uniquely for use by that trusted third party and + publishing the public key in the controlling organization's DNS + domain, thus enabling the third party to sign mail using the + signature of the controlling organization. For example, if Company A + outsources its employee benefits to a third party, it can use a + special key pair that enables the benefits company to sign mail as + "companyA.example". Because the key pair is unique to that trusted + third party, it is easy for Company A to revoke the authorization if + necessary by simply removing the public key from the companyA.example + DNS. + + A more cautious approach would be to create a dedicated subdomain + (e.g., benefits.companyA.example) to segment the outsourced mail + stream, and to publish the public key there; the signature would then + use d=benefits.companyA.example. + + + + + + + + +Hansen, et al. Informational [Page 29] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +6.4.1. DNS Delegation + + Another possibility for configuring trusted third-party access, as + discussed in Section 3.4, is to have Company A use DNS delegation and + have the designated subdomain managed directly by the trusted third + party. In this case, Company A would create a subdomain + benefits.companya.example, and delegate the DNS management of that + subdomain to the benefits company so it could maintain its own key + records. When revocation becomes necessary, Company A could simply + remove the DNS delegation record. + +6.5. Multiple Signatures + + A simple configuration for DKIM-signed mail is to have a single + signature on a given message. This works well for domains that + manage and send all of their own email from single sources, or for + cases where multiple email streams exist but each has its own unique + key pair. It also represents the case in which only one of the + participants in an email sequence is able to sign, no matter whether + it represents the author or one of the operators. + + The examples thus far have considered the implications of using + different identities in DKIM signatures, but have used only one such + identity for any given message. In some cases, it can make sense to + have more than one identity claiming responsibility for the same + message. + + There are a number of situations where applying more than one DKIM + signature to the same message might make sense. A few examples are: + + Companies with multiple subdomain identities: A company that has + multiple subdomains sending distinct categories of mail might + choose to sign with distinct subdomain identities to enable each + subdomain to manage its own identity. However, it might also want + to provide a common identity that cuts across all of the distinct + subdomains. For example, Company A can sign mail for its sales + department with a signature where d=sales.companya.example and a + second signature where d=companya.example + + Service Providers: A service provider can, as described above, + choose to sign outbound messages with either its own identity or + an identity unique to each of its clients (possibly delegated). + However, it can also do both: sign each outbound message with its + own identity as well as with the identity of each individual + client. For example, ESP A might sign mail for its client Company + B with its service provider signature d=espa.example, and a second + client-specific signature where d= either companyb.example or + companyb.espa.example. The existence of the service provider + + + +Hansen, et al. Informational [Page 30] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + signature could, for example, help cover a new client while it + establishes its own reputation, or help a very small volume client + who might never reach a volume threshold sufficient to establish + an individual reputation. + + Forwarders: Forwarded mail poses a number of challenges to email + authentication. DKIM is relatively robust in the presence of + forwarders as long as the signature is designed to avoid message + parts that are likely to be modified; however, some forwarders do + make modifications that can invalidate a DKIM signature. + + Some forwarders such as mailing lists or "forward article to a + friend" services might choose to add their own signatures to + outbound messages to vouch for them having legitimately originated + from the designated service. In this case, the signature would be + added even in the presence of a preexisting signature, and both + signatures would be relevant to the verifier. + + Any forwarder that modifies messages in ways that will break + preexisting DKIM signatures needs to sign its forwarded messages. + + Reputation Providers: Although third-party reputation providers + today use a variety of protocols to communicate their information + to receivers, it is possible that they, or other organizations + willing to put their "seal of approval" on an email stream, might + choose to use a DKIM signature to do it. In nearly all cases, + this "reputation" signature would be in addition to the author or + originator signature. + + One important caveat to the use of multiple signatures is that there + is currently no clear consensus among receivers on how they plan to + handle them. The opinions range from ignoring all but one signature + (and the specification of which of them is verified differs from + receiver to receiver), to verifying all signatures present and + applying a weighted blend of the trust assessments for those + identifiers, to verifying all signatures present and simply using the + identifier that represents the most positive trust assessment. It is + likely that the industry will evolve to accept multiple signatures + using either the second or third of these, but it can take some time + before one approach becomes pervasive. + +7. Example Usage Scenarios + + Signatures are created by different types of email actors, based on + different criteria, such as where the actor operates in the sequence + from author to recipient, whether they want different messages to be + evaluated under the same reputation or a different one, and so on. + + + + +Hansen, et al. Informational [Page 31] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + This section provides some examples of usage scenarios for DKIM + deployments; the selection is not intended to be exhaustive but to + illustrate a set of key deployment considerations. + +7.1. Author's Organization - Simple + + The simplest DKIM configuration is to have some mail from a given + organization (Company A) be signed with the same d= value (e.g., + d=companya.example). If there is a desire to associate additional + information, the AUID [RFC5672] value can become + uniqueID@companya.example, or @uniqueID.companya.example. + + In this scenario, Company A need only generate a single signing key + and publish it under their top-level domain (companya.example); the + signing module would then tailor the AUID value as needed at signing + time. + +7.2. Author's Organization - Differentiated Types of Mail + + A slight variation of the one signature case is where Company A signs + some of its mail, but it wants to differentiate among categories of + its outbound mail by using different identifiers. For example, it + might choose to distinguish marketing, billing or transactional, and + individual corporate email into marketing.companya.example, + billing.companya.example, and companya.example, respectively, where + each category is assigned a unique subdomain and unique signing keys. + +7.3. Author Domain Signing Practices + +7.3.1. Introduction + + Some domains might decide to sign all of their outgoing mail. If all + of the legitimate mail for a domain is signed, recipients can be more + aggressive in their filtering of mail that uses the domain but does + not have a valid signature from the domain; in such a configuration, + the absence of a signature would be more significant than for the + general case. It might be desirable for such domains to be able to + advertise their intent to other receivers: this is the topic of + Author Domain Signing Practices (ADSP). + + Note that ADSP is not for everyone. Sending domains that do not + control all legitimate outbound mail purporting to be from their + domain (i.e., with an RFC5322.From address in their domain) are + likely to experience delivery problems with some percentage of that + mail. Administrators evaluating ADSP for their domains needs to + carefully weigh the risk of phishing attacks against the likelihood + of undelivered mail. + + + + +Hansen, et al. Informational [Page 32] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + This section covers some examples of ADSP usage. For the complete + specification, see [RFC5617]. + +7.3.2. A Few Definitions + + In the ADSP specification, an address in the RFC5322.From header + field of a message is defined as an "Author Address", and an "Author + Domain" is defined as anything to the right of the '@' in an author + address. + + An "Author Signature" is thus any valid signature where the value of + the SDID matches an author domain in the message. + + It is important to note that unlike the DKIM specification, which + makes no correlation between the signature domain and any message + headers, the ADSP specification applies only to the author domain. + In essence, under ADSP, any non-author signatures are ignored + (treated as if they are not present). + + Signers wishing to publish an Author Domain Signing Practices (ADSP) + [RFC5617] record describing their signing practices will thus want to + include an author signature on their outbound mail to avoid ADSP + verification failures. + +7.3.3. Some ADSP Examples + + An organization (Company A) can specify its signing practices by + publishing an ADSP record with "dkim=all" or "dkim=discardable". In + order to avoid misdelivery of its mail at receivers that are + validating ADSP, Company A needs to first have done an exhaustive + analysis to determine all sources of outbound mail from its domain + (companyA.example) and ensure that they all have valid author + signatures from that domain. + + For example, email with an RFC5322.From address of bob@ + companyA.example needs to have an author signature where the SDID + value is "companyA.example" or it will fail an ADSP validation. + + Note that once an organization publishes an ADSP record using + dkim=all or dkim=discardable, any email with an RFC5322.From address + that uses the domain where the ADSP record is published that does not + have a valid author signature is at risk of being misdelivered or + discarded. For example, if a message with an RFC5322.From address of + newsletter@companyA.example has a signature with + d=marketing.companyA.example, that message will fail the ADSP check + because the signature would not be considered a valid author + signature. + + + + +Hansen, et al. Informational [Page 33] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Because the semantics of an ADSP author signature are more + constrained than the semantics of a "pure" DKIM signature, it is + important to make sure the nuances are well understood before + deploying an ADSP record. The ADSP specification [RFC5617] provides + some fairly extensive lookup examples (in Appendix A) and usage + examples (in Appendix B). + + In particular, in order to prevent mail from being negatively + impacted or even discarded at the receiver, it is essential to + perform a thorough survey of outbound mail from a domain before + publishing an ADSP policy of anything stronger than "unknown". This + includes mail that might be sent from external sources that might not + be authorized to use the domain signature, as well as mail that risks + modification in transit that might invalidate an otherwise valid + author signature (e.g., mailing lists, courtesy forwarders, and other + paths that could add or modify headers or modify the message body). + +7.4. Delegated Signing + + An organization might choose to outsource certain key services to an + independent company. For example, Company A might outsource its + benefits management, or Organization B might outsource its marketing + email. + + If Company A wants to ensure that all of the mail sent on its behalf + through the benefits providers email servers shares the Company A + reputation, as discussed in Section 6.4, it can either publish keys + designated for the use of the benefits provider under + companyA.example (preferably under a designated subdomain of + companyA.example), or it can delegate a subdomain (e.g., + benefits.companyA.example) to the provider and enable the provider to + generate the keys and manage the DNS for the designated subdomain. + + In both of these cases, mail would be physically going out of the + benefit provider's mail servers with a signature of, e.g., + d=benefits.companya.example. Note that the RFC5322.From address is + not constrained: it could be affiliated with either the benefits + company (e.g., benefits-admin@benefitprovider.example, or + benefits-provider@benefits.companya.example) or the companyA domain. + + Note that in both of the above scenarios, as discussed in + Section 3.4, security concerns dictate that the keys be generated by + the organization that plans to do the signing so that there is no + need to transfer the private key. In other words, the benefits + provider would generate keys for both of the above scenarios. + + + + + + +Hansen, et al. Informational [Page 34] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +7.5. Independent Third-Party Service Providers + + Another way to manage the service provider configuration would be to + have the service provider sign the outgoing mail on behalf of its + client, Company A, with its own (provider) identifier. For example, + an Email Service Provider (ESP A) might want to share its own mailing + reputation with its clients, and might sign all outgoing mail from + its clients with its own d= domain (e.g., d=espa.example). + + When the ESP wants to distinguish among its clients, it has two + options: + + o Share the SDID domain and use the AUID value to distinguish among + the clients, e.g., a signature on behalf of client A would have + d=espa.example and i=@clienta.espa.example (or + i=clienta@espa.example). + + o Extend the SDID domain, so there is a unique value (and subdomain) + for each client, e.g., a signature on behalf of client A would + have d=clienta.espa.example. + + Note that this scenario and the delegation scenario are not mutually + exclusive. In some cases, it can be desirable to sign the same + message with both the ESP and the ESP client identities. + +7.6. Mail Streams Based on Behavioral Assessment + + An ISP (ISP A) might want to assign signatures to outbound mail from + its users according to each user's past sending behavior + (reputation). In other words, the ISP would segment its outbound + traffic according to its own assessment of message quality, to aid + recipients in differentiating among these different streams. Since + the semantics of behavioral assessments are not valid AUID values, + ISP A (ispa.example) can configure subdomains corresponding to the + assessment categories (e.g., good.ispa.example, neutral.ispa.example, + bad.ispa.example), and use these subdomains in the d= value of the + signature. + + The signing module can also set the AUID value to have a unique user + ID (distinct from the local-part of the user's email address), for + example, user3456@neutral.domain.example. Using a user ID that is + distinct from a given email alias is useful in environments where a + single user might register multiple email aliases. + + Note that in this case, the AUID values are only partially stable. + They are stable in the sense that a given i= value will always + represent the same identity, but they are unstable in the sense that + + + + +Hansen, et al. Informational [Page 35] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + a given user can migrate among the assessment subdomains depending on + their sending behavior (i.e., the same user might have multiple AUID + values over the lifetime of a single account). + + In this scenario, ISP A can generate as many keys as there are + assessment subdomains (SDID values), so that each assessment + subdomain has its own key. The signing module would then choose its + signing key based on the assessment of the user whose mail was being + signed, and if desired, include the user ID in the AUID of the + signature. As discussed earlier, the per-user granularity of the + AUID can be ignored by verifiers; so organizations choosing to use it + ought not rely on its use for receiver side filtering results. + However, some organizations might also find the information useful + for their own purposes in processing bounces or abuse reports. + +7.7. Agent or Mediator Signatures + + Another scenario is that of an agent, usually a re-mailer of some + kind, that signs on behalf of the service or organization that it + represents. Some examples of agents might be a mailing list manager, + or the "forward article to a friend" service that many online + publications offer. In most of these cases, the signature is + asserting that the message originated with, or was relayed by, the + service asserting responsibility. In general, if the service is + configured in such a way that its forwarding would break existing + DKIM signatures, it needs to always add its own signature. + +8. Usage Considerations + +8.1. Non-Standard Submission and Delivery Scenarios + + The robustness of DKIM's verification mechanism is based on the fact + that only authorized signing modules have access to the designated + private key. This has the side effect that email submission and + delivery scenarios that originate or relay messages from outside the + domain of the authorized signing module will not have access to that + protected private key, and thus will be unable to attach the expected + domain signature to those messages. Such scenarios include mailing + lists, courtesy forwarders, MTAs at hotels, hotspot networks used by + traveling users, and other paths that could add or modify headers, or + modify the message body. + + For example, assume Joe works for Company A and has an email address + joe@companya.example. Joe also has an ISP-1 account + joe@isp1.example.com, and he uses ISP-1's multiple address feature to + attach his work email address, joe@companya.example, to email from + his ISP-1 account. When Joe sends email from his ISP-1 account and + uses joe@companya.example as his designated RFC5322.From