path: root/doc/rfc/rfc2660.txt

Network Working Group                                       E. Rescorla
Request for Comments: 2660                                   RTFM, Inc.
Category: Experimental                                     A. Schiffman
                                                   Terisa Systems, Inc.
                                                            August 1999


                 The Secure HyperText Transfer Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This memo describes a syntax for securing messages sent using the
   Hypertext Transfer Protocol (HTTP), which forms the basis for the
   World Wide Web. Secure HTTP (S-HTTP) provides independently
   applicable security services for transaction confidentiality,
   authenticity/integrity and non-repudiability of origin.

   The protocol emphasizes maximum flexibility in choice of key
   management mechanisms, security policies and cryptographic algorithms
   by supporting option negotiation between parties for each
   transaction.

Table of Contents

   1. Introduction .................................................. 3
   1.1. Summary of Features ......................................... 3
   1.2. Changes ..................................................... 4
   1.3. Processing Model ............................................ 5
   1.4. Modes of Operation .......................................... 6
   1.5. Implementation Options ...................................... 7
   2. Message Format ................................................ 7
   2.1. Notational Conventions ...................................... 8
   2.2. The Request Line ............................................ 8
   2.3. The Status Line ............................................. 8
   2.4. Secure HTTP Header Lines .................................... 8
   2.5. Content .....................................................12
   2.6. Encapsulation Format Options ................................13



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   2.6.1. Content-Privacy-Domain: CMS ...............................13
   2.6.2. Content-Privacy-Domain: MOSS ..............................14
   2.6.3. Permitted HTTP headers ....................................14
   2.6.3.2. Host ....................................................15
   2.6.3.3. Connection ..............................................15
   3. Cryptographic Parameters ......................................15
   3.1. Options Headers .............................................15
   3.2. Negotiation Options .........................................16
   3.2.1. Negotiation Overview ......................................16
   3.2.2. Negotiation Option Format .................................16
   3.2.3. Parametrization for Variable-length Key Ciphers ...........18
   3.2.4. Negotiation Syntax ........................................18
   3.3. Non-Negotiation Headers .....................................23
   3.3.1. Encryption-Identity .......................................23
   3.3.2. Certificate-Info ..........................................23
   3.3.3. Key-Assign ................................................24
   3.3.4. Nonces ....................................................25
   3.4. Grouping Headers With SHTTP-Cryptopts .......................26
   3.4.1. SHTTP-Cryptopts ...........................................26
   4. New Header Lines for HTTP .....................................26
   4.1. Security-Scheme .............................................26
   5. (Retriable) Server Status Error Reports .......................27
   5.1. Retry for Option (Re)Negotiation ............................27
   5.2. Specific Retry Behavior .....................................28
   5.3. Limitations On Automatic Retries ............................29
   6. Other Issues ..................................................30
   6.1. Compatibility of Servers with Old Clients ...................30
   6.2. URL Protocol Type ...........................................30
   6.3. Browser Presentation ........................................31
   7. Implementation Notes ..........................................32
   7.1. Preenhanced Data ............................................32
   7.2. Note:Proxy Interaction ......................................34
   7.2.1. Client-Proxy Authentication ...............................34
   8. Implementation Recommendations and Requirements ...............34
   9. Protocol Syntax Summary .......................................35
   10. An Extended Example ..........................................36
   Appendix: A Review of CMS ........................................40
   Bibliography and References ......................................41
   Security Considerations ..........................................43
   Authors' Addresses ...............................................44
   Full Copyright Statement..........................................45










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1.  Introduction

   The World Wide Web (WWW) is a distributed hypermedia system which has
   gained widespread acceptance among Internet users.  Although WWW
   browsers support other, preexisting Internet application protocols,
   the native and primary protocol used between WWW clients and servers
   is the HyperText Transfer Protocol (HTTP) [RFC-2616].  The ease of
   use of the Web has prompted its widespread employment as a
   client/server architecture for many applications.  Many such
   applications require the client and server to be able to authenticate
   each other and exchange sensitive information confidentially. The
   original HTTP specification had only modest support for the
   cryptographic mechanisms appropriate for such transactions.

   Secure HTTP (S-HTTP) provides secure communication mechanisms between
   an HTTP client-server pair in order to enable spontaneous commercial
   transactions for a wide range of applications.  Our design intent is
   to provide a flexible protocol that supports multiple orthogonal
   operation modes, key management mechanisms, trust models,
   cryptographic algorithms and encapsulation formats through option
   negotiation between parties for each transaction.

1.1.  Summary of Features

   Secure HTTP is a secure message-oriented communications protocol
   designed for use in conjunction with HTTP. It is designed to coexist
   with HTTP's messaging model and to be easily integrated with HTTP
   applications.

   Secure HTTP provides a variety of security mechanisms to HTTP clients
   and servers, providing the security service options appropriate to
   the wide range of potential end uses possible for the World-Wide Web.
   The protocol provides symmetric capabilities to both client and
   server (in that equal treatment is given to both requests and
   replies, as well as for the preferences of both parties) while
   preserving the transaction model and implementation characteristics
   of HTTP.

   Several cryptographic message format standards may be incorporated
   into S-HTTP clients and servers, particularly, but in principle not
   limited to, [CMS] and [MOSS]. S-HTTP supports interoperation among a
   variety of implementations, and is compatible with HTTP.  S-HTTP
   aware clients can communicate with S-HTTP oblivious servers and
   vice-versa, although such transactions obviously would not use S-HTTP
   security features.

   S-HTTP does not require client-side public key certificates (or
   public keys), as it supports symmetric key-only operation modes.



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   This is significant because it means that spontaneous private
   transactions can occur without requiring individual users to have
   an established public key.  While S-HTTP is able to take advantage
   of ubiquitous certification infrastructures, its deployment does
   not require it.

   S-HTTP supports end-to-end secure transactions, in contrast with the
   original HTTP authorization mechanisms which require the client to
   attempt access and be denied before the security mechanism is
   employed.  Clients may be "primed" to initiate a secure transaction
   (typically using information supplied in message headers); this may
   be used to support encryption of fill-out forms, for example. With
   S-HTTP, no sensitive data need ever be sent over the network in the
   clear.

   S-HTTP provides full flexibility of cryptographic algorithms, modes
   and parameters. Option negotiation is used to allow clients and
   servers to agree on transaction modes (e.g., should the request be
   signed or encrypted or both -- similarly for the reply?);
   cryptographic algorithms (RSA vs. DSA for signing, DES vs.
   RC2 for encrypting, etc.); and certificate selection
   (please sign with your "Block-buster Video certificate").

   S-HTTP attempts to avoid presuming a particular trust model, although
   its designers admit to a conscious effort to facilitate
   multiply-rooted hierarchical trust, and anticipate that principals may
   have many public key certificates.

   S-HTTP differs from Digest-Authentication, described in [RFC-2617] in
   that it provides support for public key cryptography and consequently
   digital signature capability, as well as providing confidentiality.

1.2.  Changes

   This document describes S-HTTP/1.4. It differs from the previous
   memo in that it differs from the previous memo in its support of
   the Cryptographic Message Syntax (CMS) [CMS], a successor to PKCS-7;
   and hence now supports the Diffie-Hellman and the (NIST) Digital
   Signature Standard cryptosystems. CMS used in RSA mode is bits on the
   wire compatible with PKCS-7.











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1.3.  Processing Model

1.3.1.  Message Preparation

   The creation of an S-HTTP message can be thought of as a a function
   with three inputs:

      1. The cleartext message. This is either an HTTP message
      or some other data object. Note that since the cleartext message
      is carried transparently, headers and all, any version of HTTP
      can be carried within an S-HTTP wrapper.
      2. The receiver's cryptographic preferences and keying material.
      This is either explicitly specified by the receiver or subject
      to some default set of preferences.
      3. The sender's cryptographic preferences and keying material.
      This input to the function can be thought of as implicit
      since it exists only in the memory of the sender.

   In order to create an S-HTTP message, then, the sender integrates the
   sender's preferences with the receiver's preferences. The result of
   this is a list of cryptographic enhancements to be applied and keying
   material to be used to apply them. This may require some user
   intervention. For instance, there might be multiple keys available to
   sign the message. (See Section 3.2.4.9.3 for more on this topic.)
   Using this data, the sender applies the enhancements to the message
   clear-text to create the S-HTTP message.

   The processing steps required to transform the cleartext message into
   the S-HTTP message are described in Sections 2 and 3. The processing
   steps required to merge the sender's and receiver's preferences are
   described in Sections 3.2.

1.3.2.  Message Recovery

   The recovery of an S-HTTP message can be thought of as a function of
   four distinct inputs:

      1. The S-HTTP message.
      2. The receiver's stated cryptographic preferences and keying
      material. The receiver has the opportunity to remember what
      cryptographic preferences it provided in order for this
      document to be dereferenced.
      3. The receiver's current cryptographic preferences and
      keying material.
      4. The sender's previously stated cryptographic options.
      The sender may have stated that he would perform certain
      cryptographic operations in this message. (Again, see
      sections 4 and 5 for details on how to do this.)



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   In order to recover an S-HTTP message, the receiver needs to read the
   headers to discover which cryptographic transformations were
   performed on the message, then remove the transformations using some
   combination of the sender's and receiver's keying material, while
   taking note of which enhancements were applied.

   The receiver may also choose to verify that the applied enhancements
   match both the enhancements that the sender said he would apply
   (input 4 above) and that the receiver requested (input 2 above) as
   well as the current preferences to see if the S-HTTP message was
   appropriately transformed. This process may require interaction with
   the user to verify that the enhancements are acceptable to the user.
   (See Section 6.4 for more on this topic.)

1.4.  Modes of Operation

   Message protection may be provided on three orthogonal axes:
   signature, authentication, and encryption. Any message may be signed,
   authenticated, encrypted, or any combination of these (including no
   protection).

