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RFC 9678: Forward Secrecy Extension to the Improved Extensible Authentication Protocol Method for Authentication and Key Agreement (EAP-AKA' FS)

  • J. Arkko,  
  • K. Norrman,  
  • J. Preuß Mattsson
Proposed Standard

Abstract

This document updates RFC 9048, "Improved Extensible Authentication Protocol Method for 3GPP Mobile Network Authentication and Key Agreement (EAP-AKA')", and its predecessor RFC 5448 with an optional extension providing ephemeral key exchange. The extension EAP-AKA' Forward Secrecy (EAP-AKA' FS), when negotiated, provides forward secrecy for the session keys generated as a part of the authentication run in EAP-AKA'. This prevents an attacker who has gained access to the long-term key from obtaining session keys established in the past. In addition, EAP-AKA' FS mitigates passive attacks (e.g., large-scale pervasive monitoring) against future sessions. This forces attackers to use active attacks instead.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on Internet Standards is available in 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 .

1. Introduction

Many different attacks have been reported as part of the revelations associated with pervasive surveillance. Some of the reported attacks involved compromising the Universal Subscriber Identity Module (USIM) card supply chain. Attacks revealing the AKA long-term key may occur, for instance:

  • during the manufacturing process of USIM cards,
  • during the transfer of the cards and associated information to the operator, and
  • when the system is running.

Since the publication of reports about such attacks (see [Heist2015]), manufacturing and provisioning processes have gained much scrutiny and have improved.

However, the danger of resourceful attackers attempting to gain information about long-term keys is still a concern because these keys are high-value targets. Note that the attacks are largely independent of the used authentication technology; the issue is not vulnerabilities in algorithms or protocols, but rather the possibility of someone gaining unauthorized access to key material. Furthermore, an explicit goal of the IETF is to ensure that we understand the surveillance concerns related to IETF protocols and take appropriate countermeasures [].

While strong protection of manufacturing and other processes is essential in mitigating surveillance and other risks associated with AKA long-term keys, there are also protocol mechanisms that can help.

This document updates [], "Improved Extensible Authentication Protocol Method for 3GPP Mobile Network Authentication and Key Agreement (EAP-AKA')", with an optional extension providing ephemeral key exchange, which minimizes the impact of long-term key compromise and strengthens the identity privacy requirements. This is important, given the large number of users of AKA in mobile networks.

The extension, when negotiated, provides Forward Secrecy (FS) [DOW1992] for the session key generated as a part of the authentication run in EAP-AKA'. This prevents an attacker who has gained access to the long-term key in a USIM card from getting access to past session keys. In addition to FS, the included Diffie-Hellman exchange forces attackers to be active if they want access to future session keys, even if they have access to the long-term key. This is beneficial because active attacks demand many more resources to launch and are easier to detect. As with other protocols, an active attacker with access to the long-term key material will, of course, be able to attack all future communications, but risks detection, particularly if done at scale.

It should also be noted that 5G network architecture [TS.33.501] includes the use of the EAP framework for authentication. While any methods can be run, the default authentication method within that context will be EAP-AKA'. As a result, improvements in EAP-AKA' security have the potential to improve security for many users.

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 [] [] when, and only when, they appear in all capitals, as shown here.

3. Protocol Design and Deployment Objectives

The extension specified here reuses large portions of the current structure of 3GPP interfaces and functions, with the rationale that this will make the construction more easily adopted. In particular, the construction keeps the interface between the USIM and the mobile terminal intact. As a consequence, there is no need to roll out new credentials to existing subscribers. The work is based on an earlier paper (see [TrustCom2015]) and uses much of the same material but is applied to EAP rather than the underlying AKA method.

It has been a goal to implement this change as an extension of the widely supported EAP-AKA' method, rather than implement a completely new authentication method. The extension is implemented as a set of new, optional attributes that are provided alongside the base attributes in EAP-AKA'. Old implementations can ignore these attributes, but their presence will nevertheless be verified as part of the base EAP-AKA' integrity verification process, helping protect against bidding down attacks. This extension does not increase the number of rounds necessary to complete the protocol.