address, + + + +Hansen, et al. Informational [Page 36] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + that email cannot have a signature with d=companya.example because + the ISP-1 servers have no access to Company A's private key. In + ISP-1's case, it will have an ISP-1 signature, but for some other + mail clients offering the same multiple address feature there might + be no signature at all on the message. + + Another example might be the use of a forward article to a friend + service. Most instances of these services today allow someone to + send an article with their email address in the RFC5322.From to their + designated recipient. If Joe used either of his two addresses + (joe@companya.example or joe@isp1.example.com), the forwarder would + be equally unable to sign with a corresponding domain. As in the + mail client case, the forwarder can either sign as its own domain or + put no signature on the message. + + A third example is the use of privately configured forwarding. + Assume that Joe has another account at ISP-2, joe@isp-2.example.com, + but he'd prefer to read his ISP-2 mail from his ISP-1 account. He + sets up his ISP-2 account to forward all incoming mail to + joe@isp1.example.com. Assume alice@companyb.example sends + joe@isp-2.example.com an email. Depending on how companyb.example + configured its signature, and depending on whether or not ISP-2 + modifies messages that it forwards, it is possible that when Alice's + message is received in Joe's ISP-1 account, the original signature + will fail verification. + +8.2. Protection of Internal Mail + + One identity is particularly amenable to easy and accurate + assessment: the organization's own identity. Members of an + organization tend to trust messages that purport to be from within + that organization. However, Internet Mail does not provide a + straightforward means of determining whether such mail is, in fact, + from within the organization. DKIM can be used to remedy this + exposure. If the organization signs all of its mail, then its + boundary MTAs can look for mail purporting to be from the + organization that does not contain a verifiable signature. + + Such mail can, in most cases, be presumed to be spurious. However, + domain managers are advised to consider the ways that mail processing + can modify messages in ways that will invalidate an existing DKIM + signature: mailing lists, courtesy forwarders, and other paths that + could add or modify headers or modify the message body (e.g., MTAs at + hotels, hotspot networks used by traveling users, and other scenarios + described in the previous section). Such breakage is particularly + relevant in the presence of Author Domain Signing Practices. + + + + + +Hansen, et al. Informational [Page 37] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +8.3. Signature Granularity + + Although DKIM's use of domain names is optimized for a scope of + organization-level signing, it is possible to administer subdomains + or otherwise adjust signatures in a way that supports per-user + identification. This user-level granularity can be specified in two + ways: either by sharing the signing identity and specifying an + extension to the i= value that has a per-user granularity or by + creating and signing with unique per-user keys. + + A subdomain or local part in the i= tag needs to be treated as an + opaque identifier and thus need not correspond directly to a DNS + subdomain or be a specific user address. + + The primary way to sign with per-user keys requires each user to have + a distinct DNS (sub)domain, where each distinct d= value has a key + published. (It is possible, although not advised, to publish the + same key in more than one distinct domain.) + + It is technically possible to publish per-user keys within a single + domain or subdomain by utilizing different selector values. This is + not advised and is unlikely to be treated uniquely by Assessors: the + primary purpose of selectors is to facilitate key management, and the + DKIM specification recommends against using them in determining or + assessing identities. + + In most cases, it would be impractical to sign email on a per-user + granularity. Such an approach would be + + likely to be ignored: In most cases today, if receivers are + verifying DKIM signatures, they are in general taking the simplest + possible approach. In many cases, maintaining reputation + information at a per-user granularity is not interesting to them, + in large part because the per-user volume is too small to be + useful or interesting. So even if senders take on the complexity + necessary to support per-user signatures, receivers are unlikely + to retain anything more than the base domain reputation. + + difficult to manage: Any scheme that involves maintenance of a + significant number of public keys might require infrastructure + enhancements or extensive administrative expertise. For domains + of any size, maintaining a valid per-user keypair, knowing when + keys need to be revoked or added due to user attrition or + onboarding, and the overhead of having the signing engine + constantly swapping keys can create significant and often + unnecessary management complexity. It is also important to note + + + + + +Hansen, et al. Informational [Page 38] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + that there is no way within the scope of the DKIM specification + for a receiver to infer that a sender intends a per-user + granularity. + + As mentioned before, what might make sense, however, is to use the + infrastructure that enables finer granularity in signatures to + identify segments smaller than a domain but much larger than a per- + user segmentation. For example, a university might want to segment + student, staff, and faculty mail into three distinct streams with + differing reputations. This can be done by creating separate + subdomains for the desired segments, and either specifying the + subdomains in the i= tag of the DKIM Signature or by adding + subdomains to the d= tag and assigning and signing with different + keys for each subdomain. + + For those who choose to represent user-level granularity in + signatures, the