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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/rfc9048.txt | |
| parent | ea76e11061bda059ae9f9ad130a9895cc85607db (diff) | |
doc: Add RFC documents
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diff --git a/doc/rfc/rfc9048.txt b/doc/rfc/rfc9048.txt new file mode 100644 index 0000000..507f078 --- /dev/null +++ b/doc/rfc/rfc9048.txt @@ -0,0 +1,2175 @@ + + + + +Internet Engineering Task Force (IETF) J. Arkko +Request for Comments: 9048 V. Lehtovirta +Updates: 4187, 5448 V. Torvinen +Category: Informational Ericsson +ISSN: 2070-1721 P. Eronen + Independent + October 2021 + + + Improved Extensible Authentication Protocol Method for 3GPP Mobile + Network Authentication and Key Agreement (EAP-AKA') + +Abstract + + The 3GPP mobile network Authentication and Key Agreement (AKA) is an + authentication mechanism for devices wishing to access mobile + networks. RFC 4187 (EAP-AKA) made the use of this mechanism possible + within the Extensible Authentication Protocol (EAP) framework. RFC + 5448 (EAP-AKA') was an improved version of EAP-AKA. + + This document is the most recent specification of EAP-AKA', + including, for instance, details about and references related to + operating EAP-AKA' in 5G networks. + + EAP-AKA' differs from EAP-AKA by providing a key derivation function + that binds the keys derived within the method to the name of the + access network. The key derivation function has been defined in the + 3rd Generation Partnership Project (3GPP). EAP-AKA' allows its use + in EAP in an interoperable manner. EAP-AKA' also updates the + algorithm used in hash functions, as it employs SHA-256 / HMAC- + SHA-256 instead of SHA-1 / HMAC-SHA-1, which is used in EAP-AKA. + + This version of the EAP-AKA' specification defines the protocol + behavior for both 4G and 5G deployments, whereas the previous version + defined protocol behavior for 4G deployments only. While EAP-AKA' as + defined in RFC 5448 is not obsolete, this document defines the most + recent and fully backwards-compatible specification of EAP-AKA'. + This document updates both RFCs 4187 and 5448. + +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 candidates for any level of Internet + Standard; see Section 2 of RFC 7841. + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + https://www.rfc-editor.org/info/rfc9048. + +Copyright Notice + + Copyright (c) 2021 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 + (https://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. + +Table of Contents + + 1. Introduction + 2. Requirements Language + 3. EAP-AKA' + 3.1. AT_KDF_INPUT + 3.2. AT_KDF + 3.3. Key Derivation + 3.4. Hash Functions + 3.4.1. PRF' + 3.4.2. AT_MAC + 3.4.3. AT_CHECKCODE + 3.5. Summary of Attributes for EAP-AKA' + 4. Bidding Down Prevention for EAP-AKA + 4.1. Summary of Attributes for EAP-AKA + 5. Peer Identities + 5.1. Username Types in EAP-AKA' Identities + 5.2. Generating Pseudonyms and Fast Re-Authentication Identities + 5.3. Identifier Usage in 5G + 5.3.1. Key Derivation + 5.3.2. EAP Identity Response and EAP-AKA' AT_IDENTITY + Attribute + 6. Exported Parameters + 7. Security Considerations + 7.1. Privacy + 7.2. Discovered Vulnerabilities + 7.3. Pervasive Monitoring + 7.4. Security Properties of Binding Network Names + 8. IANA Considerations + 8.1. Type Value + 8.2. Attribute Type Values + 8.3. Key Derivation Function Namespace + 9. References + 9.1. Normative References + 9.2. Informative References + Appendix A. Changes from RFC 5448 + Appendix B. Changes to RFC 4187 + Appendix C. Importance of Explicit Negotiation + Appendix D. Test Vectors + Acknowledgments + Contributors + Authors' Addresses + +1. Introduction + + The 3GPP mobile network Authentication and Key Agreement (AKA) is an + authentication mechanism for devices wishing to access mobile + networks. [RFC4187] (EAP-AKA) made the use of this mechanism + possible within the Extensible Authentication Protocol (EAP) + framework [RFC3748]. + + EAP-AKA' is an improved version of EAP-AKA. EAP-AKA' was defined in + RFC 5448 [RFC5448], and it updated EAP-AKA [RFC4187]. + + This document is the most recent specification of EAP-AKA', + including, for instance, details about and references related to + operating EAP-AKA' in 5G networks. This document does not obsolete + RFC 5448; however, this document is the most recent and fully + backwards-compatible specification. + + EAP-AKA' is commonly implemented in mobile phones and network + equipment. It can be used for authentication to gain network access + via Wireless LAN networks and, with 5G, also directly to mobile + networks. + + EAP-AKA' differs from EAP-AKA by providing a different key derivation + function. This function binds the keys derived within the method to + the name of the access network. This limits the effects of + compromised access network nodes and keys. EAP-AKA' also updates the + algorithm used for hash functions. + + The EAP-AKA' method employs the derived keys CK' and IK' from the + 3GPP specification [TS-3GPP.33.402] and updates the hash function + that is used to SHA-256 [FIPS.180-4] and HMAC to HMAC-SHA-256. + Otherwise, EAP-AKA' is equivalent to EAP-AKA. Given that a different + EAP method Type value is used for EAP-AKA and EAP-AKA', a mutually + supported method may be negotiated using the standard mechanisms in + EAP [RFC3748]. + + Note that any change of the key derivation must be unambiguous + to both sides in the protocol. That is, it must not be + possible to accidentally connect old equipment to new equipment + and get the key derivation wrong or to attempt to use incorrect + keys without getting a proper error message. See Appendix C + for further information. + + Note also that choices in authentication protocols should be + secure against bidding down attacks that attempt to force the + participants to use the least secure function. See Section 4 + for further information. + + This specification makes the following changes from RFC 5448: + + * Updates the reference that specifies how the Network Name field is + constructed in the protocol. This update ensures that EAP-AKA' is + compatible with 5G deployments. RFC 5448 referred to the Release + 8 version of [TS-3GPP.24.302]. This document points to the first + 5G version, Release 16. + + * Specifies how EAP and EAP-AKA' use identifiers in 5G. Additional + identifiers are introduced in 5G, and for interoperability, it is + necessary that the right identifiers are used as inputs in the key + derivation. In addition, for identity privacy it is important + that when privacy-friendly identifiers in 5G are used, no + trackable, permanent identifiers are passed in EAP-AKA', either. + + * Specifies session identifiers and other exported parameters, as + those were not specified in [RFC5448] despite requirements set + forward in [RFC5247] to do so. Also, while [RFC5247] specified + session identifiers for EAP-AKA, it only did so for the full + authentication case, not for the case of fast re-authentication. + + * Updates the requirements on generating pseudonym usernames and + fast re-authentication identities to ensure identity privacy. + + * Describes what has been learned about any vulnerabilities in AKA + or EAP-AKA'. + + * Describes the privacy and pervasive monitoring considerations + related to EAP-AKA'. + + * Adds summaries of the attributes. + + Some of the updates are small. For instance, the reference update to + [TS-3GPP.24.302] does not change the 3GPP specification number, only + the version. But this reference is crucial for the correct + calculation of the keys that result from running the EAP-AKA' method, + so an RFC update pointing to the newest version was warranted. + + Note: Any further updates in 3GPP specifications that affect, + for instance, key derivation is something that EAP-AKA' + implementations need to take into account. Upon such updates, + there will be a need to update both this specification and the + implementations. + + It is an explicit non-goal of this specification to include any other + technical modifications, addition of new features, or other changes. + The EAP-AKA' base protocol is stable and needs to stay that way. If + there are any extensions or variants, those need to be proposed as + standalone extensions or even as different authentication methods. + + The rest of this specification is structured as follows. Section 3 + defines the EAP-AKA' method. Section 4 adds support to EAP-AKA to + prevent bidding down attacks from EAP-AKA'. Section 5 specifies + requirements regarding the use of peer identities, including how 5G + identifiers are used in the EAP-AKA' context. Section 6 specifies + which parameters EAP-AKA' exports out of the method. Section 7 + explains the security differences between EAP-AKA and EAP-AKA'. + Section 8 describes the IANA considerations, and Appendix A and + Appendix B explain the updates to RFC 5448 (EAP-AKA') and RFC 4187 + (EAP-AKA) that have been made in this specification. Appendix C + explains some of the design rationale for creating EAP-AKA'. + Finally, Appendix D provides test vectors. + +2. Requirements Language + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and + "OPTIONAL" in this document are to be interpreted as described in + BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all + capitals, as shown here. + +3. EAP-AKA' + + EAP-AKA' is an EAP method that follows the EAP-AKA specification + [RFC4187] in all respects except the following: + + * It uses the Type code 0x32, not 0x17 (which is used by EAP-AKA). + + * It carries the AT_KDF_INPUT attribute, as defined in Section 3.1, + to ensure that both the peer and server know the name of the + access network. + + * It supports key derivation function negotiation via the AT_KDF + attribute (Section 3.2) to allow for future extensions. + + * It calculates keys as defined in Section 3.3, not as defined in + EAP-AKA. + + * It employs SHA-256 / HMAC-SHA-256 [FIPS.180-4], not SHA-1 / HMAC- + SHA-1 [RFC2104] (see Section 3.4). + + Figure 1 shows an example of the authentication process. Each + message AKA'-Challenge and so on represents the corresponding message + from EAP-AKA, but with the EAP-AKA' Type code. The definition of + these messages, along with the definition of attributes AT_RAND, + AT_AUTN, AT_MAC, and AT_RES can be found in [RFC4187]. + + Peer Server + | EAP-Request/Identity | + |<-------------------------------------------------------| + | | + | EAP-Response/Identity | + | (Includes user's Network Access Identifier, NAI) | + |------------------------------------------------------->| + | +--------------------------------------------------+ + | | Server determines the network name and ensures | + | | that the given access network is authorized to | + | | use the claimed name. The server then runs the | + | | AKA' algorithms generating RAND and AUTN, and | + | | derives session keys from CK' and IK'. RAND and | + | | AUTN are sent as AT_RAND and AT_AUTN attributes, | + | | whereas the network name is transported in the | + | | AT_KDF_INPUT attribute. AT_KDF signals the used | + | | key derivation function. The session keys are | + | | used in creating the AT_MAC attribute. | + | +--------------------------------------------------+ + | EAP-Request/AKA'-Challenge | + | (AT_RAND, AT_AUTN, AT_KDF, AT_KDF_INPUT, AT_MAC)| + |<-------------------------------------------------------| + +------------------------------------------------------+ | + | The peer determines what the network name should be, | | + | based on, e.g., what access technology it is using. | | + | The peer also retrieves the network name sent by | | + | the network from the AT_KDF_INPUT attribute. The | | + | two names are compared for