The use of this extension is at the discretion of the authenticating parties. It should be noted that FS and defenses against passive attacks do not solve all problems, but they can provide a partial defense that increases the cost and risk associated with pervasive surveillance.

While adding FS to the existing mobile network infrastructure can be done in multiple different ways, this document specifies a solution that is relatively easy to deploy. In particular:

  • As noted above, no new credentials are needed; there is no change to USIM cards.
  • FS property can be incorporated into any current or future system that supports EAP, without changing any network functions beyond the EAP endpoints.
  • Key generation happens at the endpoints, enabling the highest grade key material to be used both by the endpoints and the intermediate systems (such as access points that are given access to specific keys).
  • While EAP-AKA' is just one EAP method, for practical purposes, FS being available for both EAP-TLS [] [] and EAP-AKA' ensures that, for many practical systems, FS can be enabled for either all or a significant fraction of users.

4. Background

The reader is assumed to have a basic understanding of the EAP framework [].

4.1. AKA

We use the term "Authentication and Key Agreement" (or "AKA") for the main authentication and key agreement protocol used by 3GPP mobile networks from the third generation (3G) and onward. Later generations add new features to AKA, but the core remains the same. It is based on challenge-response mechanisms and symmetric cryptography. In contrast to its earlier GSM counterparts, AKA provides long key lengths and mutual authentication. The phone typically executes AKA in a USIM. A USIM is technically just an application that can reside on a removable Universal Integrated Circuit Card (UICC), an embedded UICC, or integrated in a Trusted Execution Environment (TEE). In this document, we use the term "USIM card" to refer to any Subscriber Identity Module (SIM) capable of running AKA.

The goals of AKA are to mutually authenticate the USIM and the so-called home environment, which is the authentication Server in the subscriber's home operator's network, and to establish key material between the two.

AKA works in the following manner:

  • The USIM and the home environment have agreed on a long-term symmetric key beforehand.
  • The actual authentication process starts by having the home environment produce an authentication vector, based on the long-term key and a sequence number. The authentication vector contains a random part RAND, an authenticator part AUTN used for authenticating the network to the USIM, an expected result part XRES, a 128-bit session key for the integrity check IK, and a 128-bit session key for the encryption CK.
  • The authentication vector is passed to the serving network, which uses it to authenticate the device.
  • The RAND and the AUTN are delivered to the USIM.
  • The USIM verifies the AUTN, again based on the long-term key and the sequence number. If this process is successful (the AUTN is valid and the sequence number used to generate the AUTN is within the correct range), the USIM produces an authentication result RES and sends it to the serving network.
  • The serving network verifies that the result from the USIM matches the expected value in the authentication vector. If it does, the USIM is considered authenticated, and the IK and CK can be used to protect further communications between the USIM and the home environment.

4.2. EAP-AKA' Protocol

When AKA is embedded into EAP, the authentication processing on the network side is moved to the home environment. The 3GPP Authentication Database (AD) generates authentication vectors. The 3GPP authentication Server takes the role of EAP Server. The USIM combined with the mobile phone takes the role of client. The difference between EAP-AKA [] and EAP-AKA' [] is that EAP-AKA' binds the derived keys to the name of the access network. Figure 1 describes the basic flow in the EAP-AKA' authentication process. The definition of the full protocol behavior, along with the definition of the attributes AT_RAND, AT_AUTN, AT_MAC, and AT_RES can be found in [] and []. Note the use of EAP terminology from hereon. That is, the 3GPP serving network takes on the role of an EAP access network.