   Multiple key management mechanisms are supported, including
   password-style manually shared secrets and public-key key exchange.
   In particular, provision has been made for prearranged (in an earlier
   transaction or out of band) symmetric session keys in order to send
   confidential messages to those who have no public key pair.

   Additionally, a challenge-response ("nonce") mechanism is provided to
   allow parties to assure themselves of transaction freshness.

1.4.1.  Signature

   If the digital signature enhancement is applied, an appropriate
   certificate may either be attached to the message (possibly along
   with a certificate chain) or the sender may expect the recipient to
   obtain the required certificate (chain) independently.

1.4.2.  Key Exchange and Encryption

   In support of bulk encryption, S-HTTP defines two key transfer
   mechanisms, one using public-key enveloped key exchange and another
   with externally arranged keys.

   In the former case, the symmetric-key cryptosystem parameter is
   passed encrypted under the receiver's public key.






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   In the latter mode, we encrypt the content using a prearranged
   session key, with key identification information specified on one of
   the header lines.

1.4.3.  Message Integrity and Sender Authentication

   Secure HTTP provides a means to verify message integrity and sender
   authenticity for a message via the computation of a Message
   Authentication Code (MAC), computed as a keyed hash over the document
   using a shared secret -- which could potentially have been arranged
   in a number of ways, e.g.: manual arrangement or 'inband' key
   management.  This technique requires neither the use of public key
   cryptography nor encryption.

   This mechanism is also useful for cases where it is appropriate to
   allow parties to identify each other reliably in a transaction
   without providing (third-party) non-repudiability for the
   transactions themselves. The provision of this mechanism is motivated
   by our bias that the action of "signing" a transaction should be
   explicit and conscious for the user, whereas many authentication
   needs (i.e., access control) can be met with a lighter-weight
   mechanism that retains the scalability advantages of public-key
   cryptography for key exchange.

1.4.4.  Freshness

   The protocol provides a simple challenge-response mechanism, allowing
   both parties to insure the freshness of transmissions. Additionally,
   the integrity protection provided to HTTP headers permits
   implementations to consider the Date: header allowable in HTTP
   messages as a freshness indicator, where appropriate (although this
   requires implementations to make allowances for maximum clock skew
   between parties, which we choose not to specify).

1.5.  Implementation Options

   In order to encourage widespread adoption of secure documents for the
   World-Wide Web in the face of the broad scope of application
   requirements, variability of user sophistication, and disparate
   implementation constraints, Secure HTTP deliberately caters to a
   variety of implementation options.  See Section 8 for implementation
   recommendations and requirements.

2.  Message Format

   Syntactically, Secure HTTP messages are the same as HTTP, consisting
   of a request or status line followed by headers and a body. However,
   the range of headers is different and the bodies are typically



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   cryptographically enhanced.

2.1.  Notational Conventions

   This document uses the augmented BNF from HTTP [RFC-2616]. You should
   refer to that document for a description of the syntax.

2.2.  Request Line

   In order to differentiate S-HTTP messages from HTTP messages and
   allow for special processing, the request line should use the special
   Secure" method and use the protocol designator "Secure-HTTP/1.4".
   Consequently, Secure-HTTP and HTTP processing can be intermixed on
   the same TCP port, e.g. port 80.  In order to prevent leakage of
   potentially sensitive information Request-URI should be "*". For
   example:

           Secure * Secure-HTTP/1.4

   When communicating via a proxy, the Request-URI should be consist of
   the AbsoluteURI. Typically, the rel path section should be replaced
   by "*" to minimize the information passed to in the clear.  (e.g.
   http://www.terisa.com/*); proxies should remove the appropriate
   amount of this information to minimize the threat of traffic
   analysis.  See Section 7.2.2.1 for a situation where providing more
   information is appropriate.

2.3.  The Status Line

   S-HTTP responses should use the protocol designator "Secure-
   HTTP/1.4".  For example:

           Secure-HTTP/1.4 200 OK

   Note that the status in the Secure HTTP response line does not
   indicate anything about the success or failure of the unwrapped HTTP
   request. Servers should always use 200 OK provided that the Secure
   HTTP processing is successful. This prevents analysis of success or
   failure for any request, which the correct recipient can determine
   from the encapsulated data. All case variations should be accepted.

2.4.  Secure HTTP Header Lines

   The header lines described in this section go in the header of a
   Secure HTTP message. All except 'Content-Type' and 'Content-Privacy-
   Domain' are optional. The message body shall be separated from the
   header block by two successive CRLFs.




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   All data and fields in header lines should be treated as case
   insensitive unless otherwise specified. Linear whitespace [RFC-822]
   should be used only as a token separator unless otherwise quoted.
   Long header lines may be line folded in the style of [RFC-822].

   This document refers to the header block following the S-HTTP
   request/response line and preceding the successive CRLFs collectively
   as "S-HTTP headers".

2.4.1.  Content-Privacy-Domain

   The two values defined by this document are 'MOSS' and 'CMS'.  CMS
   refers to the privacy enhancement specified in section 2.6.1. MOSS
   refers to the format defined in [RFC-1847] and [RFC-1848].

2.4.2.  Content-Type for CMS

   Under normal conditions, the terminal encapsulated content (after all
   privacy enhancements have been removed) would be an HTTP message. In
   this case, there shall be a Content-Type line reading:

           Content-Type: message/http

   The message/http content type is defined in RFC-2616.

   If the inner message is an S-HTTP message, then the content type
   shall be 'application/s-http'. (See Appendix for the definition of
   this.)

   It is intended that these types be registered with IANA as MIME
   content types.

   The terminal content may be of some other type provided that the type
   is properly indicated by the use of an appropriate Content-Type
   header line. In this case, the header fields for the encapsulation of
   the terminal content apply to the terminal content (the 'final
   headers'). But in any case, final headers should themselves always be
   S-HTTP encapsulated, so that the applicable S-HTTP/HTTP headers are
   never passed unenhanced.

   S-HTTP encapsulation of non-HTTP data is a useful mechanism for
   passing pre-enhanced data (especially presigned data) without
   requiring that the HTTP headers themselves be pre-enhanced.








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2.4.3.  Content-Type for MOSS

   The Content-Type for MOSS shall be an acceptable MIME content type
   describing the cryptographic processing applied. (e.g.
   multipart/signed). The content type of the inner content is described
   in the content type line corresponding to that inner content, and for
   HTTP messages shall be 'message/http'.

2.4.4.  Prearranged-Key-Info

   This header line is intended to convey information about a key which
   has been arranged outside of the internal cryptographic format. One
   use of this is to permit in-band communication of session keys for
   return encryption in the case where one of the parties does not have
   a key pair. However, this should also be useful in the event that the
   parties choose to use some other mechanism, for instance, a one-time
   key list.

   This specification defines two methods for exchanging named keys,
   Inband, Outband. Inband indicates that the session key was exchanged
   previously, using a Key-Assign header of the corresponding method.
   Outband arrangements imply that agents have external access to key
   materials corresponding to a given name, presumably via database
   access or perhaps supplied immediately by a user from keyboard input.
   The syntax for the header line is:

     Prearranged-Key-Info =
      "Prearranged-Key-Info" ":" Hdr-Cipher "," CoveredDEK "," CoverKey-ID
     CoverKey-ID = method ":" key-name
     CoveredDEK = *HEX
     method = "inband" |  "outband"

   While chaining ciphers require an Initialization Vector (IV) [FIPS-
   81] to start off the chaining, that information is not carried by
   this field. Rather, it should be passed internal to the cryptographic
   format being used. Likewise, the bulk cipher used is specified in
   this fashion.

   <Hdr-Cipher> should be the name of the block cipher used to encrypt
   the session key (see section 3.2.4.7)

   <CoveredDEK> is the protected Data Encryption Key (a.k.a. transaction
   key) under which the encapsulated message was encrypted. It should be
   appropriately (randomly) generated by the sending agent, then
   encrypted under the cover of the negotiated key (a.k.a. session key)
   using the indicated header cipher, and then converted into hex.





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   In order to avoid name collisions, cover key namespaces must be
   maintained separately by host and port.

   Note that some Content-Privacy-Domains, notably likely future
   revisions of MOSS and CMS may have support for symmetric key
   management.

   The Prearranged-Key-Info field need not be used in such
   circumstances.  Rather, the native syntax is preferred. Keys
   exchanged with Key-Assign, however, may be used in this situation.

2.4.5.  MAC-Info

   This header is used to supply a Message Authenticity Check, providing
   both message authentication and integrity, computed from the message
   text, the time (optional -- to prevent replay attack), and a shared
   secret between client and server. The MAC should be computed over the
   encapsulated content of the S-HTTP message.  S-HTTP/1.1 defined that
   MACs should be computed using the following algorithm ('||' means
   concatenation):

        MAC = hex(H(Message||[<time>]||<shared key>))

   The time should be represented as an unsigned 32 bit quantity
   representing seconds since 00:00:00 GMT January 1, 1970 (the UNIX
   epoch), in network byte order. The shared key format is a local
   matter.

   Recent research [VANO95] has demonstrated some weaknesses in this
   approach, and this memo introduces a new construction, derived from
   [RFC-2104]. In the name of backwards compatibility, we retain the
   previous constructions with the same names as before. However, we
   also introduce a new series of names (See Section 3.2.4.8 for the
   names) that obey a different (hopefully stronger) construction. (^
   means bitwise XOR)

   HMAC = hex(H(K' ^ pad2 || H(K' ^ pad1 ||[<time>]|| Message)))
   pad1 = the byte 0x36 repeated enough times to fill out a
                hash input block. (I.e. 64 times for both MD5 and SHA-1)
   pad2 = the byte 0x5c repeated enough times to fill out a
                hash input block.
   K' = H(<shared key>)

   The original HMAC construction is for the use of a key with length
   equal to the length of the hash output. Although it is considered
   safe to use a key of a different length (Note that strength cannot be
   increased past the length of the hash function itself, but can be
   reduced by using a shorter key.) [KRAW96b] we hash the original key



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   to permit the use of shared keys (e.g. passphrases) longer than the
   length of the hash. It is noteworthy (though obvious) that this
   technique does not increase the strength of short keys.