discrepancies, and if | | + | necessary, the authentication is aborted. Otherwise,| | + | the network name from AT_KDF_INPUT attribute is | | + | used in running the AKA' algorithms, verifying AUTN | | + | from AT_AUTN and MAC from AT_MAC attributes. The | | + | peer then generates RES. The peer also derives | | + | session keys from CK'/IK'. The AT_RES and AT_MAC | | + | attributes are constructed. | | + +------------------------------------------------------+ | + | EAP-Response/AKA'-Challenge | + | (AT_RES, AT_MAC) | + |------------------------------------------------------->| + | +--------------------------------------------------+ + | | Server checks the RES and MAC values received | + | | in AT_RES and AT_MAC, respectively. Success | + | | requires both to be found correct. | + | +--------------------------------------------------+ + | EAP-Success | + |<-------------------------------------------------------| + + Figure 1: EAP-AKA' Authentication Process + + EAP-AKA' can operate on the same credentials as EAP-AKA and employ + the same identities. However, EAP-AKA' employs different leading + characters than EAP-AKA for the conventions given in Section 4.1.1 of + [RFC4187] for usernames based on International Mobile Subscriber + Identifier (IMSI). For 4G networks, EAP-AKA' MUST use the leading + character "6" (ASCII 36 hexadecimal) instead of "0" for IMSI-based + permanent usernames. For 5G networks, the leading character "6" is + not used for IMSI-based permanent usernames. Identifier usage in 5G + is specified in Section 5.3. All other usage and processing of the + leading characters, usernames, and identities is as defined by EAP- + AKA [RFC4187]. For instance, the pseudonym and fast re- + authentication usernames need to be constructed so that the server + can recognize them. As an example, a pseudonym could begin with a + leading "7" character (ASCII 37 hexadecimal) and a fast re- + authentication username could begin with "8" (ASCII 38 hexadecimal). + Note that a server that implements only EAP-AKA may not recognize + these leading characters. According to Section 4.1.4 of [RFC4187], + such a server will re-request the identity via the EAP-Request/AKA- + Identity message, making obvious to the peer that EAP-AKA and + associated identity are expected. + +3.1. AT_KDF_INPUT + + The format of the AT_KDF_INPUT attribute is shown below. + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | AT_KDF_INPUT | Length | Actual Network Name Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + . Network Name . + . . + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + The fields are as follows: + + AT_KDF_INPUT + This is set to 23. + + Length + The length of the attribute, calculated as defined in [RFC4187], + Section 8.1. + + Actual Network Name Length + This is a 2-byte actual length field, needed due to the + requirement that the previous field is expressed in multiples of 4 + bytes per the usual EAP-AKA rules. The Actual Network Name Length + field provides the length of the network name in bytes. + + Network Name + This field contains the network name of the access network for + which the authentication is being performed. The name does not + include any terminating null characters. Because the length of + the entire attribute must be a multiple of 4 bytes, the sender + pads the name with 1, 2, or 3 bytes of all zero bits when + necessary. + + Only the server sends the AT_KDF_INPUT attribute. The value is sent + as specified in [TS-3GPP.24.302] for both non-3GPP access networks + and for 5G access networks. Per [TS-3GPP.33.402], the server always + verifies the authorization of a given access network to use a + particular name before sending it to the peer over EAP-AKA'. The + value of the AT_KDF_INPUT attribute from the server MUST be non- + empty, with a greater than zero length in the Actual Network Name + Length field. If the AT_KDF_INPUT attribute is empty, the peer + behaves as if AUTN had been incorrect and authentication fails. See + Section 3 and Figure 3 of [RFC4187] for an overview of how + authentication failures are handled. + + In addition, the peer MAY check the received value against its own + understanding of the network name. Upon detecting a discrepancy, the + peer either warns the user and continues, or fails the authentication + process. More specifically, the peer SHOULD have a configurable + policy that it can follow under these circumstances. If the policy + indicates that it can continue, the peer SHOULD log a warning message + or display it to the user. If the peer chooses to proceed, it MUST + use the network name as received in the AT_KDF_INPUT attribute. If + the policy indicates that the authentication should fail, the peer + behaves as if AUTN had been incorrect and authentication fails. + + The Network Name field contains a UTF-8 string. This string MUST be + constructed as specified in [TS-3GPP.24.302] for "Access Network + Identity". The string is structured as fields separated by colons + (:). The algorithms and mechanisms to construct the identity string + depend on the used access technology. + + On the network side, the network name construction is a configuration + issue in an access network and an authorization check in the + authentication server. On the peer, the network name is constructed + based on the local observations. For instance, the peer knows which + access technology it is using on the link, it can see information in + a link-layer beacon, and so on. The construction rules specify how + this information maps to an access network name. Typically, the + network name consists of the name of the access technology or the + name of the access technology followed by some operator identifier + that was advertised in a link-layer beacon. In all cases, + [TS-3GPP.24.302] is the normative specification for the construction + in both the network and peer side. If the peer policy allows running + EAP-AKA' over an access technology for which that specification does + not provide network name construction rules, the peer SHOULD rely + only on the information from the AT_KDF_INPUT attribute and not + perform a comparison. + + If a comparison of the locally determined network name and the one + received over EAP-AKA' is performed on the peer, it MUST be done as + follows. First, each name is broken down to the fields separated by + colons. If one of the names has more colons and fields than the + other one, the additional fields are ignored. The remaining + sequences of fields are compared, and they match only if they are + equal character by character. This algorithm allows a prefix match + where the peer would be able to match "", "FOO", and "FOO:BAR" + against the value "FOO:BAR" received from the server. This + capability is important in order to allow possible updates to the + specifications that dictate how the network names are constructed. + For instance, if a peer knows that it is running on access technology + "FOO", it can use the string "FOO" even if the server uses an + additional, more accurate description, e.g., "FOO:BAR", that contains + more information. + + The allocation procedures in [TS-3GPP.24.302] ensure that conflicts + potentially arising from using the same name in different types of + networks are avoided. The specification also has detailed rules + about how a client can determine these based on information available + to the client, such as the type of protocol used to attach to the + network, beacons sent out by the network, and so on. Information + that the client cannot directly observe (such as the type or version + of the home network) is not used by this algorithm. + + The AT_KDF_INPUT attribute MUST be sent and processed as explained + above when AT_KDF attribute has the value 1. Future definitions of + new AT_KDF values MUST define how this attribute is sent and + processed. + +3.2. AT_KDF + + AT_KDF is an attribute that the server uses to reference a specific + key derivation function. It offers a negotiation capability that can + be useful for future evolution of the key derivation functions. + + The format of the AT_KDF attribute is shown below. + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | AT_KDF | Length | Key Derivation Function | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + The fields are as follows: + + AT_KDF + This is set to 24. + + Length + The length of the attribute, calculated as defined in [RFC4187], + Section 8.1. For AT_KDF, the Length field MUST be set to 1. + + Key Derivation Function + An enumerated value representing the key derivation function that + the server (or peer) wishes to use. Value 1 represents the + default key derivation function for EAP-AKA', i.e., employing CK' + and IK' as defined in Section 3.3. + + Servers MUST send one or more AT_KDF attributes in the EAP-Request/ + AKA'-Challenge message. These attributes represent the desired + functions ordered by preference, the most preferred function being + the first attribute. + + Upon receiving a set of these attributes, if the peer supports and is + willing to use the key derivation function indicated by the first + attribute, the function is taken into use without any further + negotiation. However, if the peer does not support this function or + is unwilling to use it, it does not process the received EAP-Request/ + AKA'-Challenge in any way except by responding with the EAP-Response/ + AKA'-Challenge message that contains only one attribute, AT_KDF with + the value set to the selected alternative. If there is no suitable + alternative, the peer behaves as if AUTN had been incorrect and + authentication fails (see Figure 3 of [RFC4187]). The peer fails the + authentication also if there are any duplicate values within the list + of AT_KDF attributes (except where the duplication is due to a + request to change the key derivation function; see below for further + information). + + Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the + peer, the server checks that the suggested AT_KDF value was one of + the alternatives in its offer. The first AT_KDF value in the message + from the server is not a valid alternative since the peer should have + accepted it without further negotiation. If the peer has replied + with the first AT_KDF value, the server behaves as if AT_MAC of the + response had been incorrect and fails the authentication. For an + overview of the failed authentication process in the server side, see + Section 3 and Figure 2 of [RFC4187]. Otherwise, the server re-sends + the EAP-Response/AKA'-Challenge message, but adds the selected + alternative to the beginning of the list of AT_KDF attributes and + retains the entire list following it. Note that this means that the + selected alternative appears twice in the set of AT_KDF values. + Responding to the peer's request to change the key derivation + function is the only legal situation where such duplication may + occur. + + When the peer receives the new EAP-Request/AKA'-Challenge message, it + MUST check that the requested change, and only the requested change, + occurred in the list of AT_KDF attributes. If so, it continues with + processing the received EAP-Request/AKA'-Challenge as specified in + [RFC4187] and Section 3.1 of this document. If not, it behaves as if + AT_MAC had been incorrect and fails the authentication. If the peer + receives multiple EAP-Request/AKA'-Challenge messages with differing + AT_KDF attributes without having requested negotiation, the peer MUST + behave as if AT_MAC had been incorrect and fail the authentication. + + Note that the peer may also request sequence number resynchronization + [RFC4187]. This happens after AT_KDF negotiation has already + completed. That is, the EAP-Request/AKA'-Challenge