Peer Server EAP-Request/Identity EAP-Response/Identity (Includes user's Network Access Identifier (NAI)) The Server determines the network name and ensures that the given access network is authorized to use the claimed name. The Server then runs the EAP-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 to create 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 they do not match, the authentication is aborted. Otherwise, the network name from the AT_KDF_INPUT attribute is used in running the EAP-AKA' algorithms, verifying AUTN from AT_AUTN and Message Authentication Code (MAC) from the 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) The Server checks the RES and MAC values received in AT_RES and AT_MAC, respectively. Success requires both compared values match, respectively. EAP-Success
Figure 1: EAP-AKA' Authentication Process

4.3. Attacks Against Long-Term Keys in Smart Cards

The general security properties and potential vulnerabilities of AKA and EAP-AKA' are discussed in [].

An important question in that discussion relates to the potential compromise of long-term keys, as discussed earlier. Attacks on long-term keys are not specific to AKA or EAP-AKA', and all security systems fail, at least to some extent, if key material is stolen. However, it would be preferable to retain some security even in the face of such attacks. This document specifies a mechanism that reduces the risks of compromising key material belonging to previous sessions, before the long-term keys were compromised. It also forces attackers to be active even after the compromise.

5. Protocol Overview

Forward Secrecy (FS) for EAP-AKA' is achieved by using an Elliptic Curve Diffie-Hellman (ECDH) exchange []. To provide FS, the exchange must be run in an ephemeral manner, i.e., both sides generate temporary keys according to the negotiated ciphersuite. For example, for X25519, this is done as specified in []. This method is referred to as "ECDHE", where the last "E" stands for "Ephemeral". The two initially registered elliptic curves and their wire formats are chosen to align with the elliptic curves and formats specified for Subscription Concealed Identifier (SUCI) encryption in Appendix C.3.4 of 3GPP [TS.33.501].

The enhancements in the EAP-AKA' FS protocol are compatible with the signaling flow and other basic structures of both AKA and EAP-AKA'. The intent is to implement the enhancement as optional attributes that legacy implementations ignore.

The purpose of the protocol is to achieve mutual authentication between the EAP Server and Peer and to establish key material for secure communication between the two. This document specifies the calculation of key material, providing new properties that are not present in key material provided by EAP-AKA' in its original form.

Figure 2 describes the overall process. Since the goal has been to not require new infrastructure or credentials, the flow diagrams also show the conceptual interaction with the USIM card and the home environment. Recall that the home environment represents the 3GPP Authentication Database (AD) and Server. The details of those interactions are outside the scope of this document; however, and the reader is referred to the 3GPP specifications (for 5G, this is specified in 3GPP [TS.33.501]).

USIM Peer Server AD EAP-Req/Identity EAP-Resp/Identity (Privacy-Friendly) The Server now has an identity for the Peer. The Server then asks the help of the AD to run EAP-AKA algorithms, generating RAND, AUTN, XRES, CK, and IK. Typically, the AD performs the first part of derivations so that the authentication Server gets the CK' and IK' keys already tied to a particular network name. ID, key deriv. function, network name RAND, AUTN, XRES, CK', IK' The Server now has the needed authentication vector. It generates an ephemeral key pair, and sends the public key of that key pair and the first EAP method message to the Peer. In the message the AT_PUB_ECDHE attribute carries the public key and the AT_KDF_FS attribute carries other FS-related parameters. Both of these are skippable attributes that can be ignored if the Peer does not support this extension. EAP-Req/AKA'-Challenge AT_RAND, AT_AUTN, AT_KDF, AT_KDF_FS, AT_KDF_INPUT, AT_PUB_ECDHE, AT_MAC The Peer checks if it wants to do the FS extension. If yes, it will eventually respond with AT_PUB_ECDHE and AT_MAC. If not, it will ignore AT_PUB_ECDHE and AT_KDF_FS and base all calculations on basic EAP-AKA' attributes, continuing just as in EAP-AKA' per RFC 9048 rules. In any case, the Peer needs to query the auth parameters from the USIM card. RAND, AUTN CK, IK, RES The Peer now has everything to respond. If it wants to participate in the FS extension, it will then generate its key pair, calculate a shared key based on its key pair and the Server's public key. Finally, it proceeds to derive all EAP-AKA' key values and constructs a full response. EAP-Resp/AKA'-Challenge AT_RES, AT_PUB_ECDHE, AT_MAC The Server now has all the necessary values. It generates the ECDHE shared secret and checks the RES and MAC values received in AT_RES and AT_MAC, respectively. Success requires both to be found correct. Note that when this document is used, the keys generated from EAP-AKA' are based on CK, IK, and the ECDHE value. Even if there was an attacker who held the long-term key, only an active attacker could have determined the generated session keys; in basic EAP-AKA' the generated keys are only based on CK and IK. EAP-Success
Figure 2: EAP-AKA' FS Authentication Process