   The format of the MAC-Info line is:

   MAC-Info =
   "MAC-Info" ":"  [hex-time],
   hash-alg, hex-hash-data, key-spec
   hex-time = <unsigned seconds since Unix epoch represented as HEX>
   hash-alg = <hash algorithms from section 3.2.4.8>
   hex-hash-data = <computation as described above represented as HEX>
   Key-Spec = "null" | "dek" | Key-ID

   Key-Ids can refer either to keys bound using the Key-Assign header
   line or those bound in the same fashion as the Outband method
   described later. The use of a 'Null' key-spec implies that a zero
   length key was used, and therefore that the MAC merely represents a
   hash of the message text and (optionally) the time.  The special
   key-spec 'DEK' refers to the Data Exchange Key used to encrypt the
   following message body (it is an error to use the DEK key-spec in
   situations where the following message body is unencrypted).

   If the time is omitted from the MAC-Info line, it should simply not
   be included in the hash.

   Note that this header line can be used to provide a more advanced
   equivalent of the original HTTP Basic authentication mode in that the
   user can be asked to provide a username and password. However, the
   password remains private and message integrity can be assured.
   Moreover, this can be accomplished without encryption of any kind.

   In addition, MAC-Info permits fast message integrity verification (at
   the loss of non-repudiability) for messages, provided that the
   participants share a key (possibly passed using Key-Assign in a
   previous message).

   Note that some Content-Privacy-Domains, notably likely future
   revisions of MOSS and CMS may have support for symmetric integrity
   protection The MAC-Info field need not be used in such circumstances.
   Rather, the native syntax is preferred. Keys exchanged with Key-
   Assign, however, may be used in this situation.

2.5.  Content

   The content of the message is largely dependent upon the values of
   the Content-Privacy-Domain and Content-Transfer-Encoding fields.




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   For a CMS message, with '8BIT' Content-Transfer-Encoding, the content
   should simply be the CMS message itself.

   If the Content-Privacy-Domain is MOSS, the content should consist of
   a MOSS Security Multipart as described in RFC1847.

   It is expected that once the privacy enhancements have been removed,
   the resulting (possibly protected) contents will be a normal HTTP
   request. Alternately, the content may be another Secure-HTTP message,
   in which case privacy enhancements should be unwrapped until clear
   content is obtained or privacy enhancements can no longer be removed.
   (This permits embedding of enhancements, such as sequential Signed
   and Enveloped enhancements.) Provided that all enhancements can be
   removed, the final de-enhanced content should be a valid HTTP request
   (or response) unless otherwise specified by the Content-Type line.

   Note that this recursive encapsulation of messages potentially
   permits security enhancements to be applied (or removed) for the
   benefit of intermediaries who may be a party to the transaction
   between a client and server (e.g., a proxy requiring client
   authentication).  How such intermediaries should indicate such
   processing is described in Section 7.2.1.

2.6.  Encapsulation Format Options

2.6.1.  Content-Privacy-Domain: CMS

   Content-Privacy-Domain 'CMS' follows the form of the CMS standard
   (see Appendix).

   Message protection may proceed on two orthogonal axes: signature and
   encryption. Any message may be either signed, encrypted, both, or
   neither. Note that the 'auth' protection mode of S-HTTP is provided
   independently of CMS coding via the MAC-Info header of section 2.3.6
   since CMS does not support a 'KeyDigestedData' type, although it does
   support a 'DigestedData' type.

2.6.1.1.  Signature

   This enhancement uses the 'SignedData' type of CMS.  When digital
   signatures are used, an appropriate certificate may either be
   attached to the message (possibly along with a certificate chain) as
   specified in CMS or the sender may expect the recipient to obtain its
   certificate (and/or chain) independently.  Note that an explicitly
   allowed instance of this is a certificate signed with the private
   component corresponding to the public component being attested to.
   This shall be referred to as a self-signed certificate. What, if any,
   weight to give to such a certificate is a purely local matter.  In



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   either case, a purely signed message is precisely CMS compliant.

2.6.1.2.  Encryption

2.6.1.2.1.  Encryption -- normal, public key

   This enhancement is performed precisely as enveloping (using either '
   EnvelopedData' types) under CMS. A message encrypted in this fashion,
   signed or otherwise, is CMS compliant. To have a message which is
   both signed and encrypted, one simply creates the CMS SignedData
   production and encapsulates it in EnvelopedData as described in CMS.

2.6.1.2.2.  Encryption -- prearranged key

   This uses the 'EncryptedData' type of CMS. In this mode, we encrypt
   the content using a DEK encrypted under cover of a prearranged
   session key (how this key may be exchanged is discussed later), with
   key identification information specified on one of the header lines.
   The IV is in the EncryptedContentInfo type of the EncryptedData
   element.  To have a message which is both signed and encrypted, one
   simply creates the CMS SignedData production and encapsulates it in
   EncryptedData as described in CMS.

2.6.2.  Content-Privacy-Domain: MOSS

   The body of the message should be a MIME compliant message with
   content type that matches the Content-Type line in the S-HTTP
   headers.  Encrypted messages should use multipart/encrypted. Signed
   messages should use multipart/signed. However, since multipart/signed
   does not convey keying material, is is acceptable to use
   multipart/mixed where the first part is application/mosskey-data and
   the second part is multipart/mixed in order to convey certificates
   for use in verifying the signature.

   Implementation Note: When both encryption and signature are applied
   by the same agent, signature should in general be applied before
   encryption.

2.6.3.  Permitted HTTP headers

2.6.3.1.  Overview

   In general, HTTP [RFC-2616] headers should appear in the inner
   content (i.e. the message/http) of an S-HTTP message but should not
   appear in the S-HTTP message wrapper for security reasons. However,
   certain headers need to be visible to agents which do not have access
   to the encapsulated data. These headers may appear in the S-HTTP
   headers as well.



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   Please note that although brief descriptions of the general purposes
   of these headers are provided for clarity, the definitive reference
   is [RFC-2616].

2.6.3.2.  Host

   The host header specificies the internet host and port number of the
   resource being requested. This header should be used to disambiguate
   among multiple potential security contexts within which this message
   could be interpreted. Note that the unwrapped HTTP message will have
   it's own Host field (assuming it's an HTTP/1.1 message). If these
   fields do not match, the server should respond with a 400 status
   code.

2.6.3.3.  Connection

   The Connection field has precisely the same semantics in S-HTTP
   headers as it does in HTTP headers. This permits persistent
   connections to be used with S-HTTP.

3.  Cryptographic Parameters

3.1.  Options Headers

   As described in Section 1.3.2, every S-HTTP request is (at least
   conceptually) preconditioned by the negotiation options provided by
   the potential receiver. The two primary locations for these options
   are

           1. In the headers of an HTTP Request/Response.
           2. In the HTML which contains the anchor being dereferenced.

   There are two kinds of cryptographic options which may be provided:
   Negotiation options, as discussed in Section 3.2 convey a potential
   message recipient's cryptographic preferences. Keying options, as
   discussed in Section 3.3 provide keying material (or pointers to
   keying material) which may be of use to the sender when enhancing a
   message.

   Binding cryptographic options to anchors using HTML extensions is the
   topic of the companion document [SHTML] and will not be treated here.










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3.2.  Negotiation Options

3.2.1.  Negotiation Overview

   Both parties are able to express their requirements and preferences
   regarding what cryptographic enhancements they will permit/require
   the other party to provide. The appropriate option choices depend on
   implementation capabilities and the requirements of particular
   applications.

   A negotiation header is a sequence of specifications each conforming
   to a four-part schema detailing:

        Property -- the option being negotiated, such as bulk encryption
        algorithm.

        Value -- the value being discussed for the property, such as
        DES-CBC

        Direction -- the direction which is to be affected, namely:
        during reception or origination (from the perspective of the
        originator).

        Strength -- strength of preference, namely: required, optional,
        refused

   As an example, the header line:

           SHTTP-Symmetric-Content-Algorithms: recv-optional=DES-CBC,RC2

   could be thought to say: "You are free to use DES-CBC or RC2 for bulk
   encryption for encrypting messages to me."

   We define new headers (to be used in the encapsulated HTTP header,
   not in the S-HTTP header) to permit negotiation of these matters.

3.2.2.  Negotiation Option Format

   The general format for negotiation options is:

           Option = Field ":" Key-val ";" *(Key-val)
           Key-val = Key "=" Value *("," Value)
           Key = Mode"-"Action             ; This is represented as one
                                           ; token without whitespace
           Mode = "orig" | "recv"
           Action = "optional" | "required" | "refused"





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   The <Mode> value indicates whether this <Key-val> refers to what the
   agent's actions are upon sending privacy enhanced messages as opposed
   to upon receiving them. For any given mode-action pair, the
   interpretation to be placed on the enhancements (<Value>s) listed is:

        'recv-optional:' The agent will process the enhancement if the
        other party uses it, but will also gladly process messages
        without the enhancement.

        'recv-required:' The agent will not process messages without
        this enhancement.

        'recv-refused:' The agent will not process messages with this
        enhancement.

        'orig-optional:' When encountering an agent which refuses this
        enhancement, the agent will not provide it, and when
        encountering an agent which requires it, this agent will provide
        it.

        'orig-required:' The agent will always generate the enhancement.

        'orig-refused:' The agent will never generate the enhancement.

   The behavior of agents which discover that they are communicating
   with an incompatible agent is at the discretion of the agents. It is
   inappropriate to blindly persist in a behavior that is known to be
   unacceptable to the other party. Plausible responses include simply
   terminating the connection, or, in the case of a server response,
   returning 'Not implemented 501'.