and, possibly, + the EAP-Response/AKA'-Challenge messages are exchanged first to + determine a mutually acceptable key derivation function, and only + then is the possible AKA'-Synchronization-Failure message sent. The + AKA'-Synchronization-Failure message is sent as a response to the + newly received EAP-Request/AKA'-Challenge, which is the last message + of the AT_KDF negotiation. Note that if the first proposed KDF is + acceptable, then the first EAP-Request/AKA'-Challenge message is also + the last message. The AKA'-Synchronization-Failure message MUST + contain the AUTS parameter as specified in [RFC4187] and a copy the + AT_KDF attributes as they appeared in the last message of the AT_KDF + negotiation. If the AT_KDF attributes are found to differ from their + earlier values, the peer and server MUST behave as if AT_MAC had been + incorrect and fail the authentication. + +3.3. Key Derivation + + Both the peer and server MUST derive the keys as follows. + + AT_KDF parameter has the value 1 + In this case, MK is derived and used as follows: + + MK = PRF'(IK'|CK',"EAP-AKA'"|Identity) + K_encr = MK[0..127] + K_aut = MK[128..383] + K_re = MK[384..639] + MSK = MK[640..1151] + EMSK = MK[1152..1663] + + Here [n..m] denotes the substring from bit n to m, including bits + n and m. PRF' is a new pseudorandom function specified in + Section 3.4. The first 1664 bits from its output are used for + K_encr (encryption key, 128 bits), K_aut (authentication key, 256 + bits), K_re (re-authentication key, 256 bits), MSK (Master Session + Key, 512 bits), and EMSK (Extended Master Session Key, 512 bits). + These keys are used by the subsequent EAP-AKA' process. K_encr is + used by the AT_ENCR_DATA attribute, and K_aut by the AT_MAC + attribute. K_re is used later in this section. MSK and EMSK are + outputs from a successful EAP method run [RFC3748]. + + IK' and CK' are derived as specified in [TS-3GPP.33.402]. The + functions that derive IK' and CK' take the following parameters: + CK and IK produced by the AKA algorithm, and value of the Network + Name field comes from the AT_KDF_INPUT attribute (without length + or padding). + + The value "EAP-AKA'" is an eight-characters-long ASCII string. It + is used as is, without any trailing NUL characters. + + Identity is the peer identity as specified in Section 7 of + [RFC4187] and in Section 5.3.2 of in this document for the 5G + cases. + + When the server creates an AKA challenge and corresponding AUTN, + CK, CK', IK, and IK' values, it MUST set the Authentication + Management Field (AMF) separation bit to 1 in the AKA algorithm + [TS-3GPP.33.102]. Similarly, the peer MUST check that the AMF + separation bit is set to 1. If the bit is not set to 1, the peer + behaves as if the AUTN had been incorrect and fails the + authentication. + + On fast re-authentication, the following keys are calculated: + + MK = PRF'(K_re,"EAP-AKA' re-auth"|Identity|counter|NONCE_S) + MSK = MK[0..511] + EMSK = MK[512..1023] + + MSK and EMSK are the resulting 512-bit keys, taking the first 1024 + bits from the result of PRF'. Note that K_encr and K_aut are not + re-derived on fast re-authentication. K_re is the re- + authentication key from the preceding full authentication and + stays unchanged over any fast re-authentication(s) that may happen + based on it. The value "EAP-AKA' re-auth" is a sixteen- + characters-long ASCII string, again represented without any + trailing NUL characters. Identity is the fast re-authentication + identity, counter is the value from the AT_COUNTER attribute, + NONCE_S is the nonce value from the AT_NONCE_S attribute, all as + specified in Section 7 of [RFC4187]. To prevent the use of + compromised keys in other places, it is forbidden to change the + network name when going from the full to the fast re- + authentication process. The peer SHOULD NOT attempt fast re- + authentication when it knows that the network name in the current + access network is different from the one in the initial, full + authentication. Upon seeing a re-authentication request with a + changed network name, the server SHOULD behave as if the re- + authentication identifier had been unrecognized, and fall back to + full authentication. The server observes the change in the name + by comparing where the fast re-authentication and full + authentication EAP transactions were received at the + Authentication, Authorization, and Accounting (AAA) protocol + level. + + AT_KDF has any other value + Future variations of key derivation functions may be defined, and + they will be represented by new values of AT_KDF. If the peer + does not recognize the value, it cannot calculate the keys and + behaves as explained in Section 3.2. + + AT_KDF is missing + The peer behaves as if the AUTN had been incorrect and MUST fail + the authentication. + + If the peer supports a given key derivation function but is unwilling + to perform it for policy reasons, it refuses to calculate the keys + and behaves as explained in Section 3.2. + +3.4. Hash Functions + + EAP-AKA' uses SHA-256 / HMAC-SHA-256, not SHA-1 / HMAC-SHA-1 (see + [FIPS.180-4] and [RFC2104]) as in EAP-AKA. This requires a change to + the pseudorandom function (PRF) as well as the AT_MAC and + AT_CHECKCODE attributes. + +3.4.1. PRF' + + The PRF' construction is the same one IKEv2 uses (see Section 2.13 of + [RFC7296]; the definition of this function has not changed since + [RFC4306], which was referenced by [RFC5448]). The function takes + two arguments. K is a 256-bit value and S is a byte string of + arbitrary length. PRF' is defined as follows: + + PRF'(K,S) = T1 | T2 | T3 | T4 | ... + + where: + T1 = HMAC-SHA-256 (K, S | 0x01) + T2 = HMAC-SHA-256 (K, T1 | S | 0x02) + T3 = HMAC-SHA-256 (K, T2 | S | 0x03) + T4 = HMAC-SHA-256 (K, T3 | S | 0x04) + ... + + PRF' produces as many bits of output as is needed. HMAC-SHA-256 is + the application of HMAC [RFC2104] to SHA-256. + +3.4.2. AT_MAC + + When used within EAP-AKA', the AT_MAC attribute is changed as + follows. The MAC algorithm is HMAC-SHA-256-128, a keyed hash value. + The HMAC-SHA-256-128 value is obtained from the 32-byte HMAC-SHA-256 + value by truncating the output to the first 16 bytes. Hence, the + length of the MAC is 16 bytes. + + Otherwise, the use of AT_MAC in EAP-AKA' follows Section 10.15 of + [RFC4187]. + +3.4.3. AT_CHECKCODE + + When used within EAP-AKA', the AT_CHECKCODE attribute is changed as + follows. First, a 32-byte value is needed to accommodate a 256-bit + hash output: + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | AT_CHECKCODE | Length | Reserved | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + | Checkcode (0 or 32 bytes) | + | | + | | + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + Second, the checkcode is a hash value, calculated with SHA-256 + [FIPS.180-4], over the data specified in Section 10.13 of [RFC4187]. + +3.5. Summary of Attributes for EAP-AKA' + + Table 1 identifies which attributes may be found in which kinds of + messages, and in what quantity. + + Messages are denoted with numbers as follows: + + 1 EAP-Request/AKA-Identity + + 2 EAP-Response/AKA-Identity + + 3 EAP-Request/AKA-Challenge + + 4 EAP-Response/AKA-Challenge + + 5 EAP-Request/AKA-Notification + + 6 EAP-Response/AKA-Notification + + 7 EAP-Response/AKA-Client-Error + + 8 EAP-Request/AKA-Reauthentication + + 9 EAP-Response/AKA-Reauthentication + + 10 EAP-Response/AKA-Authentication-Reject + + 11 EAP-Response/AKA-Synchronization-Failure + + The column denoted with "E" indicates whether the attribute is a + nested attribute that MUST be included within AT_ENCR_DATA. + + In addition, the numbered columns indicate the quantity of the + attribute within the message as follows: + + "0" Indicates that the attribute MUST NOT be included in the + message. + + "1" Indicates that the attribute MUST be included in the message. + + "0-1" Indicates that the attribute is sometimes included in the + message + + "0+" Indicates that zero or more copies of the attribute MAY be + included in the message. + + "1+" Indicates that there MUST be at least one attribute in the + message but more than one MAY be included in the message. + + "0*" Indicates that the attribute is not included in the message + in cases specified in this document, but MAY be included in + the future versions of the protocol. + + The attribute table is shown below. The table is largely the same as + in the EAP-AKA attribute table ([RFC4187], Section 10.1), but changes + how many times AT_MAC may appear in an EAP-Response/AKA'-Challenge + message as it does not appear there when AT_KDF has to be sent from + the peer to the server. The table also adds the AT_KDF and + AT_KDF_INPUT attributes. + + +======================+===+===+===+===+===+===+=+====+=====+==+==+=+ + | Attribute |1 |2 |3 |4 |5 |6 |7|8 | 9 |10|11|E| + +======================+===+===+===+===+===+===+=+====+=====+==+==+=+ + | AT_PERMANENT_ID_REQ |0-1|0 |0 |0 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_ANY_ID_REQ |0-1|0 |0 |0 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_FULLAUTH_ID_REQ |0-1|0 |0 |0 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_IDENTITY |0 |0-1|0 |0 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_RAND |0 |0 |1 |0 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_AUTN |0 |0 |1 |0 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_RES |0 |0 |0 |1 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_AUTS |0 |0 |0 |0 |0 |0 |0|0 | 0 |0 |1 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_NEXT_PSEUDONYM |0 |0 |0-1|0 |0 |0 |0|0 | 0 |0 |0 |Y| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_NEXT_REAUTH_ID |0 |0 |0-1|0 |0 |0 |0|0-1 | 0 |0 |0 |Y| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_IV |0 |0 |0-1|0* |0-1|0-1|0|1 | 1 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_ENCR_DATA |0 |0 |0-1|0* |0-1|0-1|0|1 | 1 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_PADDING |0 |0 |0-1|0* |0-1|0-1|0|0-1 | 0-1 |0 |0 |Y| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_CHECKCODE |0 |0 |0-1|0-1|0 |0 |0|0-1 | 0-1 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_RESULT_IND |0 |0 |0-1|0-1|0 |0 |0|0-1 | 0-1 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_MAC |0 |0 |1 |0-1|0-1|0-1|0|1 | 1 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_COUNTER |0 |0 |0 |0 |0-1|0-1|0|1 | 1 |0 |0 |Y| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_COUNTER_TOO_SMALL |0 |0 |0 |0 |0 |0 |0|0 | 0-1 |0 |0 |Y| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_NONCE_S |0 |0 |0 |0 |0 |0 |0|1 | 0 |0 |0 |Y| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_NOTIFICATION |0 |0 |0 |0 |1 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_CLIENT_ERROR_CODE |0 |0 |0 |0 |0 |0 |1|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_KDF |0 |0 |1+ |0+ |0 |0 |0|0 | 0 |0 |1+|N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + | AT_KDF_INPUT |0 |0 |1 |0 |0 |0 |0|0 | 0 |0 |0 |N| + +----------------------+---+---+---+---+---+---+-+----+-----+--+--+-+ + + Table 1: The Attribute Table + +4. Bidding Down Prevention for EAP-AKA + + As discussed in [RFC3748], negotiation of methods within EAP is + insecure. That is, a man-in-the-middle attacker may force the + endpoints to use a method that is not the strongest that they both + support. This is a problem, as we expect EAP-AKA and EAP-AKA' to be + negotiated via EAP. + + In order to prevent such attacks, this RFC specifies a mechanism for + EAP-AKA that allows the endpoints to securely discover the + capabilities of each other. This mechanism comes in the form of the + AT_BIDDING attribute. This allows both endpoints to communicate + their desire and support for EAP-AKA' when exchanging EAP-AKA + messages. This attribute is not included