performance and management considerations above + suggest that it would be more effective to do so by specifying a + local part or subdomain extension in the i= tag rather than by + extending the d= domain and publishing individual keys. + +8.4. Email Infrastructure Agents + + It is expected that the most common venue for a DKIM implementation + will be within the infrastructure of an organization's email service, + such as a department or a boundary MTA. What follows are some + general recommendations for the Email Infrastructure. + + Outbound: An MSA (Mail Submission Agent) or an outbound MTA used + for mail submission needs to ensure that the message sent is in + compliance with the advertised email sending policy. It needs + to also be able to generate an operator alert if it determines + that the email messages do not comply with the published DKIM + sending policy. + + An MSA needs to be aware that some MUAs might add their own + signatures. If the MSA needs to perform operations on a + message to make it comply with its email sending policy, if at + all possible, it needs to do so in a way that would not break + those signatures. + + MUAs equipped with the ability to sign ought not to be + encouraged. In terms of security, MUAs are generally not under + the direct control of those in responsible roles within an + organization and are thus more vulnerable to attack and + compromise, which would expose private signing keys to + intruders and thus jeopardize the integrity and reputation of + the organization. + + + +Hansen, et al. Informational [Page 39] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Inbound: When an organization deploys DKIM, it needs to make + sure that its email infrastructure components that do not have + primary roles in DKIM handling do not modify message in ways + that prevent subsequent verification. + + An inbound MTA or an MDA can incorporate an indication of the + verification results into the message, such as using an + Authentication-Results header field [RFC5451]. + + Intermediaries: An email intermediary is both an inbound and + outbound MTA. Each of the requirements outlined in the + sections relating to MTAs apply. If the intermediary modifies + a message in a way that breaks the signature, the intermediary. + + + needs to deploy abuse filtering measures on the inbound + mail, and + + + probably also needs to remove all signatures that will be + broken. + + In addition, the intermediary can: + + + verify the message signature prior to modification. + + + incorporate an indication of the verification results into + the message, such as using an Authentication-Results header + field [RFC5451]. + + + sign the modified message including the verification results + (e.g., the Authentication-Results header field). + +8.5. Mail User Agent + + The DKIM specification is expected to be used primarily between + Boundary MTAs, or other infrastructure components of the originating + and receiving ADMDs. However, there is nothing in DKIM that is + specific to those venues. In particular, MUAs can also support DKIM + signing and verifying directly. + + Outbound: An MUA can support signing even if mail is to be + relayed through an outbound MSA. In this case, the signature + applied by the MUA will be in addition to any signature added + by the MSA. However, the warnings in the previous section need + to be taken into consideration. + + + + + + + +Hansen, et al. Informational [Page 40] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Some user software goes beyond simple user functionality and + also performs MSA and MTA functions. When this is employed for + sending directly to a receiving ADMD, the user software needs + to be considered an outbound MTA. + + Inbound: An MUA can rely on a report of a DKIM signature + verification that took place at some point in the inbound MTA/ + MDA path (e.g., an Authentication-Results header field), or an + MUA can perform DKIM signature verification directly. A + verifying MUA needs to allow for the case where mail has been + modified in the inbound MTA path; if a signature fails, the + message is to be treated the same as a message that does not + have a signature. + + An MUA that looks for an Authentication-Results header field + needs to be configurable to choose which Authentication-Results + header fields are considered trustable. The MUA developer is + encouraged to re-read the Security Considerations of [RFC5451]. + + DKIM requires that all verifiers treat messages with signatures + that do not verify as if they are unsigned. + + If verification in the client is to be acceptable to users, it + is essential that successful verification of a signature not + result in a less than satisfactory user experience compared to + leaving the message unsigned. The mere presence of a verified + DKIM signature cannot be used by itself by an MUA to indicate + that a message is to be treated better than a message without a + verified DKIM signature. However, the fact that a DKIM + signature was verified can be used as input into a reputation + system (i.e., a whitelist of domains and users) for + presentation of such indicators. + + It is common for components of an ADMD's email infrastructure to do + violence to a message, such that a DKIM signature might be rendered + invalid. Hence, users of MUAs that support DKIM signing and/or + verifying need a basis for knowing that their associated email + infrastructure will not break a signature. + +9. Security Considerations + + The security considerations of the DKIM protocol are described in the + DKIM base specification [RFC4871]. + +10. Acknowledgements + + The effort of the DKIM Working Group is gratefully acknowledged. + + + + +Hansen, et al. Informational [Page 41] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +11. References + +11.1. Normative References + + [RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, + J., and M. Thomas, "DomainKeys Identified Mail (DKIM) + Signatures", RFC 4871, May 2007. + + [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, + October 2008. + + [RFC5451] Kucherawy, M., "Message Header Field for Indicating + Message Authentication Status", RFC 5451, April 