6. Extensions to EAP-AKA'

6.1. AT_PUB_ECDHE

The AT_PUB_ECDHE attribute carries an ECDHE value.

The format of the AT_PUB_ECDHE 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_PUB_ECDHE Length Value

The fields are as follows:

AT_PUB_ECDHE:
This is set to 152.
Length:
This is the length of the attribute, set as other attributes in EAP-AKA []. The length is expressed in multiples of 4 bytes. The length includes the attribute type field, the Length field itself, and the Value field (along with any padding).
Value:

This value is the sender's ECDHE public key. The value depends on the AT_KDF_FS attribute and is calculated as follows:

  • For X25519, the length of this value is 32 bytes, encoded as specified in Section 5 of [].

  • For P-256, the length of this value is 33 bytes, encoded using the compressed form specified in Section 2.3.3 of [SEC1].

Because the length of the attribute must be a multiple of 4 bytes, the sender pads the Value field with zero bytes when necessary. To retain the security of the keys, the sender SHALL generate a fresh value for each run of the protocol.

6.2. AT_KDF_FS

The AT_KDF_FS attribute indicates the used or desired FS key generation function, if the FS extension is used. It will also indicate the used or desired ECDHE group. A new attribute is needed to carry this information, as AT_KDF carries the basic KDF value that is still used together with the FS KDF value. The basic KDF value is also used by those EAP Peers that cannot or do not want to use this extension.

This document only specifies the behavior relating to the following combinations of basic KDF values and FS KDF values:

  • the basic KDF value in AT_KDF is 1, as specified in [] and [] and
  • the FS KDF values in AT_KDF_FS are 1 or 2, as specified below and in Section 6.3.

Any future specifications that add either new basic KDFs or new FS KDF values need to specify how they are treated and what combinations are allowed. This requirement is an update to how [] and [] may be extended in the future.

The format of the AT_KDF_FS 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_FS Length FS Key Derivation Function

The fields are as follows:

AT_KDF_FS:
This is set to 153.
Length:
This is the length of the attribute; it MUST be set to 1.
FS Key Derivation Function:
This is an enumerated value representing the FS Key Derivation Function (KDF) that the Server (or Peer) wishes to use. See Section 6.3 for the functions specified in this document. Note: this field has a different name space than the similar field in the AT_KDF attribute KDF defined in [].

Servers MUST send one or more AT_KDF_FS attributes in the EAP-Request/AKA'-Challenge message. These attributes represent the desired functions ordered by preference, with the most preferred function being the first attribute. The most preferred function is the only one that the Server includes a public key value for, however. So, for a set of AT_KDF_FS attributes, there is always only one AT_PUB_ECDHE attribute.

Upon receiving a set of these attributes:

  • If the Peer supports and is willing to use the FS KDF indicated by the first AT_KDF_FS attribute, and is willing and able to use the extension defined in this document, the function will be used without any further negotiation.
  • If the Peer does not support this function or is unwilling to use it, it responds to the Server with an indication that a different function is needed. Similarly, with the negotiation process defined in [] for AT_KDF, the Peer sends an EAP-Response/AKA'-Challenge message that contains only one attribute, AT_KDF_FS, with the value set to the desired alternative function from among the ones suggested by the Server earlier. If there is no suitable alternative, the Peer has a choice of either falling back to EAP-AKA' or behaving as if the AUTN had been incorrect and failing authentication (see Figure 3 of []). The Peer MUST fail the authentication if there are any duplicate values within the list of AT_KDF_FS attributes (except where the duplication is due to a request to change the KDF; see below for further information).
  • If the Peer does not recognize the extension defined in this document or is unwilling to use it, it ignores the AT_KDF_FS attribute.