in EAP-AKA' messages. It + is only included in EAP-AKA messages, which are protected with the + AT_MAC attribute. This approach is based on the assumption that EAP- + AKA' is always preferable (see Section 7). If during the EAP-AKA + authentication process it is discovered that both endpoints would + have been able to use EAP-AKA', the authentication process SHOULD be + aborted, as a bidding down attack may have happened. + + The format of the AT_BIDDING attribute is shown below. + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | AT_BIDDING | Length |D| Reserved | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + The fields are as follows: + + AT_BIDDING + This is set to 136. + + Length + The length of the attribute, calculated as defined in [RFC4187], + Section 8.1. For AT_BIDDING, the Length MUST be set to 1. + + D + This bit is set to 1 if the sender supports EAP-AKA', is willing + to use it, and prefers it over EAP-AKA. Otherwise, it should be + set to zero. + + Reserved + This field MUST be set to zero when sent and ignored on receipt. + + The server sends this attribute in the EAP-Request/AKA-Challenge + message. If the peer supports EAP-AKA', it compares the received + value to its own capabilities. If it turns out that both the server + and peer would have been able to use EAP-AKA' and preferred it over + EAP-AKA, the peer behaves as if AUTN had been incorrect and fails the + authentication (see Figure 3 of [RFC4187]). A peer not supporting + EAP-AKA' will simply ignore this attribute. In all cases, the + attribute is protected by the integrity mechanisms of EAP-AKA, so it + cannot be removed by a man-in-the-middle attacker. + + Note that we assume (Section 7) that EAP-AKA' is always stronger than + EAP-AKA. As a result, this specification does not provide protection + against bidding "down" attacks in the other direction, i.e., + attackers forcing the endpoints to use EAP-AKA'. + +4.1. Summary of Attributes for EAP-AKA + + The appearance of the AT_BIDDING attribute in EAP-AKA exchanges is + shown below, using the notation from Section 3.5: + + +============+===+===+===+===+===+===+===+===+===+====+====+===+ + | Attribute | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | E | + +============+===+===+===+===+===+===+===+===+===+====+====+===+ + | AT_BIDDING | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | N | + +------------+---+---+---+---+---+---+---+---+---+----+----+---+ + + Table 2: AT_BIDDING Attribute Appearance + +5. Peer Identities + + EAP-AKA' peer identities are as specified in [RFC4187], Section 4.1, + with the addition of some requirements specified in this section. + + EAP-AKA' includes optional identity privacy support that can be used + to hide the cleartext permanent identity and thereby make the + subscriber's EAP exchanges untraceable to eavesdroppers. EAP-AKA' + can also use the privacy-friendly identifiers specified for 5G + networks. + + The permanent identity is usually based on the IMSI. Exposing the + IMSI is undesirable because, as a permanent identity, it is easily + trackable. In addition, since IMSIs may be used in other contexts as + well, there would be additional opportunities for such tracking. + + In EAP-AKA', identity privacy is based on temporary usernames or + pseudonym usernames. These are similar to, but separate from, the + Temporary Mobile Subscriber Identities (TMSI) that are used on + cellular networks. + +5.1. Username Types in EAP-AKA' Identities + + Section 4.1.1.3 of [RFC4187] specifies that there are three types of + usernames: permanent, pseudonym, and fast re-authentication + usernames. This specification extends this definition as follows. + There are four types of usernames: + + (1) Regular usernames. These are external names given to EAP-AKA' + peers. The regular usernames are further subdivided into to + categories: + + (a) Permanent usernames, for instance, IMSI-based usernames. + + (b) Privacy-friendly temporary usernames, for instance, 5G GUTI + (5G Globally Unique Temporary Identifier) or 5G privacy + identifiers (see Section 5.3.2) such as SUCI (Subscription + Concealed Identifier). + + (2) EAP-AKA' pseudonym usernames. For example, + 2s7ah6n9q@example.com might be a valid pseudonym identity. In + this example, 2s7ah6n9q is the pseudonym username. + + (3) EAP-AKA' fast re-authentication usernames. For example, + 43953754@example.com might be a valid fast re-authentication + identity and 43953754 the fast re-authentication username. + + The permanent, privacy-friendly temporary, and pseudonym usernames + are only used with full authentication, and fast re-authentication + usernames only with fast re-authentication. Unlike permanent + usernames and pseudonym usernames, privacy-friendly temporary + usernames and fast re-authentication usernames are one-time + identifiers, which are not reused across EAP exchanges. + +5.2. Generating Pseudonyms and Fast Re-Authentication Identities + + This section provides some additional guidance to implementations for + producing secure pseudonyms and fast re-authentication identities. + It does not impact backwards compatibility because each server + consumes only the identities that it generates itself. However, + adherence to the guidance will provide better security. + + As specified by [RFC4187], Section 4.1.1.7, pseudonym usernames and + fast re-authentication identities are generated by the EAP server in + an implementation-dependent manner. RFC 4187 provides some general + requirements on how these identities are transported, how they map to + the NAI syntax, how they are distinguished from each other, and so + on. + + However, to enhance privacy, some additional requirements need to be + applied. + + The pseudonym usernames and fast re-authentication identities MUST be + generated in a cryptographically secure way so that it is + computationally infeasible for an attacker to differentiate two + identities belonging to the same user from two identities belonging + to different users. This can be achieved, for instance, by using + random or pseudorandom identifiers such as random byte strings or + ciphertexts. See also [RFC4086] for guidance on random number + generation. + + Note that the pseudonym and fast re-authentication usernames also + MUST NOT include substrings that can be used to relate the username + to a particular entity or a particular permanent identity. For + instance, the usernames cannot include any subscriber-identifying + part of an IMSI or other permanent identifier. Similarly, no part of + the username can be formed by a fixed mapping that stays the same + across multiple different pseudonyms or fast re-authentication + identities for the same subscriber. + + When the identifier used to identify a subscriber in an EAP-AKA' + authentication exchange is a privacy-friendly identifier that is used + only once, the EAP-AKA' peer MUST NOT use a pseudonym provided in + that authentication exchange in subsequent exchanges more than once. + To ensure that this does not happen, the EAP-AKA' server MAY decline + to provide a pseudonym in such authentication exchanges. An + important case where such privacy-friendly identifiers are used is in + 5G networks (see Section 5.3). + +5.3. Identifier Usage in 5G + + In EAP-AKA', the peer identity may be communicated to the server in + one of three ways: + + * As a part of link-layer establishment procedures, externally to + EAP. + + * With the EAP-Response/Identity message in the beginning of the EAP + exchange, but before the selection of EAP-AKA'. + + * Transmitted from the peer to the server using EAP-AKA' messages + instead of EAP-Response/Identity. In this case, the server + includes an identity-requesting attribute (AT_ANY_ID_REQ, + AT_FULLAUTH_ID_REQ, or AT_PERMANENT_ID_REQ) in the EAP-Request/ + AKA-Identity message, and the peer includes the AT_IDENTITY + attribute, which contains the peer's identity, in the EAP- + Response/AKA-Identity message. + + The identity carried above may be a permanent identity, privacy- + friendly identity, pseudonym identity, or fast re-authentication + identity as defined in Section 5.1. + + 5G supports the concept of privacy identifiers, and it is important + for interoperability that the right type of identifier is used. + + 5G defines the SUbscription Permanent Identifier (SUPI) and + SUbscription Concealed Identifier (SUCI) [TS-3GPP.23.501] + [TS-3GPP.33.501] [TS-3GPP.23.003]. SUPI is globally unique and + allocated to each subscriber. However, it is only used internally in + the 5G network and is privacy sensitive. The SUCI is a privacy- + preserving identifier containing the concealed SUPI, using public key + cryptography to encrypt the SUPI. + + Given the choice between these two types of identifiers, EAP-AKA' + ensures interoperability as follows: + + * Where identifiers are used within EAP-AKA' (such as key + derivation) determine the exact values of the identity to be used, + to avoid ambiguity (see Section 5.3.1). + + * Where identifiers are carried within EAP-AKA' packets (such as in + the AT_IDENTITY attribute) determine which identifiers should be + filled in (see Section 5.3.2). + + In 5G, the normal mode of operation is that identifiers are only + transmitted outside EAP. However, in a system involving terminals + from many generations and several connectivity options via 5G and + other mechanisms, implementations and the EAP-AKA' specification need + to prepare for many different situations, including sometimes having + to communicate identities within EAP. + + The following sections clarify which identifiers are used and how. + +5.3.1. Key Derivation + + In EAP-AKA', the peer identity is used in the key derivation formula + found in Section 3.3. + + The identity needs to be represented in exactly the correct format + for the key derivation formula to produce correct results. + + If the AT_KDF_INPUT parameter contains the prefix "5G:", the AT_KDF + parameter has the value 1, and this authentication is not a fast re- + authentication, then the peer identity used in the key derivation + MUST be as specified in Annex F.3 of [TS-3GPP.33.501] and Clause 2.2 + of [TS-3GPP.23.003]. This is in contrast to [RFC5448], which uses + the identity as communicated in EAP and represented as a NAI. Also, + in contrast to [RFC5448], in 5G EAP-AKA' does not use the "0" nor the + "6" prefix in front of the identifier. + + For an example of the format of the identity, see Clause 2.2 of + [TS-3GPP.23.003]. + + In all other cases, the following applies: + + The identity used in the key derivation formula MUST be exactly + the one sent in the EAP-AKA' AT_IDENTITY attribute, if one was + sent, regardless of the kind of identity that it may have been. + If no AT_IDENTITY was sent, the identity MUST be exactly the + one sent in the generic EAP Identity exchange, if one was made. + + If no identity was communicated inside EAP, then the identity + is the one communicated outside EAP in link-layer messaging. + + In this case, the used identity MUST be the identity most + recently communicated by the peer to the network, again + regardless of what type of identity it may have been. + +5.3.2. EAP Identity Response and EAP-AKA' AT_IDENTITY Attribute + + The EAP authentication option is only available in 5G when the new 5G + core network is also in use. However, in other networks, an EAP-AKA' + peer may be connecting to other types of networks and existing + equipment. + + When the EAP server is in a 5G network, the 5G procedures for EAP- + AKA' apply. [TS-3GPP.33.501] specifies when the EAP server is in a + 5G network. + + Note: Currently, the following conditions are specified: when + the EAP peer uses the 5G Non-Access Stratum (NAS) protocol + [TS-3GPP.24.501] or when