   Optional values are considered to be listed in decreasing order of
   preference. Agents are free to choose any member of the intersection
   of the optional lists (or none) however.

   If any <Key-Val> is left undefined, it should be assumed to be set to
   the default. Any key which is specified by an agent shall override
   any appearance of that key in any <Key-Val> in the default for that
   field.












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3.2.3.  Parametrization for Variable-length Key Ciphers

   For ciphers with variable key lengths, values may be parametrized
   using the syntax <cipher>'['<length>']'

   For example, 'RSA[1024]' represents a 1024 bit key for RSA. Ranges
   may be represented as

           <cipher>'['<bound1>'-'<bound2>']'

   For purposes of preferences, this notation should be treated as if it
   read (assuming x and y are integers)

           <cipher>[x], <cipher>[x+1],...<cipher>[y] (if x<y)

   and

           <cipher>[x], <cipher>[x-1],...<cipher>[y] (if x>y)

   The special value 'inf' may be used to denote infinite length.

   Using simply <cipher> for such a cipher shall be read as the maximum
   range possible with the given cipher.

3.2.4.  Negotiation Syntax

3.2.4.1.  SHTTP-Privacy-Domains

   This header refers to the Content-Privacy-Domain type of section
   2.3.1. Acceptable values are as listed there. For instance,

                   SHTTP-Privacy-Domains: orig-required=cms;
                                          recv-optional=cms,MOSS

   would indicate that the agent always generates CMS compliant
   messages, but can read CMS or MOSS (or, unenhanced messages).

3.2.4.2.  SHTTP-Certificate-Types

   This indicates what sort of Public Key certificates the agent will
   accept. Currently defined values are 'X.509' and 'X.509v3'.

3.2.4.3.  SHTTP-Key-Exchange-Algorithms

   This header indicates which algorithms may be used for key exchange.
   Defined values are 'DH', 'RSA', 'Outband' and 'Inband'. DH refers to
   Diffie-Hellman X9.42 style enveloping. [DH] RSA refers to RSA
   enveloping. Outband refers to some sort of external key agreement.



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   Inband refers to section 3.3.3.1.

   The expected common configuration of clients having no certificates
   and servers having certificates would look like this (in a message
   sent by the server):

           SHTTP-Key-Exchange-Algorithms: orig-optional=Inband, DH;
                                         recv-required=DH

3.2.4.4.  SHTTP-Signature-Algorithms

   This header indicates what Digital Signature algorithms may be used.
   Defined values are 'RSA' [PKCS-1] and 'NIST-DSS' [FIPS-186] Since
   NIST-DSS and RSA use variable length moduli the parametrization
   syntax of section 3.2.3 should be used.  Note that a key length
   specification may interact with the acceptability of a given
   certificate, since keys (and their lengths) are specified in public-
   key certificates.

3.2.4.5.  SHTTP-Message-Digest-Algorithms

   This indicates what message digest algorithms may be used.
   Previously defined values are 'RSA-MD2' [RFC-1319], 'RSA-MD5' [RFC-
   1321], 'NIST-SHS' [FIPS-180].

3.2.4.6.  SHTTP-Symmetric-Content-Algorithms

   This header specifies the symmetric-key bulk cipher used to encrypt
   message content.  Defined values are:

   DES-CBC -- DES in Cipher Block Chaining (CBC) mode [FIPS-81]
   DES-EDE-CBC -- 2 Key 3DES using Encrypt-Decrypt-Encrypt in outer
                  CBC mode
   DES-EDE3-CBC -- 3 Key 3DES using Encrypt-Decrypt-Encrypt in outer
                   CBC mode
   DESX-CBC -- RSA's DESX in CBC mode
   IDEA-CBC -- IDEA in CBC mode
   RC2-CBC -- RSA's RC2 in CBC mode
   CDMF-CBC -- IBM's CDMF (weakened key DES) [JOHN93] in CBC mode

   Since RC2 keys are variable length, the syntax of section 3.2.3
   should be used.









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3.2.4.7.  SHTTP-Symmetric-Header-Algorithms

   This header specifies the symmetric-key cipher used to encrypt
   message headers.

   DES-ECB -- DES in Electronic Codebook (ECB) mode [FIPS-81]
   DES-EDE-ECB -- 2 Key 3DES using Encrypt-Decrypt-Encrypt in ECB mode
   DES-EDE3-ECB -- 3 Key 3DES using Encrypt-Decrypt-Encrypt in ECB mode
   DESX-ECB -- RSA's DESX in ECB mode
   IDEA-ECB -- IDEA
   RC2-ECB -- RSA's RC2 in ECB mode
   CDMF-ECB -- IBM's CDMF in ECB mode

   Since RC2 is variable length, the syntax of section 3.2.3 should be
   used.

3.2.4.8.  SHTTP-MAC-Algorithms

   This header indicates what algorithms are acceptable for use in
   providing a symmetric key MAC. 'RSA-MD2', 'RSA-MD5' and 'NIST-SHS'
   persist from S-HTTP/1.1 using the old MAC construction. The tokens '
   RSA-MD2-HMAC', 'RSA-MD5-HMAC' and 'NIST-SHS-HMAC' indicate the new
   HMAC construction of 2.3.6 with the MD2, MD5, and SHA-1 algorithms
   respectively.

3.2.4.9.  SHTTP-Privacy-Enhancements

   This header indicates security enhancements to apply.  Possible
   values are 'sign', 'encrypt' and 'auth' indicating whether messages
   are signed, encrypted, or authenticated (i.e., provided with a MAC),
   respectively.

3.2.4.10.  Your-Key-Pattern

   This is a generalized pattern match syntax to describe identifiers
   for a large number of types of keying material. The general syntax
   is:

        Your-Key-Pattern =
                "Your-Key-Pattern" ":" key-use "," pattern-info
        key-use = "cover-key" | "auth-key" | "signing-key"










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3.2.4.10.1.  Cover Key Patterns

   This header specifies desired values for key names used for
   encryption of transaction keys using the Prearranged-Key-Info syntax
   of section 2.3.5.  The pattern-info syntax consists of a series of
   comma separated regular expressions. Commas should be escaped with
   backslashes if they appear in the regexps. The first pattern should
   be assumed to be the most preferred.

3.2.4.10.2.  Auth key patterns

   Auth-key patterns specify name forms desired for use for MAC
   authenticators.  The pattern-info syntax consists of a series of
   comma separated regular expressions. Commas should be escaped with
   backslashes if they appear in the regexps. The first pattern should
   be assumed to be the most preferred.

3.2.4.10.3.  Signing Key Pattern

   This parameter describes a pattern or patterns for what keys are
   acceptable for signing for the digital signature enhancement.  The
   pattern-info syntax for signing-key is:

           pattern-info = name-domain "," pattern-data

   The only currently defined name-domain is 'DN-1779'.  This parameter
   specifies desired values for fields of Distinguished Names.  DNs are
   considered to be represented as specified in RFC1779, the order of
   fields and whitespace between fields is not significant.

   All RFC1779 values should use ',' as a separator rather than ';',
   since ';' is used as a statement separator in S-HTTP.

   Pattern-data is a modified RFC1779 string, with regular expressions
   permitted as field values.  Pattern match is performed field-wise,
   unspecified fields match any value (and therefore leaving the DN-
   Pattern entirely unspecified allows for any DN). Certificate chains
   may be matched as well (to allow for certificates without name
   subordination). DN chains are considered to be ordered left-to-right
   with the issuer of a given certificate on its immediate right,
   although issuers need not be specified. A trailing '.' indicates that
   the sequence of DNs is absolute. I.e. that the one furthest to the
   right is a root.








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   The syntax for the pattern values is,

        Value = DN-spec *("," Dn-spec)["."]
        Dn-spec = "/" *(Field-spec) "/"
        Field-spec := Attr = "Pattern"
        Attr = "CN" | "L" | "ST" | "O" |
                   "OU" | "C" | <or as appropriate>
        Pattern = <POSIX 1003.2 regular expressions>

   For example, to request that the other agent sign with a key
   certified by the RSA Persona CA (which uses name subordination) one
   could use the expression below.  Note the use of RFC1779 quoting to
   protect the comma (an RFC1779 field separator) and the POSIX 1003.2
   quoting to protect the dot (a regular expression metacharacter).

      Your-Key-Pattern: signing-key, DN-1779,
                   /OU=Persona Certificate, O="RSA Data Security,
   Inc\."/

3.2.4.11.  Example

   A representative header block for a server follows.

        SHTTP-Privacy-Domains: recv-optional=MOSS, CMS;
              orig-required=CMS
        SHTTP-Certificate-Types: recv-optional=X.509;
              orig-required=X.509
        SHTTP-Key-Exchange-Algorithms: recv-required=DH;
              orig-optional=Inband,DH
        SHTTP-Signature-Algorithms: orig-required=NIST-DSS;
              recv-required=NIST-DSS
        SHTTP-Privacy-Enhancements: orig-required=sign;
              orig-optional=encrypt

3.2.4.12.  Defaults

   Explicit negotiation parameters take precedence over default values.
   For a given negotiation option type, defaults for a given mode-action
   pair (such as 'orig-required') are implicitly merged unless
   explicitly overridden.