2009. + + [RFC5585] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys + Identified Mail (DKIM) Service Overview", RFC 5585, + July 2009. + + [RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine, + "DomainKeys Identified Mail (DKIM) Author Domain Signing + Practices (ADSP)", RFC 5617, August 2009. + + [RFC5672] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM) + Signatures -- Update", RFC 5672, August 2009. + +11.2. Informative References + + [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. + Rose, "Resource Records for the DNS Security Extensions", + RFC 4034, March 2005. + + [RFC4870] Delany, M., "Domain-Based Email Authentication Using + Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, + May 2007. + + [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS + Security (DNSSEC) Hashed Authenticated Denial of + Existence", RFC 5155, March 2008. + + + + + + + + + + + + + +Hansen, et al. Informational [Page 42] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +Appendix A. Migration Strategies + + There are three migration occasions worth noting in particular for + DKIM: + + 1. Migrating from DomainKeys to DKIM. + + 2. Migrating from a current hash algorithm to a new standardized + hash algorithm. + + 3. Migrating from a current signing algorithm to a new standardized + signing algorithm. + + The case of deploying a new key selector record is described + elsewhere (Section 3.5). + + As with any migration, the steps required will be determined by who + is doing the migration and their assessment of: + + o the users of what they are generating, or + + o the providers of what they are consuming. + + Signers and verifiers have different considerations. + +A.1. Migrating from DomainKeys + + DKIM replaces the earlier DomainKeys (DK) specification. Selector + files are mostly compatible between the two specifications. + +A.1.1. Signers + + A signer that currently signs with DK will go through various stages + as it migrates to using DKIM, not all of which are required for all + signers. The real questions that a signer needs to ask are: + + 1. how many receivers or what types of receivers are *only* looking + at the DK signatures and not the DKIM signatures, and + + 2. how much does the signer care about those receivers? + + If no one is looking at the DK signature any more, then it's no + longer necessary to sign with DK. Or if all "large players" are + looking at DKIM in addition to or instead of DK, a signer can choose + to stop signing with DK. + + + + + + +Hansen, et al. Informational [Page 43] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + With respect to signing policies, a reasonable, initial approach is + to use DKIM signatures in the same way that DomainKeys signatures are + already being used. In particular, the same selectors and DNS key + records can be used for both, after verifying that they are + compatible as discussed below. + + Each secondary step in all of the following scenarios is to be + prefaced with the gating factor "test, then when comfortable with the + previous step's results, continue". + + One migration strategy is to: + + o ensure that the current selector DNS key record is compatible with + both DK and DKIM + + o sign messages with both DK and DKIM signatures + + o when it's decided that DK signatures are no longer necessary, stop + signing with DK + + Another migration strategy is to: + + o add a new selector DNS key record only for DKIM signatures + + o sign messages with both DK (using the old DNS key record) and DKIM + signatures (using the new DNS key record) + + o when it's decided that DK signatures are no longer necessary, stop + signing with DK + + o eventually remove the old DK selector DNS record + + A combined migration strategy is to: + + o ensure that the current selector DNS key record is compatible with + both DK and DKIM + + o start signing messages with both DK and DKIM signatures + + o add a new selector DNS key record for DKIM signatures + + o switch the DKIM signatures to use the new selector + + o when it's decided that DK signatures are no longer necessary, stop + signing with DK + + o eventually remove the old DK selector DNS record + + + + +Hansen, et al. Informational [Page 44] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + Another migration strategy is to: + + o add a new selector DNS key record for DKIM signatures + + o do a flash cut and replace the DK signatures with DKIM signatures + + o eventually remove the old DK selector DNS record + + Another migration strategy is to: + + o ensure that the current selector DNS key record is compatible with + both DK and DKIM + + o do a flash cut and replace the DK signatures with DKIM signatures + + Note that when you have separate key records for DK and DKIM, you can + use the same public key for both. + +A.1.1.1. DNS Selector Key Records + + The first step in some of the above scenarios is ensuring that the + selector DNS key records are compatible for both DK and DKIM. The + format of the DNS key record was intentionally meant to be backwardly + compatible between the two systems, but not necessarily upwardly + compatible. DKIM has enhanced the DK DNS key record format by adding + several optional parameters, which DK needs to ignore. However, + there is one critical difference between DK and DKIM DNS key records. + The definitions of the "g" fields: + + g= granularity of the key: In both DK and DKIM, this is an optional + field that is used to constrain which sending address(es) can + legitimately use this selector. Unfortunately, the treatment of + an empty field ("g=;") is different. DKIM allows wildcards where + DK does not. For DK, an empty field is the same as a missing + value, and is treated as allowing any sending address. For DKIM, + an empty field only matches an empty local part. In DKIM, both a + missing value and "g=*;" mean to allow any sending address. + + Also, in DomainKeys, the "g" field is required to match the + address in "From:"/"Sender:", while in DKIM, it is required to + match i=. This might or might not affect transition. + + If your DK DNS key record has an empty "g" field in it ("g=;"), + your best course of action is to modify the record to