the EAP peer attaches to a network + that advertises 5G connectivity without NAS [TS-3GPP.23.501]. + Possible future conditions may also be specified by 3GPP. + + When the 5G procedures for EAP-AKA' apply, EAP identity exchanges are + generally not used as the identity is already made available on + previous link-layer exchanges. + + In this situation, the EAP Identity Response and EAP-AKA' AT_IDENTITY + attribute are handled as specified in Annex F.2 of [TS-3GPP.33.501]. + + When used in EAP-AKA', the format of the SUCI MUST be as specified in + [TS-3GPP.23.003], Section 28.7.3, with the semantics defined in + [TS-3GPP.23.003], Section 2.2B. Also, in contrast to [RFC5448], in + 5G EAP-AKA' does not use the "0" nor the "6" prefix in front of the + identifier. + + For an example of an IMSI in NAI format, see [TS-3GPP.23.003], + Section 28.7.3. + + Otherwise, the peer SHOULD employ an IMSI, SUPI, or NAI [RFC7542] as + it is configured to use. + +6. Exported Parameters + + When not using fast re-authentication, the EAP-AKA' Session-Id is the + concatenation of the EAP-AKA' Type value (0x32, one byte) with the + contents of the RAND field from the AT_RAND attribute followed by the + contents of the AUTN field in the AT_AUTN attribute: + + Session-Id = 0x32 || RAND || AUTN + + When using fast re-authentication, the EAP-AKA' Session-Id is the + concatenation of the EAP-AKA' Type value (0x32) with the contents of + the NONCE_S field from the AT_NONCE_S attribute followed by the + contents of the MAC field from the AT_MAC attribute from the EAP- + Request/AKA-Reauthentication: + + Session-Id = 0x32 || NONCE_S || MAC + + The Peer-Id is the contents of the Identity field from the + AT_IDENTITY attribute, using only the Actual Identity Length bytes + from the beginning. Note that the contents are used as they are + transmitted, regardless of whether the transmitted identity was a + permanent, pseudonym, or fast EAP re-authentication identity. If no + AT_IDENTITY attribute was exchanged, the exported Peer-Id is the + identity provided from the EAP Identity Response packet. If no EAP + Identity Response was provided either, the exported Peer-Id is the + null string (zero length). + + The Server-Id is the null string (zero length). + +7. Security Considerations + + A summary of the security properties of EAP-AKA' follows. These + properties are very similar to those in EAP-AKA. We assume that HMAC + SHA-256 is at least as secure as HMAC SHA-1 (see also [RFC6194]). + This is called the SHA-256 assumption in the remainder of this + section. Under this assumption, EAP-AKA' is at least as secure as + EAP-AKA. + + If the AT_KDF attribute has value 1, then the security properties of + EAP-AKA' are as follows: + + Protected ciphersuite negotiation + EAP-AKA' has no ciphersuite negotiation mechanisms. It does have + a negotiation mechanism for selecting the key derivation + functions. This mechanism is secure against bidding down attacks + from EAP-AKA' to EAP-AKA. The negotiation mechanism allows + changing the offered key derivation function, but the change is + visible in the final EAP-Request/AKA'-Challenge message that the + server sends to the peer. This message is authenticated via the + AT_MAC attribute, and carries both the chosen alternative and the + initially offered list. The peer refuses to accept a change it + did not initiate. As a result, both parties are aware that a + change is being made and what the original offer was. + + Per assumptions in Section 4, there is no protection against + bidding down attacks from EAP-AKA to EAP-AKA' should EAP-AKA' + somehow be considered less secure some day than EAP-AKA. Such + protection was not provided in RFC 5448 implementations and + consequently neither does this specification provide it. If such + support is needed, it would have to be added as a separate new + feature. + + In general, it is expected that the current negotiation + capabilities in EAP-AKA' are sufficient for some types of + extensions, including adding Perfect Forward Secrecy [EMU-AKA-PFS] + and perhaps others. However, some larger changes may require a + new EAP method type, which is how EAP-AKA' itself happened. One + example of such change would be the introduction of new + algorithms. + + Mutual authentication + Under the SHA-256 assumption, the properties of EAP-AKA' are at + least as good as those of EAP-AKA in this respect. Refer to + [RFC4187], Section 12, for further details. + + Integrity protection + Under the SHA-256 assumption, the properties of EAP-AKA' are at + least as good (most likely better) as those of EAP-AKA in this + respect. Refer to [RFC4187], Section 12, for further details. + The only difference is that a stronger hash algorithm and keyed + MAC, SHA-256 / HMAC-SHA-256, is used instead of SHA-1 / HMAC-SHA- + 1. + + Replay protection + Under the SHA-256 assumption, the properties of EAP-AKA' are at + least as good as those of EAP-AKA in this respect. Refer to + [RFC4187], Section 12, for further details. + + Confidentiality + The properties of EAP-AKA' are exactly the same as those of EAP- + AKA in this respect. Refer to [RFC4187], Section 12, for further + details. + + Key derivation + EAP-AKA' supports key derivation with an effective key strength + against brute-force attacks equal to the minimum of the length of + the derived keys and the length of the AKA base key, i.e., 128 + bits or more. The key hierarchy is specified in Section 3.3. + + The Transient EAP Keys used to protect EAP-AKA packets (K_encr, + K_aut, K_re), the MSK, and the EMSK are cryptographically + separate. If we make the assumption that SHA-256 behaves as a + pseudorandom function, an attacker is incapable of deriving any + non-trivial information about any of these keys based on the other + keys. An attacker also cannot calculate the pre-shared secret + from IK, CK, IK', CK', K_encr, K_aut, K_re, MSK, or EMSK by any + practically feasible means. + + EAP-AKA' adds an additional layer of key derivation functions + within itself to protect against the use of compromised keys. + This is discussed further in Section 7.4. + + EAP-AKA' uses a pseudorandom function modeled after the one used + in IKEv2 [RFC7296] together with SHA-256. + + Key strength + See above. + + Dictionary attack resistance + Under the SHA-256 assumption, the properties of EAP-AKA' are at + least as good as those of EAP-AKA in this respect. Refer to + [RFC4187], Section 12, for further details. + + Fast reconnect + Under the SHA-256 assumption, the properties of EAP-AKA' are at + least as good as those of EAP-AKA in this respect. Refer to + [RFC4187], Section 12, for further details. Note that + implementations MUST prevent performing a fast reconnect across + method types. + + Cryptographic binding + Note that this term refers to a very specific form of binding, + something that is performed between two layers of authentication. + It is not the same as the binding to a particular network name. + The properties of EAP-AKA' are exactly the same as those of EAP- + AKA in this respect, i.e., as it is not a tunnel method, this + property is not applicable to it. Refer to [RFC4187], Section 12, + for further details. + + Session independence + The properties of EAP-AKA' are exactly the same as those of EAP- + AKA in this respect. Refer to [RFC4187], Section 12, for further + details. + + Fragmentation + The properties of EAP-AKA' are exactly the same as those of EAP- + AKA in this respect. Refer to [RFC4187], Section 12, for further + details. + + Channel binding + EAP-AKA', like EAP-AKA, does not provide channel bindings as + they're defined in [RFC3748] and [RFC5247]. New skippable + attributes can be used to add channel binding support in the + future, if required. + + However, including the Network Name field in the AKA' algorithms + (which are also used for other purposes than EAP-AKA') provides a + form of cryptographic separation between different network names, + which resembles channel bindings. However, the network name does + not typically identify the EAP (pass-through) authenticator. See + Section 7.4 for more discussion. + +7.1. Privacy + + [RFC6973] suggests that the privacy considerations of IETF protocols + be documented. + + The confidentiality properties of EAP-AKA' itself have been discussed + above under "Confidentiality" (Section 7). + + EAP-AKA' uses several different types of identifiers to identify the + authenticating peer. It is strongly RECOMMENDED to use the privacy- + friendly temporary or hidden identifiers, i.e., the 5G GUTI or SUCI, + pseudonym usernames, and fast re-authentication usernames. The use + of permanent identifiers such as the IMSI or SUPI may lead to an + ability to track the peer and/or user associated with the peer. The + use of permanent identifiers such as the IMSI or SUPI is strongly NOT + RECOMMENDED. + + As discussed in Section 5.3, when authenticating to a 5G network, + only the SUCI identifier is normally used. The use of EAP-AKA' + pseudonyms in this situation is at best limited because the SUCI + already provides a stronger mechanism. In fact, reusing the same + pseudonym multiple times will result in a tracking opportunity for + observers that see the pseudonym pass by. To avoid this, the peer + and server need to follow the guidelines given in Section 5.2. + + When authenticating to a 5G network, per Section 5.3.1, both the EAP- + AKA' peer and server need to employ the permanent identifier SUPI as + an input to key derivation. However, this use of the SUPI is only + internal. As such, the SUPI need not be communicated in EAP + messages. Therefore, SUPI MUST NOT be communicated in EAP-AKA' when + authenticating to a 5G network. + + While the use of SUCI in 5G networks generally provides identity + privacy, this is not true if the null-scheme encryption is used to + construct the SUCI (see [TS-3GPP.33.501], Annex C). The use of this + scheme makes the use of SUCI equivalent to the use of SUPI or IMSI. + The use of the null scheme is NOT RECOMMENDED where identity privacy + is important. + + The use of fast re-authentication identities when authenticating to a + 5G network does not have the same problems as the use of pseudonyms, + as long as the 5G authentication server generates the fast re- + authentication identifiers in a proper manner specified in + Section 5.2. + + Outside 5G, the peer can freely choose between the use of permanent, + pseudonym, or fast re-authentication identifiers: + + * A peer that has not yet performed any EAP-AKA' exchanges does not + typically have a pseudonym available. If the peer does not have a + pseudonym available, then the privacy mechanism cannot be used, + and the permanent identity will have to be sent in the clear. + + The terminal SHOULD store the pseudonym in nonvolatile memory so + that it can be maintained across reboots. An active attacker that + impersonates the network may use the AT_PERMANENT_ID_REQ attribute + ([RFC4187], Section 4.1.2) to learn the subscriber's IMSI. + However, as discussed in [RFC4187], Section 4.1.2, the terminal + can refuse to send the cleartext permanent identity if it believes + that the network should be able to recognize the pseudonym. + + * When pseudonyms and fast re-authentication identities are used, + the peer relies on the properly created identifiers by the server. + + It is essential that an attacker cannot link a privacy-friendly + identifier to the user in any way or determine that two + identifiers belong to the same user as outlined in Section 5.2. + The pseudonym usernames and fast re-authentication identities MUST + NOT be used for other purposes (e.g., in other protocols). + + If the peer and server cannot guarantee that SUCI can be used or that + pseudonyms