   The default values (these may be negotiated downward or upward) are:

        SHTTP-Privacy-Domains: orig-optional=CMS;
                               recv-optional=CMS
        SHTTP-Certificate-Types: orig-optional=X.509;
                                 recv-optional=X.509
        SHTTP-Key-Exchange-Algorithms: orig-optional=DH,Inband,Outband;



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                                       recv-optional=DH,Inband,Outband
        SHTTP-Signature-Algorithms: orig-optional=NIST-DSS;
                                    recv-optional=NIST-DSS
        SHTTP-Message-Digest-Algorithms: orig-optional=RSA-MD5;
                                         recv-optional=RSA-MD5
        SHTTP-Symmetric-Content-Algorithms: orig-optional=DES-CBC;
                                            recv-optional=DES-CBC
        SHTTP-Symmetric-Header-Algorithms: orig-optional=DES-ECB;
                                           recv-optional=DES-ECB
        SHTTP-Privacy-Enhancements: orig-optional=sign,encrypt, auth;
                                            recv-required=encrypt;
                                            recv-optional=sign, auth
3.3.  Non-Negotiation Headers

   There are a number of options that are used to communicate or
   identify the potential recipient's keying material.

3.3.1.  Encryption-Identity

   This header identifies a potential principal for whom the message
   described by these options could be encrypted; Note that this
   explicitly permits return encryption under (say) public key without
   the other agent signing first (or under a different key than that of
   the signature). The syntax of the Encryption-Identity line is:

           Encryption-Identity =
                   "Encryption Identity" ":" name-class,key-sel,name-arg
           name-class = "DN-1779" | MOSS name forms

   The name-class is an ASCII string representing the domain within
   which the name is to be interpreted, in the spirit of MOSS. In
   addition to the MOSS name forms of RFC1848, we add the DN-1779 name
   form to represent a more convenient form of distinguished name.

3.3.1.1.  DN-1779 Name Class

   The argument is an RFC-1779 encoded DN.

3.3.2.  Certificate-Info

   In order to permit public key operations on DNs specified by
   Encryption-Identity headers without explicit certificate fetches by
   the receiver, the sender may include certification information in the
   Certificate-Info option. The format of this option is:

           Certificate-Info: <Cert-Fmt>','<Cert-Group>

   <Cert-Fmt> should be the type of <Cert-Group> being presented.



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   Defined values are 'PEM' and 'CMS'. CMS certificate groups are
   provided as a base-64 encoded CMS SignedData message containing
   sequences of certificates with or without the SignerInfo field. A PEM
   format certificate group is a list of comma-separated base64-encoded
   PEM certificates.

   Multiple Certificate-Info lines may be defined.

3.3.3.  Key-Assign

   This option serves to indicate that the agent wishes to bind a key to
   a symbolic name for (presumably) later reference.

   The general syntax of the key-assign header is:

        Key-Assign =
                "Key-Assign" ":" Method "," Key-Name ","
                Lifetime "," Ciphers ";" Method-args

        Key-name = string
        Lifetime = "this" | "reply" | ""
        Method ="inband"
        Ciphers = "null" | Cipher+
        Cipher" = <Header cipher from section 3.2.4.7>
        kv = "4" | "5"

   Key-Name is the symbolic name to which this key is to be bound.
   Ciphers is a list of ciphers for which this key is potentially
   applicable (see the list of header ciphers in section 3.2.4.7). The
   keyword 'null' should be used to indicate that it is inappropriate
   for use with ANY cipher. This is potentially useful for exchanging
   keys for MAC computation.

   Lifetime is a representation of the longest period of time during
   which the recipient of this message can expect the sender to accept
   that key. 'this' indicates that it is likely to be valid only for
   reading this transmission. 'reply' indicates that it is useful for a
   reply to this message.  If a Key-Assign with the reply lifetime
   appears in a CRYPTOPTS block, it indicates that it is good for at
   least one (but perhaps only one) dereference of this anchor.  An
   unspecified lifetime implies that this key may be reused for an
   indefinite number of transactions.

   Method should be one of a number of key exchange methods.  The only
   currently defined value is 'inband' referring to Inband keys (i.e.,
   direct assignment).





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   This header line may appear either in an unencapsulated header or in
   an encapsulated message, though when an uncovered key is being
   directly assigned, it may only appear in an encrypted encapsulated
   content. Assigning to a key that already exists causes that key to be
   overwritten.

   Keys defined by this header are referred to elsewhere in this
   specification as Key-IDs, which have the syntax:

        Key-ID = method ":" key-name

3.3.3.1.  Inband Key Assignment

   This refers to the direct assignment of an uncovered key to a
   symbolic name. Method-args should be just the desired session key
   encoded in hexidecimal as in:

        Key-Assign: inband,akey,reply,DES-ECB;0123456789abcdef


   Short keys should be derived from long keys by reading bits from left
   to right.

   Note that inband key assignment is especially important in order to
   permit confidential spontaneous communication between agents where
   one (but not both) of the agents have key pairs.  However, this
   mechanism is also useful to permit key changes without public key
   computations. The key information is carried in this header line must
   be in the inner secured HTTP request, therefore use in unencrypted
   messages is not permitted.

3.3.4.  Nonces

   Nonces are opaque, transient, session-oriented identifiers which may
   be used to provide demonstrations of freshness. Nonce values are a
   local matter, although they are might well be simply random numbers
   generated by the originator. The value is supplied simply to be
   returned by the recipient.

3.3.4.1.  Nonce

   This header is used by an originator to specify what value is to be
   returned in the reply. The field may be any value. Multiple nonces
   may be supplied, each to be echoed independently.

   The Nonce should be returned in a Nonce-Echo header line. See section
   4.1.1.




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3.4.  Grouping Headers With SHTTP-Cryptopts

   In order for servers to bind a group of headers to an HTML anchor, it
   is possible to combine a number of headers on a single S-HTTP
   Cryptopts header line. The names of the anchors to which these
   headers apply is indicated with a 'scope' parameter.

3.4.1.  SHTTP-Cryptopts

   This option provides a set of cryptopts and a list of references to
   which it applies. (For HTML, these references would be named using
   the NAME tag). The names are provided in the scope attribute as a
   comma separated list and separated from the next header line by a
   semicolon. The format for the SHTTP-Cryptopts line is:

SHTTP-Cryptopts =
                   "SHTTP-Cryptopts" ":" scope ";" cryptopt-list
scope = "scope="<tag-spec>    ; This is all one token without whitespace
tag-spec = tag *("," tag) | ""
cryptopt-list = cryptopt *(";" cryptopt)
cryptopt = <S-HTTP cryptopt lines described below>
tag = <value used in HTML anchor NAME attribute>

      For example:

SHTTP-Cryptopts: scope=tag1,tag2;
                   SHTTP-Privacy-Domains:
                   orig-required=cms; recv-optional=cms,MOSS

   If a message contains both S-HTTP negotiation headers and headers
   grouped on SHTTP-Cryptopts line(s), the other headers shall be taken
   to apply to all anchors not bound on the SHTTP-Cryptopts line(s).
   Note that this is an all-or-nothing proposition. That is, if a
   SHTTP-Cryptopts header binds options to a reference, then none of
   these global options apply, even if some of the options headers do
   not appear in the bound options. Rather, the S-HTTP defaults found in
   Section 3.2.4.11 apply.

4.  New Header Lines for HTTP

   Two non-negotiation header lines for HTTP are defined here.

4.1.  Security-Scheme

   All S-HTTP compliant agents must generate the Security-Scheme header
   in the headers of all HTTP messages they generate. This header
   permits other agents to detect that they are communicating with an
   S-HTTP compliant agent and generate the appropriate cryptographic



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   options headers.

   For implementations compliant with this specification, the value must
   be 'S-HTTP/1.4'.

4.1.1.  Nonce-Echo

   The header is used to return the value provided in a previously
   received Nonce: field. This has to go in the encapsulated headers so
   that it an be cryptographically protected.

5.  (Retriable) Server Status Error Reports

   We describe here the special processing appropriate for client
   retries in the face of servers returning an error status.

5.1.  Retry for Option (Re)Negotiation

   A server may respond to a client request with an error code that
   indicates that the request has not completely failed but rather that
   the client may possibly achieve satisfaction through another request.
   HTTP already has this concept with the 3XX redirection codes.

   In the case of S-HTTP, it is conceivable (and indeed likely) that the
   server expects the client to retry his request using another set of
   cryptographic options. E.g., the document which contains the anchor
   that the client is dereferencing is old and did not require digital
   signature for the request in question, but the server now has a
   policy requiring signature for dereferencing this URL. These options
   should be carried in the header of the encapsulated HTTP message,
   precisely as client options are carried.

   The general idea is that the client will perform the retry in the
   manner indicated by the combination of the original request and the
   precise nature of the error and the cryptographic enhancements
   depending on the options carried in the server response.

   The guiding principle in client response to these errors should be to
   provide the user with the same sort of informed choice with regard to
   dereference of these anchors as with normal anchor dereference. For
   instance, in the case above, it would be inappropriate for the client
   to sign the request without requesting permission for the action.









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5.2.  Specific Retry Behavior

5.2.1.  Unauthorized 401, PaymentRequired 402

   The HTTP errors 'Unauthorized 401', 'PaymentRequired 402' represent
   failures of HTTP style authentication and payment schemes. While S-
   HTTP has no explicit support for these mechanisms, they can be
   performed under S-HTTP while taking advantage of the privacy services
   offered by S-HTTP. (There are other errors for S-HTTP specific
   authentication errors.)

5.2.2.  420 SecurityRetry

   This server status reply is provided so that the server may inform
   the client that although the current request is rejected, a retried
   request with different cryptographic enhancements is worth
   attempting. This header shall also be used in the case where an HTTP
   request has been made but an S-HTTP request should have been made.
   Obviously, this serves no useful purpose other than signalling an
   error if the original request should have been encrypted, but in
   other situations (e.g. access control) may be useful.

5.2.2.1.  SecurityRetries for S-HTTP Requests

   In the case of a request that was made as an SHTTP request, it
   indicates that for some reason the cryptographic enhancements applied
   to the request were unsatisfactory and that the request should be
   repeated with the options found in the response header.  Note that
   this can be used as a way to force a new public key negotiation if
   the session key in use has expired or to supply a unique nonce for
   the purposes of ensuring request freshness.