remove the + empty field. In that way, the DK semantics will remain the same, + and the DKIM semantics will match. + + + + + +Hansen, et al. Informational [Page 45] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + If your DNS key record does not have an empty "g" field in it + ("g=;"), it's probable that the record can be left alone. But the + best course of action would still be to make sure that it has a + "v" field. When the decision is made to stop supporting + DomainKeys and to only support DKIM, it is important to verify + that the "g" field is compatible with DKIM, and typically having + "v=DKIM1;" in it. It is strongly encouraged that if use of an + empty "g" field in the DKIM selector, include the "v" field. + +A.1.1.2. Removing DomainKeys Signatures + + The principal use of DomainKeys is at boundary MTAs. Because no + operational transition is ever instantaneous, it is advisable to + continue performing DomainKeys signing until it is determined that + DomainKeys receive-side support is no longer used, or is sufficiently + reduced. That is, a signer needs to add a DKIM signature to a + message that also has a DomainKeys signature and keep it there until + they decide it is deemed no longer useful. The signer can do its + transitions in a straightforward manner, or more gradually. Note + that because digital signatures are not free, there is a cost to + performing both signing algorithms, so signing with both algorithms + ought not be needlessly prolonged. + + The tricky part is deciding when DK signatures are no longer + necessary. The real questions are: how many DomainKeys verifiers are + there that do *not* also do DKIM verification, which of those are + important, and how can you track their usage? Most of the early + adopters of DK verification have added DKIM verification, but not all + yet. If a verifier finds a message with both DK and DKIM, it can + choose to verify both signatures, or just one or the other. + + Many DNS services offer tracking statistics so it can be determined + how often a DNS record has been accessed. By using separate DNS + selector key records for your signatures, you can chart the use of + your records over time, and watch the trends. An additional + distinguishing factor to track would take into account the verifiers + that verify both the DK and DKIM signatures, and discount those from + counts of DK selector usage. When the number for DK selector access + reaches a low-enough level, that's the time to consider discontinuing + signing with DK. + + Note, this level of rigor is not required. It is perfectly + reasonable for a DK signer to decide to follow the "flash cut" + scenario described above. + + + + + + + +Hansen, et al. Informational [Page 46] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +A.1.2. Verifiers + + As a verifier, several issues need to be considered: + +A.1.2.1. Ought DK signature verification be performed? + + At the time of writing, there is still a significant number of sites + that are only producing DK signatures. Over time, it is expected + that this number will go to zero, but it might take several years. + So it would be prudent for the foreseeable future for a verifier to + look for and verify both DKIM and DK signatures. + +A.1.2.2. Ought both DK and DKIM signatures be evaluated on a single + message? + + For a period of time, there will be sites that sign with both DK and + DKIM. A verifier receiving a message that has both types of + signatures can verify both signatures, or just one. One disadvantage + of verifying both signatures is that signers will have a more + difficult time deciding how many verifiers are still using their DK + selectors. One transition strategy is to verify the DKIM signature, + then only verify the DK signature if the DKIM verification fails. + +A.1.2.3. DNS Selector Key Records + + The format of the DNS key record was intentionally meant to be + backwardly compatible between DK and DKIM, but not necessarily + upwardly compatible. DKIM has enhanced the DK DNS key record format + by adding several optional parameters, which DK needs to ignore. + However, there is one key difference between DK and DKIM DNS key + records. The definitions of the g fields: + + g= granularity of the key: In both DK and DKIM, this is an optional + field that is used to constrain which sending address(es) can + legitimately use this selector. Unfortunately, the treatment of + an empty field ("g=;") is different. For DK, an empty field is + the same as a missing value, and is treated as allowing any + sending address. For DKIM, an empty field only matches an empty + local part. + + v= version of the selector It is advised that a DKIM selector have + "v=DKIM1;" at its beginning, but it is not required. + + If a DKIM verifier finds a selector record that has an empty "g" + field ("g=;") and it does not have a "v" field ("v=DKIM1;") at its + beginning, it is faced with deciding if this record was: + + + + + +Hansen, et al. Informational [Page 47] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + 1. from a DK signer that transitioned to supporting DKIM but forgot + to remove the "g" field (so that it could be used by both DK and + DKIM verifiers); or + + 2. from a DKIM signer that truly meant to use the empty "g" field + but forgot to put in the "v" field. It is advised that you treat + such records using the first interpretation, and treat such + records as if the signer did not have a "g" field in the record. + +A.2. Migrating Hash Algorithms + + [RFC4871] defines the use of two hash algorithms: SHA-1 and SHA-256. + The security of all hash algorithms is constantly under attack, and + SHA-1 has already shown weaknesses as of this writing. Migrating + from SHA-1 to SHA-256 is not an issue, because all verifiers are + already required to support SHA-256. But when it becomes necessary + to replace SHA-256 with a more secure algorithm, there will be a + migratory period. In the following, "NEWHASH" is used to represent a + new hash algorithm. Section 4.1 of [RFC4871] briefly discusses this + scenario. + +A.2.1. Signers + + As with migrating from DK to DKIM, migrating hash algorithms is + dependent on the signer's best guess