will be available, generated properly, and maintained + reliably, and identity privacy is required, then additional + protection from an external security mechanism such as tunneled EAP + methods like Tunneled Transport Layer Security (TTLS) [RFC5281] or + Tunnel Extensible Authentication Protocol (TEAP) [RFC7170] may be + used. The benefits and the security considerations of using an + external security mechanism with EAP-AKA are beyond the scope of this + document. + + Finally, as with other EAP methods, even when privacy-friendly + identifiers or EAP tunneling is used, typically the domain part of an + identifier (e.g., the home operator) is visible to external parties. + +7.2. Discovered Vulnerabilities + + There have been no published attacks that violate the primary secrecy + or authentication properties defined for Authentication and Key + Agreement (AKA) under the originally assumed trust model. The same + is true of EAP-AKA'. + + However, there have been attacks when a different trust model is in + use, with characteristics not originally provided by the design, or + when participants in the protocol leak information to outsiders on + purpose, and there have been some privacy-related attacks. + + For instance, the original AKA protocol does not prevent an insider + supplying keys to a third party, e.g., as described by Mjølsnes and + Tsay in [MT2012] where a serving network lets an authentication run + succeed, but then it misuses the session keys to send traffic on the + authenticated user's behalf. This particular attack is not different + from any on-path entity (such as a router) pretending to send + traffic, but the general issue of insider attacks can be a problem, + particularly in a large group of collaborating operators. + + Another class of attacks is the use of tunneling of traffic from one + place to another, e.g., as done by Zhang and Fang in [ZF2005] to + leverage security policy differences between different operator + networks, for instance. To gain something in such an attack, the + attacker needs to trick the user into believing it is in another + location. If policies between locations differ, for instance, if + payload traffic is not required to be encrypted in some location, the + attacker may trick the user into opening a vulnerability. As an + authentication mechanism, EAP-AKA' is not directly affected by most + of these attacks. EAP-AKA' network name binding can also help + alleviate some of the attacks. In any case, it is recommended that + EAP-AKA' configuration not be dependent on the location of request + origin, unless the location information can be cryptographically + confirmed, e.g., with the network name binding. + + Zhang and Fang also looked at denial-of-service attacks [ZF2005]. A + serving network may request large numbers of authentication runs for + a particular subscriber from a home network. While the + resynchronization process can help recover from this, eventually it + is possible to exhaust the sequence number space and render the + subscriber's card unusable. This attack is possible for both + original AKA and EAP-AKA'. However, it requires the collaboration of + a serving network in an attack. It is recommended that EAP-AKA' + implementations provide the means to track, detect, and limit + excessive authentication attempts to combat this problem. + + There have also been attacks related to the use of AKA without the + generated session keys (e.g., [BT2013]). Some of those attacks + relate to the use of HTTP Digest AKAv1 [RFC3310], which was + originally vulnerable to man-in-the-middle attacks. This has since + been corrected in [RFC4169]. The EAP-AKA' protocol uses session keys + and provides channel binding, and as such, it is resistant to the + above attacks except where the protocol participants leak information + to outsiders. + + Basin, et al. [Basin2018] have performed formal analysis and + concluded that the AKA protocol would have benefited from additional + security requirements such as key confirmation. + + In the context of pervasive monitoring revelations, there were also + reports of compromised long-term pre-shared keys used in SIM and AKA + [Heist2015]. While no protocol can survive the theft of key material + associated with its credentials, there are some things that alleviate + the impacts in such situations. These are discussed further in + Section 7.3. + + Arapinis, et al. [Arapinis2012] describe an attack that uses the AKA + resynchronization protocol to attempt to detect whether a particular + subscriber is in a given area. This attack depends on the attacker + setting up a false base station in the given area and on the + subscriber performing at least one authentication between the time + the attack is set up and run. + + Borgaonkar, et al. discovered that the AKA resynchronization protocol + may also be used to predict the authentication frequency of a + subscriber if a non-time-based sequence number (SQN) generation + scheme is used [Borgaonkar2018]. The attacker can force the reuse of + the keystream that is used to protect the SQN in the AKA + resynchronization protocol. The attacker then guesses the + authentication frequency based on the lowest bits of two XORed SQNs. + The researchers' concern was that the authentication frequency would + reveal some information about the phone usage behavior, e.g., number + of phone calls made or number of SMS messages sent. There are a + number of possible triggers for authentication, so such an + information leak is not direct, but it can be a concern. The impact + of the attack differs depending on whether the SQN generation scheme + that is used is time-based or not. + + Similar attacks are possible outside AKA in the cellular paging + protocols where the attacker can simply send application-layer data, + send short messages, or make phone calls to the intended victim and + observe the air interface (e.g., [Kune2012] and [Shaik2016]). + Hussain, et al. demonstrated a slightly more sophisticated version of + the attack that exploits the fact that the 4G paging protocol uses + the IMSI to calculate the paging timeslot [Hussain2019]. As this + attack is outside AKA, it does not impact EAP-AKA'. + + Finally, bad implementations of EAP-AKA' may not produce pseudonym + usernames or fast re-authentication identities in a manner that is + sufficiently secure. While it is not a problem with the protocol + itself, following the recommendations in Section 5.2 can mitigate + this concern. + +7.3. Pervasive Monitoring + + As required by [RFC7258], work on IETF protocols needs to consider + the effects of pervasive monitoring and mitigate them when possible. + + As described in Section 7.2, after the publication of RFC 5448, new + information has come to light regarding the use of pervasive + monitoring techniques against many security technologies, including + AKA-based authentication. + + For AKA, these attacks relate to theft of the long-term, shared- + secret key material stored on the cards. Such attacks are + conceivable, for instance, during the manufacturing process of cards, + through coercion of the card manufacturers, or during the transfer of + cards and associated information to an operator. Since the + publication of reports about such attacks, manufacturing and + provisioning processes have gained much scrutiny and have improved. + + In particular, it is crucial that manufacturers limit access to the + secret information and the cards only to necessary systems and + personnel. It is also crucial that secure mechanisms be used to + store and communicate the secrets between the manufacturer and the + operator that adopts those cards for their customers. + + Beyond these operational considerations, there are also technical + means to improve resistance to these attacks. One approach is to + provide Perfect Forward Secrecy (PFS). This would prevent any + passive attacks merely based on the long-term secrets and observation + of traffic. Such a mechanism can be defined as a backwards- + compatible extension of EAP-AKA' and is pursued separately from this + specification [EMU-AKA-PFS]. Alternatively, EAP-AKA' authentication + can be run inside a PFS-capable, tunneled authentication method. In + any case, the use of some PFS-capable mechanism is recommended. + +7.4. Security Properties of Binding Network Names + + The ability of EAP-AKA' to bind the network name into the used keys + provides some additional protection against key leakage to + inappropriate parties. The keys used in the protocol are specific to + a particular network name. If key leakage occurs due to an accident, + access node compromise, or another attack, the leaked keys are only + useful when providing access with that name. For instance, a + malicious access point cannot claim to be network Y if it has stolen + keys from network X. Obviously, if an access point is compromised, + the malicious node can still represent the compromised node. As a + result, neither EAP-AKA' nor any other extension can prevent such + attacks; however, the binding to a particular name limits the + attacker's choices, allows better tracking of attacks, makes it + possible to identify compromised networks, and applies good + cryptographic hygiene. + + The server receives the EAP transaction from a given access network, + and verifies that the claim from the access network corresponds to + the name that this access network should be using. It becomes + impossible for an access network to claim over AAA that it is another + access network. In addition, if the peer checks that the information + it has received locally over the network-access link-layer matches + with the information the server has given it via EAP-AKA', it becomes + impossible for the access network to tell one story to the AAA + network and another one to the peer. These checks prevent some + "lying NAS" (Network Access Server) attacks. For instance, a roaming + partner, R, might claim that it is the home network H in an effort to + lure peers to connect to itself. Such an attack would be beneficial + for the roaming partner if it can attract more users, and damaging + for the users if their access costs in R are higher than those in + other alternative networks, such as H. + + Any attacker who gets hold of the keys CK and IK, produced by the AKA + algorithm, can compute the keys CK' and IK' and, hence, the Master + Key (MK) according to the rules in Section 3.3. The attacker could + then act as a lying NAS. In 3GPP systems in general, the keys CK and + IK have been distributed to, for instance, nodes in a visited access + network where they may be vulnerable. In order to reduce this risk, + the AKA algorithm MUST be computed with the AMF separation bit set to + 1, and the peer MUST check that this is indeed the case whenever it + runs EAP-AKA'. Furthermore, [TS-3GPP.33.402] requires that no CK or + IK keys computed in this way ever leave the home subscriber system. + + The additional security benefits obtained from the binding depend + obviously on the way names are assigned to different access networks. + This is specified in [TS-3GPP.24.302]. See also [TS-3GPP.23.003]. + Ideally, the names allow separating each different access technology, + each different access network, and each different NAS within a + domain. If this is not possible, the full benefits may not be + achieved. For instance, if the names identify just an access + technology, use of compromised keys in a different technology can be + prevented, but it is not possible to prevent their use by other + domains or devices using the same technology. + +8. IANA Considerations + + IANA has updated the "Extensible Authentication Protocol (EAP) + Registry" and the "EAP-AKA and EAP-SIM