Upon receiving an EAP-Response/AKA'-Challenge message with an AT_KDF_FS attribute from the Peer, the Server checks that the suggested AT_KDF_FS value was one of the alternatives in its offer. The first AT_KDF_FS value in the message from the Server is not a valid alternative. If the Peer has replied with the first AT_KDF_FS value, the Server behaves as if the 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 in []. 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_FS 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 FS KDF is the only valid 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_FS attributes. If so, it continues. If not, it behaves as if AT_MAC were incorrect and fails the authentication. If the Peer receives multiple EAP-Request/AKA'-Challenge messages with differing AT_KDF_FS attributes without having requested negotiation, the Peer MUST behave as if AT_MAC were incorrect and fail the authentication.

6.3. Forward Secrecy Key Derivation Functions

Two new FS KDF types are defined for "EAP-AKA' with ECDHE and X25519", represented by value 1, and "EAP-AKA' with ECDHE and P-256", represented by value 2. These values represent a particular choice of KDF and, at the same time, select an ECDHE group to be used.

The FS KDF type value is only used in the AT_KDF_FS attribute. When the FS extension is used, the AT_KDF_FS attribute determines how to derive the MK_ECDHE key, K_re key, Master Session Key (MSK), and Extended Master Session Key (EMSK). The AT_KDF_FS attribute should not be confused with the different range of KDFs that can be represented in the AT_KDF attribute as defined in []. When the FS extension is used, the AT_KDF attribute only specifies how to derive the Master Key (MK), the K_encr key, and the K_aut key.

Key derivation in this extension produces exactly the same keys for internal use within one authentication run as EAP-AKA' [] does. For instance, the K_aut that is used in AT_MAC is still exactly as it was in EAP-AKA'. The only change to key derivation is in the re-authentication keys and keys exported out of the EAP method, MSK and EMSK. As a result, EAP-AKA' attributes such as AT_MAC continue to be usable even when this extension is in use.

When the FS KDF field in the AT_KDF_FS attribute is set to 1 or 2 and the KDF field in the AT_KDF attribute is set to 1, the MK and accompanying keys are derived as follows:

MK       = PRF'(IK'|CK',"EAP-AKA'"|Identity)
MK_ECDHE = PRF'(IK'|CK'|SHARED_SECRET,"EAP-AKA' FS"|Identity)
K_encr   = MK[0..127]
K_aut    = MK[128..383]
K_re     = MK_ECDHE[0..255]
MSK      = MK_ECDHE[256..767]
EMSK     = MK_ECDHE[768..1279]

An explanation of the notation used above is copied here:

  • [n..m] denotes the substring from bit n to m.
  • PRF' is a new pseudorandom function specified in [].
  • K_encr is the encryption key (128 bits).
  • K_aut is the authentication key (256 bits).
  • K_re is the re-authentication key (256 bits).
  • MSK is the Master Session Key (512 bits).
  • EMSK is the Extended Master Session Key (512 bits).

Note: MSK and EMSK are outputs from a successful EAP method run [].

The CK and IK are produced by the AKA algorithm. The IK' and CK' are derived as specified in [] from the IK and CK.

The value "EAP-AKA'" is an ASCII string that is 8 characters long. It is used as is, without any trailing NUL characters. Similarly, "EAP-AKA' FS" is an ASCII string that is 11 characters long, also used as is.

Requirements for how to securely generate, validate, and process the ephemeral public keys depend on the elliptic curve.