5.2.2.2.  SecurityRetries for HTTP Requests

   If the 420 code is returned in response to an HTTP request, it
   indicates that the request should be retried using S-HTTP and the
   cryptographic options indicated in the response header.

5.2.3.  421 BogusHeader

   This error code indicates that something about the S-HTTP request was
   bad. The error code is to be followed by an appropriate explanation,
   e.g.:

           421 BogusHeader Content-Privacy-Domain must be specified






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5.2.4.  422 SHTTP Proxy Authentication Required

   This response is analagous to the 420 response except that the
   options in the message refer to enhancements that the client must
   perform in order to satisfy the proxy.

5.2.5.  320 SHTTP Not Modifed

   This response code is specifically for use with proxy-server
   interaction where the proxy has placed the If-Modified-Since header
   in the S-HTTP headers of its request. This response indicates that
   the following S-HTTP message contains sufficient keying material for
   the proxy to forward the cached document for the new requestor.

   In general, this takes the form of an S-HTTP message where the actual
   enhanced content is missing, but all the headers and keying material
   are retained. (I.e. the optional content section of the CMS message
   has been removed.) So, if the original response was encrypted, the
   response contains the original DEK re-covered for the new recipient.
   (Notice that the server performs the same processing as it would have
   in the server side caching case of 7.1 except that the message body
   is elided.)

5.2.6.  Redirection 3XX

   These headers are again internal to HTTP, but may contain S-HTTP
   negotiation options of significance to S-HTTP. The request should be
   redirected in the sense of HTTP, with appropriate cryptographic
   precautions being observed.

5.3.  Limitations On Automatic Retries

   Permitting automatic client retry in response to this sort of server
   response permits several forms of attack.  Consider for the moment
   the simple credit card case:

       The user views a document which requires his credit card.  The
       user verifies that the DN of the intended recipient is acceptable
       and that the request will be encrypted and dereferences the
       anchor.  The attacker intercepts the server's reply and responds
       with a message encrypted under the client's public key containing
       the Moved 301 header. If the client were to automatically perform
       this redirect it would allow compromise of the user's credit
       card.







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5.3.1.  Automatic Encryption Retry

   This shows one possible danger of automatic retries -- potential
   compromise of encrypted information. While it is impossible to
   consider all possible cases, clients should never automatically
   reencrypt data unless the server requesting the retry proves that he
   already has the data. So, situations in which it would be acceptable
   to reencrypt would be if:

       1. The retry response was returned encrypted under an inband key
       freshly generated for the original request.
       2. The retry response was signed by the intended recipient of the
       original request.
       3. The original request used an outband key and the response is
       encrypted under that key.

   This is not an exhaustive list, however the browser author would be
   well advised to consider carefully before implementing automatic
   reencryption in other cases. Note that an appropriate behavior in
   cases where automatic reencryption is not appropriate is to query the
   user for permission.

5.3.2.  Automatic Signature Retry

   Since we discourage automatic (without user confirmation) signing in
   even the usual case, and given the dangers described above, it is
   prohibited to automatically retry signature enchancement.

5.3.3.  Automatic MAC Authentication Retry

   Assuming that all the other conditions are followed, it is
   permissible to automatically retry MAC authentication.

6.  Other Issues

6.1.  Compatibility of Servers with Old Clients

   Servers which receive requests in the clear which should be secured
   should return 'SecurityRetry 420' with header lines set to indicate
   the required privacy enhancements.

6.2.  URL Protocol Type

   We define a new URL protocol designator, 'shttp'. Use of this
   designator as part of an anchor URL implies that the target server is
   S-HTTP capable, and that a dereference of this URL should undergo S-
   HTTP processing.




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   Note that S-HTTP oblivious agents should not be willing to
   dereference a URL with an unknown protocol specifier, and hence
   sensitive data will not be accidentally sent in the clear by users of
   non-secure clients.

6.3.  Browser Presentation

6.3.1.  Transaction Security Status

   While preparing a secure message, the browser should provide a visual
   indication of the security of the transaction, as well as an
   indication of the party who will be able to read the message. While
   reading a signed and/or enveloped message, the browser should
   indicate this and (if applicable) the identity of the signer. Self-
   signed certificates should be clearly differentiated from those
   validated by a certification hierarchy.

6.3.2.  Failure Reporting

   Failure to authenticate or decrypt an S-HTTP message should be
   presented differently from a failure to retrieve the document.
   Compliant clients may at their option display unverifiable documents
   but must clearly indicate that they were unverifiable in a way
   clearly distinct from the manner in which they display documents
   which possessed no digital signatures or documents with verifiable
   signatures.

6.3.3.  Certificate Management

   Clients shall provide a method for determining that HTTP requests are
   to be signed and for determining which (assuming there are many)
   certificate is to be used for signature. It is suggested that users
   be presented with some sort of selection list from which they may
   choose a default. No signing should be performed without some sort of
   explicit user interface action, though such action may take the form
   of a persistent setting via a user preferences mechanism (although
   this is discouraged.)

6.3.4.  Anchor Dereference

   Clients shall provide a method to display the DN and certificate
   chain associated with a given anchor to be dereferenced so that users
   may determine for whom their data is being encrypted.  This should be
   distinct from the method for displaying who has signed the document
   containing the anchor since these are orthogonal pieces of encryption
   information.





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7.  Implementation Notes

7.1.  Preenhanced Data

   While S-HTTP has always supported preenhanced documents, in previous
   versions it was never made clear how to actually implement them.
   This section describes two methods for doing so: preenhancing the
   HTTP request/response and preenhancing the underlying data.

7.1.1.  Motivation

   The two primary motivations for preenhanced documents are security
   and performance. These advantages primarily accrue to signing but may
   also under special circumstances apply to confidentiality or
   repudiable (MAC-based) authentication.

   Consider the case of a server which repeatedly serves the same
   content to multiple clients. One such example would be a server which
   serves catalogs or price lists. Clearly, customers would like to be
   able to verify that these are actual prices. However, since the
   prices are typically the same to all comers, confidentiality is not
   an issue. (Note: see Section 7.1.5 below for how to deal with this
   case as well).

   Consequently, the server might wish to sign the document once and
   simply send the cached signed document out when a client makes a new
   request, avoiding the overhead of a private key operation each time.
   Note that conceivably, the signed document might have been generated
   by a third party and placed in the server's cache. The server might
   not even have the signing key! This illustrates the security benefit
   of presigning: Untrusted servers can serve authenticated data without
   risk even if the server is compromised.

7.1.2.  Presigned Requests/Responses

   The obvious implementation is simply to take a single
   request/response, cache it, and send it out in situations where a new
   message would otherwise be generated.

7.1.3.  Presigned Documents

   It is also possible using S-HTTP to sign the underlying data and send
   it as an S-HTTP messsage. In order to do this, one would take the
   signed document (a CMS or MOSS message) and attach both S-HTTP
   headers (e.g. the S-HTTP request/response line, the Content-Privacy-
   Domain) and the necessary HTTP headers (including a Content-Type that
   reflects the inner content).




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           SECURE * Secure-HTTP/1.4
           Content-Type: text/html
           Content-Privacy-Domain: CMS

           Random signed message here...

   This message itself cannot be sent, but needs to be recursively
   encapsulated, as described in the next section.

7.1.4.  Recursive Encapsulation

   As required by Section 7.3, the result above needs to be itself
   encapsulated to protect the HTTP headers. the obvious case [and the
   one illustrated here] is when confidentiality is required, but the
   auth enhancement or even the null transform might be applied instead.
   That is, the message shown above can be used as the inner content of
   a new S-HTTP message, like so:

           SECURE * Secure-HTTP/1.4
           Content-Type: application/s-http
           Content-Privacy-Domain: CMS

           Encrypted version of the message above...

   To unfold this, the receiver would decode the outer S-HTTP message,
   reenter the (S-)HTTP parsing loop to process the new message, see
   that that too was S-HTTP, decode that, and recover the inner content.

   Note that this approach can also be used to provide freshness of
   server activity (though not of the document itself) while still
   providing nonrepudiation of the document data if a NONCE is included
   in the request.

7.1.5.  Preencrypted Messages

   Although preenhancement works best with signature, it can also be
   used with encryption under certain conditions. Consider the situation
   where the same confidential document is to be sent out repeatedly.
   The time spent to encrypt can be saved by caching the ciphertext and
   simply generating a new key exchange block for each recipient. [Note
   that this is logically equivalent to a multi- recipient message as
   defined in both MOSS and CMS and so care must be taken to use proper
   PKCS-1 padding if RSA is being used since otherwise, one may be open
   to a low encryption exponent attack [HAST96].







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7.2.  Proxy Interaction

   The use of S-HTTP presents implementation issues to the use of HTTP
   proxies. While simply having the proxy blindly forward responses is
   straightforward, it would be preferable if S-HTTP aware proxies were
   still able to cache responses in at least some circumstances. In
   addition, S-HTTP services should be usable to protect client-proxy
   authentication. This section describes how to achieve those goals
   using the mechanisms described above.

7.2.1.  Client-Proxy Authentication

   When an S-HTTP aware proxy receives a request (HTTP or S-HTTP) that
   (by whatever access control rules it uses) it requires to be S-HTTP
   authenticated (and if it isn't already so), it should return the 422
   response code (5.7.4).

   When the client receives the 422 response code, it should read the
   cryptographic options that the proxy sent and determine whether or
   not it is willing to apply that enhancement to the message. If the
   client is willing to meet these requirements, it should recursively
   encapsulate the request it previously sent using the appropriate
   options.  (Note that since the enhancement is recursively applied,
   even clients which are unwilling to send requests to servers in the
   clear may be willing to send the already encrypted message to the
   proxy without further encryption.) (See Section 7.1 for another
   example of a recursively encapsulated message)

   When the proxy receives such a message, it should strip the outer
   encapsulation to recover the message which should be sent to the
   server.