as to the utility of continuing + to sign with the older algorithms and the expected support for the + newer algorithm by verifiers. The utility of continuing to sign with + the older algorithms is also based on how broken the existing hash + algorithms are considered and how important that is to the signers. + + One strategy is to wait until it's determined that there is a large + enough base of verifiers available that support NEWHASH, and then + flash cut to the new algorithm. + + Another strategy is to sign with both the old and new hash algorithms + for a period of time. This is particularly useful for testing the + new code to support the new hash algorithm, as verifiers will + continue to accept the signature for the older hash algorithm and + ought to ignore any signature that fails because the code is slightly + wrong. Once the signer has determined that the new code is correct + AND it's determined that there is a large enough base of verifiers + available that support NEWHASH, the signer can flash cut to the new + algorithm. + + One advantage migrating hash algorithms has is that the selector can + be completely compatible for all hash algorithms. The key selector + has an optional "h=" field that can be used to list the hash + algorithms being used; it also is used to limit the algorithms that a + + + +Hansen, et al. Informational [Page 48] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + + verifier will accept. If the signer is not currently using the key + selector "h=" field, no change is required. If the signer is + currently using the key selector "h=" field, NEWHASH will need to be + added to the list, as in "h=sha256:NEWHASH;". (When the signer is no + longer using SHA-256, it can be removed from the "h=" list.) + +A.2.2. Verifiers + + When a new hash algorithm becomes standardized, it is best for a + verifier to start supporting it as quickly as possible. + +A.3. Migrating Signing Algorithms + + [RFC4871] defines the use of the RSA signing algorithm. Similar to + hashes, signing algorithms are constantly under attack, and when it + becomes necessary to replace RSA with a newer signing algorithm, + there will be a migratory period. In the following, "NEWALG" is used + to represent a new signing algorithm. + +A.3.1. Signers + + As with the other migration issues discussed above, migrating signing + algorithms is dependent on the signer's best guess as to the utility + of continuing to sign with the older algorithms and the expected + support for the newer algorithm by verifiers. The utility of + continuing to sign with the older algorithms is also based on how + broken the existing signing algorithms are considered and how + important that is to the signers. + + As before, the two basic strategies are to 1) wait until there is + sufficient base of verifiers available that support NEWALG and then + do a flash cut to NEWALG, and 2) use a phased approach by signing + with both the old and new algorithms before removing support for the + old algorithm. + + It is unlikely that a new algorithm would be able to use the same + public key as "rsa", so using the same selector DNS record for both + algorithms' keys is ruled out. Therefore, in order to use the new + algorithm, a new DNS selector record would need to be deployed in + parallel with the existing DNS selector record for the existing + algorithm. The new DNS selector record would specify a different + "k=" value to reflect the use of NEWALG. + + + + + + + + + +Hansen, et al. Informational [Page 49] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +A.3.2. Verifiers + + When a new hash algorithm becomes standardized, it is best for a + verifier to start supporting it as quickly as possible. + +Appendix B. General Coding Criteria for Cryptographic Applications + + NOTE: This section could possibly be changed into a reference to + something else, such as another RFC. + + Correct implementation of a cryptographic algorithm is a necessary + but not a sufficient condition for the coding of cryptographic + applications. Coding of cryptographic libraries requires close + attention to security considerations that are unique to cryptographic + applications. + + In addition to the usual security coding considerations, such as + avoiding buffer or integer overflow and underflow, implementers need + to pay close attention to management of cryptographic private keys + and session keys, ensuring that these are correctly initialized and + disposed of. + + Operating system mechanisms that permit the confidentiality of + private keys to be protected against other processes ought to be used + when available. In particular, great care needs to be taken when + releasing memory pages to the operating system to ensure that private + key information is not disclosed to other processes. + + Certain implementations of public key algorithms such as RSA can be + vulnerable to a timing analysis attack. + + Support for cryptographic hardware providing key management + capabilities is strongly encouraged. In addition to offering + performance benefits, many cryptographic hardware devices provide + robust and verifiable management of private keys. + + Fortunately, appropriately designed and coded cryptographic libraries + are available for most operating system platforms under license terms + compatible with commercial, open source and free software license + terms. Use of standard cryptographic libraries is strongly + encouraged. These have been extensively tested, reduce development + time and support a wide range of cryptographic hardware. + + + + + + + + + +Hansen, et al. Informational [Page 50] + +RFC 5863 DKIM Development/Deployment/Operations May 2010 + + +Authors' Addresses + + Tony Hansen + AT&T Laboratories + 200 Laurel Ave. South + Middletown, NJ 07748 + USA + + EMail: tony+dkimov@maillennium.att.com + + + Ellen Siegel + Consultant + + EMail: dkim@esiegel.net + + + Phillip Hallam-Baker + Default Deny Security, Inc. + + EMail: phillip@hallambaker.com + + + Dave Crocker + Brandenburg InternetWorking + 675 Spruce Dr. + Sunnyvale, CA 94086 + USA + + EMail: dcrocker@bbiw.net + + + + + + + + + + + + + + + + + + + + + +Hansen, et al. Informational [Page 51] + |