Parameters" registry so that + entries that pointed to RFC 5448 now point to this RFC instead. + +8.1. Type Value + + IANA has updated the reference for EAP-AKA' (0x32) in the "Method + Types" subregistry under the "Extensible Authentication Protocol + (EAP) Registry" to point to this document. Per Section 6.2 of + [RFC3748], this allocation can be made with Specification Required + [RFC8126]. + +8.2. Attribute Type Values + + EAP-AKA' shares its attribute space and subtypes with EAP-SIM + [RFC4186] and EAP-AKA [RFC4187]. No new registries are needed. + + IANA has updated the reference for AT_KDF_INPUT (23) and AT_KDF (24) + in the "Attribute Types (Non-Skippable Attributes 0-127)" subregistry + under the "EAP-AKA and EAP-SIM Parameters" registry to point to this + document. AT_KDF_INPUT and AT_KDF are defined in Sections 3.1 and + 3.2, respectively, of this document. + + IANA has also updated the reference for AT_BIDDING (136) in the + "Attribute Types (Skippable Attributes 128-255)" subregistry of the + "EAP-AKA and EAP-SIM Parameters" registry to point to this document. + AT_BIDDING is defined in Section 4. + +8.3. Key Derivation Function Namespace + + IANA has updated the reference for the "EAP-AKA' AT_KDF Key + Derivation Function Values" subregistry to point to this document. + This subregistry appears under the "EAP-AKA and EAP-SIM Parameters" + registry. The references for following entries have also been + updated to point to this document. New values can be created through + the Specification Required policy [RFC8126]. + + +=======+=======================+===========+ + | Value | Description | Reference | + +=======+=======================+===========+ + | 0 | Reserved | RFC 9048 | + +-------+-----------------------+-----------+ + | 1 | EAP-AKA' with CK'/IK' | RFC 9048 | + +-------+-----------------------+-----------+ + + Table 3: EAP-AKA' AT_KDF Key Derivation + Function Values + +9. References + +9.1. Normative References + + [FIPS.180-4] + National Institute of Standards and Technology, "Secure + Hash Standard", FIPS PUB 180-4, + DOI 10.6028/NIST.FIPS.180-4, August 2015, + <https://nvlpubs.nist.gov/nistpubs/FIPS/ + NIST.FIPS.180-4.pdf>. + + [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- + Hashing for Message Authentication", RFC 2104, + DOI 10.17487/RFC2104, February 1997, + <https://www.rfc-editor.org/info/rfc2104>. + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, + DOI 10.17487/RFC2119, March 1997, + <https://www.rfc-editor.org/info/rfc2119>. + + [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. + Levkowetz, Ed., "Extensible Authentication Protocol + (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004, + <https://www.rfc-editor.org/info/rfc3748>. + + [RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication + Protocol Method for 3rd Generation Authentication and Key + Agreement (EAP-AKA)", RFC 4187, DOI 10.17487/RFC4187, + January 2006, <https://www.rfc-editor.org/info/rfc4187>. + + [RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542, + DOI 10.17487/RFC7542, May 2015, + <https://www.rfc-editor.org/info/rfc7542>. + + [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for + Writing an IANA Considerations Section in RFCs", BCP 26, + RFC 8126, DOI 10.17487/RFC8126, June 2017, + <https://www.rfc-editor.org/info/rfc8126>. + + [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC + 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, + May 2017, <https://www.rfc-editor.org/info/rfc8174>. + + [TS-3GPP.23.003] + 3GPP, "3rd Generation Partnership Project; Technical + Specification Group Core Network and Terminals; Numbering, + addressing and identification (Release 16)", Version + 16.7.0, 3GPP Technical Specification 23.003, June 2021. + + [TS-3GPP.23.501] + 3GPP, "3rd Generation Partnership Project; Technical + Specification Group Services and System Aspects; System + architecture for the 5G System (5GS); (Release 16)", + Version 16.9.0, 3GPP Technical Specification 23.501, June + 2021. + + [TS-3GPP.24.302] + 3GPP, "3rd Generation Partnership Project; Technical + Specification Group Core Network and Terminals; Access to + the 3GPP Evolved Packet Core (EPC) via non-3GPP access + networks; Stage 3; (Release 16)", Version 16.4.0, 3GPP + Technical Specification 24.302, July 2020. + + [TS-3GPP.24.501] + 3GPP, "3rd Generation Partnership Project; Technical + Specification Group Core Network and Terminals; Non- + Access-Stratum (NAS) protocol for 5G System (5GS); Stage + 3; (Release 16)", Version 16.9.0, 3GPP Draft Technical + Specification 24.501, June 2021. + + [TS-3GPP.33.102] + 3GPP, "3rd Generation Partnership Project; Technical + Specification Group Services and System Aspects; 3G + Security; Security architecture (Release 16)", Version + 16.0.0, 3GPP Technical Specification 33.102, July 2020. + + [TS-3GPP.33.402] + 3GPP, "3GPP System Architecture Evolution (SAE); Security + aspects of non-3GPP accesses (Release 16)", Version + 16.0.0, 3GPP Technical Specification 33.402, July 2020. + + [TS-3GPP.33.501] + 3GPP, "3rd Generation Partnership Project; Technical + Specification Group Services and System Aspects; 3G + Security; Security architecture and procedures for 5G + System (Release 16)", Version 16.7.1, 3GPP Technical + Specification 33.501, July 2021. + +9.2. Informative References + + [Arapinis2012] + Arapinis, M., Mancini, L., Ritter, E., Ryan, M., Golde, + N., Redon, R., and R. Borgaonkar, "New Privacy Issues in + Mobile Telephony: Fix and Verification", in CCS '12: + Proceedings of the 2012 ACM Conference on Computer and + Communications Security, Raleigh, North Carolina, USA, + DOI 10.1145/2382196.2382221, October 2012, + <https://doi.org/10.1145/2382196.2382221>. + + [Basin2018] + Basin, D., Dreier, J., Hirschi, L., Radomirović, S., + Sasse, R., and V. Stettler, "A Formal Analysis of 5G + Authentication", arXiv:1806.10360, + DOI 10.1145/3243734.3243846, August 2018, + <https://doi.org/10.1145/3243734.3243846>. + + [Borgaonkar2018] + Borgaonkar, R., Hirschi, L., Park, S., and A. Shaik, "New + Privacy Threat on 3G, 4G, and Upcoming 5G AKA Protocols", + in IACR Cryptology ePrint Archive, 2018. + + [BT2013] Beekman, J. G. and C. Thompson, "Breaking Cell Phone + Authentication: Vulnerabilities in AKA, IMS and Android", + in 7th USENIX Workshop on Offensive Technologies, WOOT + '13, August 2013. + + [EMU-AKA-PFS] + Arkko, J., Norrman, K., and V. Torvinen, "Perfect-Forward + Secrecy for the Extensible Authentication Protocol Method + for Authentication and Key Agreement (EAP-AKA' PFS)", Work + in Progress, Internet-Draft, draft-ietf-emu-aka-pfs-05, 30 + October 2020, <https://datatracker.ietf.org/doc/html/ + draft-ietf-emu-aka-pfs-05>. + + [FIPS.180-1] + National Institute of Standards and Technology, "Secure + Hash Standard", FIPS PUB 180-1, + DOI 10.6028/NIST.FIPS.180-1, April 1995, + <https://csrc.nist.gov/publications/detail/fips/180/1/ + archive/1995-04-17>. + + [FIPS.180-2] + National Institute of Standards and Technology, "Secure + Hash Standard", FIPS PUB 180-2, August 2002, + <https://csrc.nist.gov/publications/detail/fips/180/2/ + archive/2002-08-01>. + + [Heist2015] + Scahill, J. and J. Begley, "How Spies Stole the Keys to + the Encryption Castle", February 2015, + <https://firstlook.org/theintercept/2015/02/19/great-sim- + heist/>. + + [Hussain2019] + Hussain, S., Echeverria, M., Chowdhury, O., Li, N., and E. + Bertino, "Privacy Attacks to the 4G and 5G Cellular Paging + Protocols Using Side Channel Information", in the + proceedings of NDSS '19, held 24-27 February, 2019, San + Diego, California, 2019. + + [Kune2012] Kune, D., Koelndorfer, J., Hopper, N., and Y. Kim, + "Location Leaks on the GSM Air Interface", in the + proceedings of NDSS '12, held 5-8 February, 2012, San + Diego, California, 2012. + + [MT2012] Mjølsnes, S. F. and J-K. Tsay, "A Vulnerability in the + UMTS and LTE Authentication and Key Agreement Protocols", + in Computer Network Security, Proceedings of the 6th + International Conference on Mathematical Methods, Models + and Architectures for Computer Network Security, Lecture + Notes in Computer Science, Vol. 7531, pp. 65-76, + DOI 10.1007/978-3-642-33704-8_6, October 2012, + <https://doi.org/10.1007/978-3-642-33704-8_6>. + + [RFC3310] Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer + Protocol (HTTP) Digest Authentication Using Authentication + and Key Agreement (AKA)", RFC 3310, DOI 10.17487/RFC3310, + September 2002, <https://www.rfc-editor.org/info/rfc3310>. + + [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, + "Randomness Requirements for Security", BCP 106, RFC 4086, + DOI 10.17487/RFC4086, June 2005, + <https://www.rfc-editor.org/info/rfc4086>. + + [RFC4169] Torvinen, V., Arkko, J., and M. Naslund, "Hypertext + Transfer Protocol (HTTP) Digest Authentication Using + Authentication and Key Agreement (AKA) Version-2", + RFC 4169, DOI 10.17487/RFC4169, November 2005, + <https://www.rfc-editor.org/info/rfc4169>. + + [RFC4186] Haverinen, H., Ed. and J. Salowey, Ed., "Extensible + Authentication Protocol Method for Global System for + Mobile Communications (GSM) Subscriber Identity Modules + (EAP-SIM)", RFC 4186, DOI 10.17487/RFC4186, January 2006, + <https://www.rfc-editor.org/info/rfc4186>. + + [RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity + Selection Hints for the Extensible Authentication Protocol + (EAP)", RFC 4284, DOI 10.17487/RFC4284, January 2006, + <https://www.rfc-editor.org/info/rfc4284>. + + [RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) + Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005, + <https://www.rfc-editor.org/info/rfc4306>. + + [RFC5113] Arkko, J., Aboba, B., Korhonen, J., Ed., and F. Bari, + "Network Discovery and Selection Problem", RFC 5113, + DOI 10.17487/RFC5113, January 2008, + <https://www.rfc-editor.org/info/rfc5113>. + + [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible + Authentication Protocol (EAP) Key Management Framework", + RFC 5247, DOI 10.17487/RFC5247, August 2008, + <https://www.rfc-editor.org/info/rfc5247>. + + [RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication + Protocol Tunneled Transport Layer Security Authenticated + Protocol Version 0 (EAP-TTLSv0)", RFC 5281, + DOI 10.17487/RFC5281, August 2008, + <https://www.rfc-editor.org/info/rfc5281>. + + [RFC5448] Arkko, J., Lehtovirta, V., and P. Eronen, "Improved + Extensible Authentication Protocol Method for 3rd + Generation Authentication and Key Agreement (EAP-AKA')", + RFC 5448, DOI 10.17487/RFC5448, May 2009, + <https://www.rfc-editor.org/info/rfc5448>. + + [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security + Considerations for the SHA-0 and SHA-1 Message-Digest + Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, + <https://www.rfc-editor.org/info/rfc6194>. + + [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., + Morris, J., Hansen, M., and R. Smith, "Privacy + Considerations for Internet Protocols", RFC 6973, + DOI 10.17487/RFC6973, July 2013, + <https://www.rfc-editor.org/info/rfc6973>. + + [RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna, + "Tunnel Extensible Authentication Protocol (TEAP) Version + 1", RFC 7170, DOI 10.17487/RFC7170, May 2014, + <https://www.rfc-editor.org/info/rfc7170>. + + [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an + Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May + 2014, <https://www.rfc-editor.org/info/rfc7258>. + + [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. + Kivinen, "Internet Key Exchange Protocol Version 2 + (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October + 2014, <https://www.rfc-editor.org/info/rfc7296>. + + [Shaik2016] + Shaik, A., Seifert, J., Borgaonkar, R., Asokan, N., and V. + Niemi, "Practical attacks against