8.  Implementation Recommendations and Requirements

   All S-HTTP agents must support the MD5 message digest and MAC
   authentication. As of S-HTTP/1.4, all agents must also support the
   RSA-MD5-HMAC construction.

   All S-HTTP agents must support Outband, Inband, and DH key exchange.

   All agents must support encryption using DES-CBC.

   Agents must support signature generation and verification using
   NIST-DSS.







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9.  Protocol Syntax Summary

   We present below a summary of the main syntactic features of S-
   HTTP/1.4, excluding message encapsulation proper.

9.1.  S-HTTP (Unencapsulated) Headers

   Content-Privacy-Domain: ('CMS' | 'MOSS')
   Prearranged-Key-Info: <Hdr-Cipher>,<Key>,<Key-ID>
   Content-Type: 'message/http'
   MAC-Info: [hex(timeofday)',']<hash-alg>','hex(<hash-data>)','
           <key-spec>

9.2.  HTTP (Encapsulated) Non-negotiation Options

   Key-Assign: <Method>','<Key-Name>','<Lifetime>','
           <Ciphers>';'<Method-args>
   Encryption-Identity: <name-class>','<key-sel>','<name-args>
   Certificate-Info: <Cert-Fmt>','<Cert-Group>
   Nonce: <string>
   Nonce-Echo: <string>

9.3.  Encapsulated Negotiation Options

   SHTTP-Cryptopts: <scope>';'<string>(,<string>)*
   SHTTP-Privacy-Domains: ('CMS' | 'MOSS')
   SHTTP-Certificate-Types: ('X.509')
   SHTTP-Key-Exchange-Algorithms: ('DH', 'RSA' | 'Inband' | 'Outband')
   SHTTP-Signature-Algorithms: ('RSA' | 'NIST-DSS')
   SHTTP-Message-Digest-Algorithms:  ('RSA-MD2' | 'RSA-MD5' | 'NIST-SHS'
           'RSA-MD2-HMAC', 'RSA-MD5-HMAC', 'NIST-SHS-HMAC')
   SHTTP-Symmetric-Content-Algorithms: ('DES-CBC' | 'DES-EDE-CBC' |
           'DES-EDE3-CBC' | 'DESX-CBC' | 'CDMF-CBC' | 'IDEA-CBC' |
           'RC2-CBC' )
   SHTTP-Symmetric-Header-Algorithms: ('DES-ECB' | 'DES-EDE-ECB' |
           'DES-EDE3-EBC' | 'DESX-ECB' | 'CDMF-ECB' | 'IDEA-ECB' |
           'RC2-ECB')
   SHTTP-Privacy-Enhancements: ('sign' | 'encrypt' | 'auth')
   Your-Key-Pattern: <key-use>','<pattern-info>

9.4.  HTTP Methods

   Secure * Secure-HTTP/1.4








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9.5.  Server Status Reports

   Secure-HTTP/1.4 200 OK
   SecurityRetry 420
   BogusHeader 421 <reason>

10.  An Extended Example

   We provide here a contrived example of a series of S-HTTP requests
   and replies. Rows of equal signs are used to set off the narrative
   from sample message traces. Note that the actual encrypted or signed
   message bodies would normally be binary garbage. In an attempt to
   preserve readability while still using (mostly) genuine messages, the
   bodies of the requests have been base64 encoded. To regenerate actual
   S-HTTP messages, it is necessary to remove the base64 encoding from
   the message body.

10.1.  A request using RSA key exchange with Inband key reply

   Alice, using an S-HTTP-capable client, begins by making an HTTP
   request which yields the following response page:

   ============================================================
   200 OK HTTP/1.0
   Server-Name: Navaho-0.1.3.3alpha
   Certificate-Info: CMS,MIAGCSqGSIb3DQEHAqCAMIACAQExADCABgkqh
           kiG9w0BBwEAAKCAM
           IIBrTCCAUkCAgC2MA0GCSqGSIb3DQEBAgUAME0xCzAJBgNVBAYTAlVTMSAwH
           gYDVQQKExdSU0EgRGF0YSBTZWN1cml0eSwgSW5jLjEcMBoGA1UECxMTUGVyc
           29uYSBDZXJ0aWZpY2F0ZTAeFw05NDA0MDkwMDUwMzdaFw05NDA4MDIxODM4N
           TdaMGcxCzAJBgNVBAYTAlVTMSAwHgYDVQQKExdSU0EgRGF0YSBTZWN1cml0e
           SwgSW5jLjEcMBoGA1UECxMTUGVyc29uYSBDZXJ0aWZpY2F0ZTEYMBYGA1UEA
           xMPU2V0ZWMgQXN0cm9ub215MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAMy8Q
           cW7RMrB4sTdQ8Nmb2DFmJmkWn+el+NdeamIDElX/qw9mIQu4xNj1FfepfJNx
           zPvA0OtMKhy6+bkrlyMEU8CAwEAATANBgkqhkiG9w0BAQIFAANPAAYn7jDgi
           rhiIL4wnP8nGzUisGSpsFsF4/7z2P2wqne6Qk8Cg/Dstu3RyaN78vAMGP8d8
           2H5+Ndfhi2mRp4YHiGHz0HlK6VbPfnyvS2wdjCCAccwggFRAgUCQAAAFDANB
           gkqhkiG9w0BAQIFADBfMQswCQYDVQQGEwJVUzEgMB4GA1UEChMXUlNBIERhd
           GEgU2VjdXJpdHksIEluYy4xLjAsBgNVBAsTJUxvdyBBc3N1cmFuY2UgQ2Vyd
           GlmaWNhdGlvbiBBdXRob3JpdHkwHhcNOTQwMTA3MDAwMDAwWhcNOTYwMTA3M
           jM1OTU5WjBNMQswCQYDVQQGEwJVUzEgMB4GA1UEChMXUlNBIERhdGEgU2Vjd
           XJpdHksIEluYy4xHDAaBgNVBAsTE1BlcnNvbmEgQ2VydGlmaWNhdGUwaTANB
           gkqhkiG9w0BAQEFAANYADBVAk4GqghQDa9Xi/2zAdYEqJVIcYhlLN1FpI9tX
           Q1m6zZ39PYXK8Uhoj0Es7kWRv8hC04vqkOKwndWbzVtvoHQOmP8nOkkuBi+A
           QvgFoRcgOUCAwEAATANBgkqhkiG9w0BAQIFAANhAD/5Uo7xDdp49oZm9GoNc
           PhZcW1e+nojLvHXWAU/CBkwfcR+FSf4hQ5eFu1AjYv6Wqf430Xe9Et5+jgnM
           Tiq4LnwgTdA8xQX4elJz9QzQobkE3XVOjVAtCFcmiin80RB8AAAMYAAAAAAA
           AAAAA==



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   Encryption-Identity: DN-1779, null, CN=Setec Astronomy, OU=Persona
           Certificate,O="RSA Data Security, Inc.", C=US;
   SHTTP-Privacy-Enhancements: recv-required=encrypt

   <A name=tag1 HREF="shttp://www.setec.com/secret">
   Don't read this. </A>
   ============================================================

   An appropriate HTTP request to dereference this URL would be:

   ============================================================
   GET /secret HTTP/1.0
   Security-Scheme: S-HTTP/1.4
   User-Agent: Web-O-Vision 1.2beta
   Accept: *.*
   Key-Assign: Inband,1,reply,des-ecb;7878787878787878

   ============================================================

   The added Key-Assign line that would not have been in an ordinary
   HTTP request permits Bob (the server) to encrypt his reply to Alice,
   even though Alice does not have a public key, since they would share
   a key after the request is received by Bob.  This request has the
   following S-HTTP encapsulation:

   ============================================================
   Secure * Secure-HTTP/1.4
   Content-Type: message/http
   Content-Privacy-Domain: CMS

   MIAGCSqGSIb3DQEHA6CAMIACAQAxgDCBqQIBADBTME0xCzAJBgNVBAYTAlVTMSAw
   HgYDVQQKExdSU0EgRGF0YSBTZWN1cml0eSwgSW5jLjEcMBoGA1UECxMTUGVyc29u
   YSBDZXJ0aWZpY2F0ZQICALYwDQYJKoZIhvcNAQEBBQAEQCU/R+YCJSUsV6XLilHG
   cNVzwqKcWzmT/rZ+duOv8Ggb7oO/d8H3xUVGQ2LsX4kYGq2szwj8Q6eWhsmhf4oz
   lvMAADCABgkqhkiG9w0BBwEwEQYFKw4DAgcECFif7BadXlw3oIAEgZBNcMexKe16
   +mNxx8YQPukBCL0bWqS86lvws/AgRkKPELmysBi5lco8MBCsWK/fCyrnxIRHs1oK
   BXBVlsAhKkkusk1kCf/GbXSAphdSgG+d6LxrNZwHbBFOX6A2hYS63Iczd5bOVDDW
   Op2gcgUtMJq6k2LFrs4L7HHqRPPlqNJ6j5mFP4xkzOCNIQynpD1rV6EECMIk/T7k
   1JLSAAAAAAAAAAAAAA==
   ============================================================

   The data between the delimiters is a CMS message, RSA enveloped for
   Setec Astronomy.

   Bob decrypts the request, finds the document in question, and is
   ready to serve it back to Alice.





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   An appropriate HTTP server response would be:

   ============================================================
   HTTP/1.0 200 OK
   Security-Scheme: S-HTTP/1.4
   Content-Type: text/html

   Congratulations, you've won.
   <A href="/prize.html"
    CRYPTOPTS="Key-Assign: Inband,alice1,reply,des-ecb;020406080a0c0e0f;
    SHTTP-Privacy-Enhancements: recv-required=auth">Click here to
   claim your prize</A>
   ============================================================

   This HTTP response, encapsulated as an S-HTTP message becomes:

   ============================================================
   Secure * Secure-HTTP/1.4
   Content-Type: message/http
   Prearranged-Key-Info: des-ecb,697fa820df8a6e53,inband:1
   Content-Privacy-Domain: CMS

   MIAGCSqGSIb3DQEHBqCAMIACAQAwgAYJKoZIhvcNAQcBMBEGBSsOAwIHBAifqtdy
   x6uIMYCCARgvFzJtOZBn773DtmXlx037ck3giqnV0WC0QAx5f+fesAiGaxMqWcir
   r9XvT0nT0LgSQ/8tiLCDBEKdyCNgdcJAduy3D0r2sb5sNTT0TyL9uydG3w55vTnW
   aPbCPCWLudArI1UHDZbnoJICrVehxG/sYX069M8v6VO8PsJS7//hh1yM+0nekzQ5
   l1p0j7uWKu4W0csrlGqhLvEJanj6dQAGSTNCOoH3jzEXGQXntgesk8poFPfHdtj0
   5RH4MuJRajDmoEjlrNcnGl/BdHAd2JaCo6uZWGcnGAgVJ/TVfSVSwN5nlCK87tXl
   nL7DJwaPRYwxb3mnPKNq7ATiJPf5u162MbwxrddmiE7e3sST7naSN+GS0ateY5X7
   AAAAAAAAAAA=
   ============================================================

   The data between the delimiters is a CMS message encrypted under a
   randomly-chosen DEK which can be recovered by computing:

           DES-DECRYPT(inband:1,697fa820df8a6e53)

   where 'inband:1' is the key exchanged in the Key-Assign line in the
   original request.












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10.2.  A request using the auth enhancement

   There is a link on the HTML page that was just returned, which Alice
   dereferences, creating the HTTP message:

============================================================
GET /prize.html HTTP/1.0
Security-Scheme: S-HTTP/1.4
User-Agent: Web-O-Vision 1.1beta
Accept: *.*

============================================================

Which, when encapsulated as an S-HTTP message, becomes:

============================================================
Secure * Secure-HTTP/1.4
Content-Type: message/http
MAC-Info:31ff8122,rsa-md5,b3ca4575b841b5fc7553e69b0896c416,inband:alice1
Content-Privacy-Domain: CMS

MIAGCSqGSIb3DQEHAaCABGNHRVQgL3ByaXplLmh0bWwgSFRUUC8xLjAKU2VjdXJp
dHktU2NoZW1lOiBTLUhUVFAvMS4xClVzZXItQWdlbnQ6IFdlYi1PLVZpc2lvbiAx
LjFiZXRhCkFjY2VwdDogKi4qCgoAAAAA
============================================================

   The data between the delimiters is a CMS 'Data' representation of the
   request.























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Appendix: A Review of CMS

   CMS ("Cryptographic Message Syntax Standard") is a cryptographic
   message encapsulation format, similar to PEM, based on RSA's PKCS-7
   cryptographic messaging syntax.

   CMS is only one of two encapsulation formats supported by S-HTTP, but
   it is to be preferred since it permits the least restricted set of
   negotiable options, and permits binary encoding.  In the interest of
   making this specification more self-contained, we summarize CMS here.

   CMS is defined in terms of OSI's Abstract Syntax Notation (ASN.1,
   defined in X.208), and is concretely represented using ASN.1's Basic
   Encoding Rules (BER, defined in X.209).  A CMS message is a sequence
   of typed content parts. There are six content types, recursively
   composable:

           Data -- Some bytes, with no enhancement.

           SignedData -- A content part, with zero or more signature
           blocks, and associated keying materials. Keying materials
           can be transported via the degenerate case of no signature
           blocks and no data.

           EnvelopedData -- One or more (per recipient) key exchange
           blocks and an encrypted content part.

           DigestedData -- A content part with a single digest block.

           EncryptedData -- An encrypted content part, with key
           materials externally provided.

   Here we will dispense with convention for the sake of ASN.1-impaired
   readers, and present a syntax for CMS in informal BNF (with much
   gloss).  In the actual encoding, most productions have explicit tag
   and length fields.

   Message = *Content
   Content = Data | SignedData | EnvelopedData |
                   DigestedData | EncryptedData
   Data = Bytes
   SignedData = *DigestAlg Content *Certificates
                    *CRLs SignerInfo*
   EnvelopedData = *RecipientInfo BulkCryptAlg
                   Encrypted(Content)






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   DigestedData = DigestAlg Content DigestBytes
   EncryptedData = BulkCryptAlg Encrypted(Bytes)
   SignerInfo = CertID ... Encrypted(DigestBytes) ...
   RecipientInfo = CertID KeyCryptAlg Encrypted(DEK)

Appendix: Internet Media Type message/s-http

   In addition to defining the S-HTTP/1.4 protocol, this document serves
   as the specification for the Internet media type "message/s-http".
   The following is to be registered with IANA.

           Media Type name:        message
           Media subtype name:     s-http
           Required parameters:    none
           Optional parameters:    version, msgtype

             version: The S-HTTP version number of the enclosed message
             (e.g. "1.4"). If not present, the version can be
                   determined from the first line of the body.

             msgtype: The message type -- "request" or "response".
                   If not present, the type can be determined from the
                   first line of the body.

             Encoding considerations: only "7bit", "8bit", or "binary"
                   are permitted.

             Security considerations: this is a security protocol.

Bibliography and References

   [BELL96]    Bellare, M., Canetti, R., Krawczyk, H., "Keying Hash
               Functions for Message Authentication", Preprint.

   [FIPS-46-1] Federal Information Processing Standards Publication
               (FIPS PUB) 46-1, Data Encryption Standard, Reaffirmed
               1988 January 22 (supersedes FIPS PUB 46, 1977 January
               15).

   [FIPS-81]   Federal Information Processing Standards Publication
               (FIPS PUB) 81, DES Modes of Operation, 1980 December 2.

   [FIPS-180]  Federal Information Processing Standards Publication
               (FIPS PUB) 180-1, "Secure Hash Standard", 1995 April 17.

   [FIPS-186]  Federal Information Processing Standards Publication
               (FIPS PUB) 186, Digital Signature Standard, 1994 May 19.




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   [HAST86]    Hastad, J., "On Using RSA With Low Exponents in a Public
               Key Network," Advances in Cryptology-CRYPTO 95
               Proceedings, Springer-Verlag, 1986.

   [JOHN93]    Johnson, D.B., Matyas, S.M., Le, A.V., Wilkins, J.D.,
               "Design of the Commercial Data Masking Facility Data
               Privacy Algorithm," Proceedings 1st ACM Conference on
               Computer & Communications Security, November 1993,
               Fairfax, VA., pp. 93-96.

   [KRAW96b]   Krawczyk, H. personal communication.

   [LAI92]     Lai, X. "On the Design and Security of Block Ciphers,"
               ETH Series in Information Processing, v. 1, Konstanz:
               Hartung-Gorre Verlag, 1992.

   [PKCS-6]    RSA Data Security, Inc. "Extended Certificate Syntax
               Standard", PKCS-6, Nov 1, 1993.

   [CMS]       Housley, R., "Cryptographic Message Syntax", RFC 2630,
               June 1999.

   [RFC-822]   Crocker, D., "Standard For The Format Of ARPA Internet
               Text Messages", STD 11, RFC 822, August 1982.

   [RFC-1319]  Kaliski, B., "The MD2 Message-Digest Algorithm", RFC
               1319, April 1992.

   [RFC-1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
               April 1992.

   [RFC-1421]  Linn, J., "Privacy Enhancement for Internet Electronic
               Mail:  Part I: Message Encryption and Authentication
               Procedures", RFC 1421, February 1993.

   [RFC-1422]  Kent, S., "Privacy Enhancement for Internet Electronic
               Mail:  Part II: Certificate-Based Key Management", RFC
               1422, February 1993.

   [RFC-1779]  Kille, S., "A String Representation of Distinguished
               Names", RFC 1779, March 1995.

   [RFC-2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, September 1993.

   [RFC-1738]  T. Berners-Lee, "Uniform Resource Locators (URLs)", RFC
               1738, December 1994.



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   [RFC-1847]  Galvin, J., Murphy, S., Crocker, S., and N. Freed,
               "Security Muliparts for MIME: Multipart/Signed and
               Multipart/Encrypted", RFC 1847, October 1995.

   [RFC-1848]  Crocker, S., Freed, N., Galvin, J., and S. Murphy, "MIME
               Object Security Services", RFC 1848, October 1995.

   [RFC-1864]  Myers, J.  and M. Rose, "The Content-MD5 Header Field",
               RFC 1864, October 1995.

   [RFC-2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
               Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
               Transfer Protocol -- HTTP/1.1" RFC 2616, June 1999.

   [RFC-2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
               Luotonen, A. and L. Stewart, "HTTP Authentication: Basic
               and Digest Access Authentication", RFC 2617, June 1999.

   [RFC-2104]  Krawczyk, H., Bellare, M. and R.  Canetti, "HMAC: Keyed-
               Hashing for Message Authentication", RFC 2104, February
               1997.

   [SHTML]     Rescorla, E. and A. Schiffman, "Security Extensions For
               HTML", RFC 2659, August 1999.

   [VANO95]    B. Prennel and P. van Oorschot, "On the security of two
               MAC algorithms", to appear Eurocrypt'96.

   [X509]      CCITT Recommendation X.509 (1988), "The Directory -
               Authentication Framework".

Security Considerations

   This entire document is about security.

















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Authors' Addresses

   Eric Rescorla
   RTFM, Inc.
   30 Newell Road, #16
   East Palo Alto, CA 94303

   Phone: (650) 328-8631
   EMail: ekr@rtfm.com


   Allan M. Schiffman
   SPYRUS/Terisa
   5303 Betsy Ross Drive
   Santa Clara, CA 95054

   Phone: (408) 327-1901
   EMail: ams@terisa.com

































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15.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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