Privacy and Availability + in 4G/LTE Mobile Communication Systems", in the + proceedings of NDSS '16 held 21-24 February, 2016, San + Diego, California, 2012. + + [TS-3GPP.35.208] + 3GPP, "3rd Generation Partnership Project; Technical + Specification Group Services and System Aspects; 3G + Security; Specification of the MILENAGE Algorithm Set: An + example algorithm set for the 3GPP authentication and key + generation functions f1, f1*, f2, f3, f4, f5 and f5*; + Document 4: Design Conformance Test Data (Release 14)", + Version 16.0.0, 3GPP Technical Specification 35.208, July + 2020. + + [ZF2005] Zhang, M. and Y. Fang, "Security analysis and enhancements + of 3GPP authentication and key agreement protocol", IEEE + Transactions on Wireless Communications, Vol. 4, No. 2, + DOI 10.1109/TWC.2004.842941, March 2005, + <https://doi.org/10.1109/TWC.2004.842941>. + +Appendix A. Changes from RFC 5448 + + The change from RFC 5448 was to refer to a newer version of + [TS-3GPP.24.302]. This RFC includes an updated definition of the + Network Name field to include 5G. + + Identifier usage for 5G has been specified in Section 5.3. Also, the + requirements for generating pseudonym usernames and fast re- + authentication identities have been updated from the original + definition in RFC 5448, which referenced RFC 4187. See Section 5. + + Exported parameters for EAP-AKA' have been defined in Section 6, as + required by [RFC5247], including the definition of those parameters + for both full authentication and fast re-authentication. + + The security, privacy, and pervasive monitoring considerations have + been updated or added. See Section 7. + + The references to [RFC2119], [RFC4306], [RFC7296], [FIPS.180-1] and + [FIPS.180-2] have been updated to their most recent versions, and + language in this document has been changed accordingly. However, + these are merely reference updates to newer specifications; the + actual protocol functions are the same as defined in the earlier + RFCs. + + Similarly, references to all 3GPP technical specifications have been + updated to their 5G versions (Release 16) or otherwise most recent + version when there has not been a 5G-related update. + + Finally, a number of clarifications have been made, including a + summary of where attributes may appear. + +Appendix B. Changes to RFC 4187 + + In addition to specifying EAP-AKA', this document also mandates a + change to another EAP method -- EAP-AKA that was defined in RFC 4187. + This change was already mandated in RFC 5448 but repeated here to + ensure that the latest EAP-AKA' specification contains the + instructions about the necessary bidding down prevention feature in + EAP-AKA as well. + + The changes to RFC 4187 relate only to the bidding down prevention + support defined in Section 4. In particular, this document does not + change how the Master Key (MK) is calculated or any other aspect of + EAP-AKA. The provisions in this specification for EAP-AKA' do not + apply to EAP-AKA, outside of Section 4. + +Appendix C. Importance of Explicit Negotiation + + Choosing between the traditional and revised AKA key derivation + functions is easy when their use is unambiguously tied to a + particular radio access network, e.g., Long Term Evolution (LTE) as + defined by 3GPP or evolved High Rate Packet Data (eHRPD) as defined + by 3GPP2. There is no possibility for interoperability problems if + this radio access network is always used in conjunction with new + protocols that cannot be mixed with the old ones; clients will always + know whether they are connecting to the old or new system. + + However, using the new key derivation functions over EAP introduces + several degrees of separation, making the choice of the correct key + derivation functions much harder. Many different types of networks + employ EAP. Most of these networks have no means to carry any + information about what is expected from the authentication process. + EAP itself is severely limited in carrying any additional + information, as noted in [RFC4284] and [RFC5113]. Even if these + networks or EAP were extended to carry additional information, it + would not affect millions of deployed access networks and clients + attaching to them. + + Simply changing the key derivation functions that EAP-AKA [RFC4187] + uses would cause interoperability problems with all of the existing + implementations. Perhaps it would be possible to employ strict + separation into domain names that should be used by the new clients + and networks. Only these new devices would then employ the new key + derivation function. While this can be made to work for specific + cases, it would be an extremely brittle mechanism, ripe to result in + problems whenever client configuration, routing of authentication + requests, or server configuration does not match expectations. It + also does not help to assume that the EAP client and server are + running a particular release of 3GPP network specifications. Network + vendors often provide features from future releases early or do not + provide all features of the current release. And obviously, there + are many EAP and even some EAP-AKA implementations that are not + bundled with the 3GPP network offerings. In general, these + approaches are expected to lead to hard-to-diagnose problems and + increased support calls. + +Appendix D. Test Vectors + + Test vectors are provided below for four different cases. The test + vectors may be useful for testing implementations. In the first two + cases, we employ the MILENAGE algorithm and the algorithm + configuration parameters (the subscriber key K and operator algorithm + variant configuration value OP) from test set 19 in [TS-3GPP.35.208]. + + The last two cases use artificial values as the output of AKA, which + are useful only for testing the computation of values within EAP- + AKA', not AKA itself. + + Case 1 + + The parameters for the AKA run are as follows: + + Identity: "0555444333222111" + + Network name: "WLAN" + + RAND: 81e9 2b6c 0ee0 e12e bceb a8d9 2a99 dfa5 + + AUTN: bb52 e91c 747a c3ab 2a5c 23d1 5ee3 51d5 + + IK: 9744 871a d32b f9bb d1dd 5ce5 4e3e 2e5a + + CK: 5349 fbe0 9864 9f94 8f5d 2e97 3a81 c00f + + RES: 28d7 b0f2 a2ec 3de5 + + Then the derived keys are generated as follows: + + CK': 0093 962d 0dd8 4aa5 684b 045c 9edf fa04 + + IK': ccfc 230c a74f cc96 c0a5 d611 64f5 a76c + + K_encr: 766f a0a6 c317 174b 812d 52fb cd11 a179 + + K_aut: 0842 ea72 2ff6 835b fa20 3249 9fc3 ec23 + c2f0 e388 b4f0 7543 ffc6 77f1 696d 71ea + + K_re: cf83 aa8b c7e0 aced 892a cc98 e76a 9b20 + 95b5 58c7 795c 7094 715c b339 3aa7 d17a + + MSK: 67c4 2d9a a56c 1b79 e295 e345 9fc3 d187 + d42b e0bf 818d 3070 e362 c5e9 67a4 d544 + e8ec fe19 358a b303 9aff 03b7 c930 588c + 055b abee 58a0 2650 b067 ec4e 9347 c75a + + EMSK: f861 703c d775 590e 16c7 679e a387 4ada + 8663 11de 2907 64d7 60cf 76df 647e a01c + 313f 6992 4bdd 7650 ca9b ac14 1ea0 75c4 + ef9e 8029 c0e2 90cd bad5 638b 63bc 23fb + + Case 2 + + The parameters for the AKA run are as follows: + + Identity: "0555444333222111" + + Network name: "HRPD" + + RAND: 81e9 2b6c 0ee0 e12e bceb a8d9 2a99 dfa5 + + AUTN: bb52 e91c 747a c3ab 2a5c 23d1 5ee3 51d5 + + IK: 9744 871a d32b f9bb d1dd 5ce5 4e3e 2e5a + + CK: 5349 fbe0 9864 9f94 8f5d 2e97 3a81 c00f + + RES: 28d7 b0f2 a2ec 3de5 + + Then the derived keys are generated as follows: + + CK': 3820 f027 7fa5 f777 32b1 fb1d 90c1 a0da + + IK': db94 a0ab 557e f6c9 ab48 619c a05b 9a9f + + K_encr: 05ad 73ac 915f ce89 ac77 e152 0d82 187b + + K_aut: 5b4a caef 62c6 ebb8 882b 2f3d 534c 4b35 + 2773 37a0 0184 f20f f25d 224c 04be 2afd + + K_re: 3f90 bf5c 6e5e f325 ff04 eb5e f653 9fa8 + cca8 3981 94fb d00b e425 b3f4 0dba 10ac + + MSK: 87b3 2157 0117 cd6c 95ab 6c43 6fb5 073f + f15c f855 05d2 bc5b b735 5fc2 1ea8 a757 + 57e8 f86a 2b13 8002 e057 5291 3bb4 3b82 + f868 a961 17e9 1a2d 95f5 2667 7d57 2900 + + EMSK: c891 d5f2 0f14 8a10 0755 3e2d ea55 5c9c + b672 e967 5f4a 66b4 bafa 0273 79f9 3aee + 539a 5979 d0a0 042b 9d2a e28b ed3b 17a3 + 1dc8 ab75 072b 80bd 0c1d a612 466e 402c + + Case 3 + + The parameters for the AKA run are as follows: + + Identity: "0555444333222111" + + Network name: "WLAN" + + RAND: e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 + + AUTN: a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 + + IK: b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 + + CK: c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 + + RES: d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 + + Then the derived keys are generated as follows: + + CK': cd4c 8e5c 68f5 7dd1 d7d7 dfd0 c538 e577 + + IK': 3ece 6b70 5dbb f7df c459 a112 80c6 5524 + + K_encr: 897d 302f a284 7416 488c 28e2 0dcb 7be4 + + K_aut: c407 00e7 7224 83ae 3dc7 139e b0b8 8bb5 + 58cb 3081 eccd 057f 9207 d128 6ee7 dd53 + + K_re: 0a59 1a22 dd8b 5b1c f29e 3d50 8c91 dbbd + b4ae e230 5189 2c42 b6a2 de66 ea50 4473 + + MSK: 9f7d ca9e 37bb 2202 9ed9 86e7 cd09 d4a7 + 0d1a c76d 9553 5c5c ac40 a750 4699 bb89 + 61a2 9ef6 f3e9 0f18 3de5 861a d1be dc81 + ce99 1639 1b40 1aa0 06c9 8785 a575 6df7 + + EMSK: 724d e00b db9e 5681 87be 3fe7 4611 4557 + d501 8779 537e e37f 4d3c 6c73 8cb9 7b9d + c651 bc19 bfad c344 ffe2 b52c a78b d831 + 6b51 dacc 5f2b 1440 cb95 1552 1cc7 ba23 + + Case 4 + + The parameters for the AKA run are as follows: + + Identity: "0555444333222111" + + Network name: "HRPD" + + RAND: e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 + + AUTN: a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 + + IK: b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 + + CK: c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 + + RES: d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 + + Then the derived keys are generated as follows: + + CK': 8310 a71c e6f7 5488 9613 da8f 64d5 fb46 + + IK': 5adf 1436 0ae8 3819 2db2 3f6f cb7f 8c76 + + K_encr: 745e 7439 ba23 8f50 fcac 4d15 d47c d1d9 + + K_aut: 3e1d 2aa4 e677 025c fd86 2a4b e183 61a1 + 3a64 5765 5714 63df 833a 9759 e809 9879 + + K_re: 99da 835e 2ae8 2462 576f e651 6fad 1f80 + 2f0f a119 1655 dd0a 273d a96d 04e0 fcd3 + + MSK: c6d3 a6e0 ceea 951e b20d 74f3 2c30 61d0 + 680a 04b0 b086 ee87 00ac e3e0 b95f a026 + 83c2 87be ee44 4322 94ff 98af 26d2 cc78 + 3bac e75c 4b0a f7fd feb5 511b a8e4 cbd0 + + EMSK: 7fb5 6813 838a dafa 99d1 40c2 f198 f6da + cebf b6af ee44 4961 1054 02b5 08c7 f363 + 352c b291 9644 b504 63e6 a693 5415 0147 + ae09 cbc5 4b8a 651d 8787 a689 3ed8 536d + +Acknowledgments + + The authors would like to thank Guenther Horn, Joe Salowey, Mats + Naslund, Adrian Escott, Brian Rosenberg, Laksminath Dondeti, Ahmad + Muhanna, Stefan Rommer, Miguel Garcia, Jan Kall, Ankur Agarwal, Jouni + Malinen, John Mattsson, Jesus De Gregorio, Brian Weis, Russ Housley, + Alfred Hoenes, Anand Palanigounder, Michael Richardson, Roman + Danyliw, Dan Romascanu, Kyle Rose, Benjamin Kaduk, Alissa Cooper, + Erik Kline, Murray Kucherawy, Robert Wilton, Warren Kumari, Andreas + Kunz, Marcus Wong, Kalle Jarvinen, Daniel Migault, and Mohit Sethi + for their in-depth reviews and interesting discussions in this + problem space. + +Contributors + + The test vectors in Appendix D were provided by Yogendra Pal and + Jouni Malinen, based on two independent implementations of this + specification. + + Jouni Malinen provided suggested text for Section 6. John Mattsson + provided much of the text for Section 7.1. Karl Norrman was the + source of much of the information in Section 7.2. + +Authors' Addresses + + Jari Arkko + Ericsson + FI-02420 Jorvas + Finland + + Email: jari.arkko@piuha.net + + + Vesa Lehtovirta + Ericsson + FI-02420 Jorvas + Finland + + Email: vesa.lehtovirta@ericsson.com + + + Vesa Torvinen + Ericsson + FI-02420 Jorvas + Finland + + Email: vesa.torvinen@ericsson.com + + + Pasi Eronen + Independent + Finland + + Email: pe@iki.fi |