For P-256, the SHARED_SECRET is the shared secret computed as specified in Section 5.7.1.2 of [SP-800-56A]. Requirements are defined in Section 5 of [SP-800-56A], in particular, Sections 5.6.2.3.4, 5.6.3.1, and 5.6.3.3. At least partial public key validation MUST be done for the ephemeral public keys. The uncompressed y-coordinate can be computed as described in Section 2.3.4 of [SEC1].

For X25519, the SHARED_SECRET is the shared secret computed as specified in Section 6.1 of []. Both the Peer and the Server MAY check for the zero-value shared secret as specified in Section 6.1 of [].

If validation of the other party's ephemeral public key or the shared secret fails, a party MUST behave as if the current EAP-AKA' process starts again from the beginning.

The rest of the computation proceeds as defined in Section 3.3 of [].

6.4. ECDHE Groups

The selection of suitable groups for the elliptic curve computation is necessary. The choice of a group is made at the same time as the decision to use a particular KDF in the AT_KDF_FS attribute.

For "EAP-AKA' with ECDHE and X25519", the group is the Curve25519 group specified in []. The support for this group is REQUIRED.

For "EAP-AKA' with ECDHE and P-256", the group is the NIST P-256 group (SEC group secp256r1), specified in Section 3.2.1.3 of [SP-800-186] or alternatively, Section 2.4.2 of [SEC2]. The support for this group is REQUIRED.

The term "support" here means that the group MUST be implemented.

6.5. Message Processing

This section specifies the changes related to message processing when this extension is used in EAP-AKA'. It specifies when a message may be transmitted or accepted, which attributes are allowed in a message, which attributes are required in a message, and other message-specific details, where those details are different for this extension than the base EAP-AKA' or EAP-AKA protocol. Unless otherwise specified here, the rules from [] or [] apply.

6.5.1. EAP-Request/AKA'-Identity

There are no changes for the EAP-Request/AKA'-Identity, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST NOT be added to this message. The appearance of these attributes in a received message MUST be ignored.

6.5.2. EAP-Response/AKA'-Identity

There are no changes for the EAP-Response/AKA'-Identity, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST NOT be added to this message. The appearance of these attributes in a received message MUST be ignored. The Peer identifier SHALL comply with the privacy-friendly requirements of []. An example of a compliant way of constructing a privacy-friendly Peer identifier is using a non-null SUCI [TS.33.501].

6.5.3. EAP-Request/AKA'-Challenge

The Server sends the EAP-Request/AKA'-Challenge on full authentication as specified by [] and []. The attributes AT_RAND, AT_AUTN, and AT_MAC MUST be included and checked on reception as specified in []. They are also necessary for backwards compatibility.

In the EAP-Request/AKA'-Challenge, there is no message-specific data covered by the MAC for the AT_MAC attribute. The AT_KDF_FS and AT_PUB_ECDHE attributes MUST be included. The AT_PUB_ECDHE attribute carries the Server's public Diffie-Hellman key. If either AT_KDF_FS or AT_PUB_ECDHE is missing on reception, the Peer MUST treat it as if neither one was sent and assume that the extension defined in this document is not in use.

The AT_RESULT_IND, AT_CHECKCODE, AT_IV, AT_ENCR_DATA, AT_PADDING, AT_NEXT_PSEUDONYM, AT_NEXT_REAUTH_ID, and other attributes may be included as specified in Section 9.3 of [].

When processing this message, the Peer MUST process AT_RAND, AT_AUTN, AT_KDF_FS, and AT_PUB_ECDHE before processing other attributes. The Peer derives keys and verifies AT_MAC only if these attributes are verified to be valid. If the Peer is unable or unwilling to perform the extension specified in this document, it proceeds as defined in []. Finally, if there is an error, see Section 6.3.1 of [].

6.5.4. EAP-Response/AKA'-Challenge

The Peer sends an EAP-Response/AKA'-Challenge in response to a valid EAP-Request/AKA'-Challenge message, as specified by [] and []. If the Peer supports and is willing to perform the extension specified in this protocol, and the Server had made a valid request involving the attributes specified in Section 6.5.3, the Peer responds per the rules specified below. Otherwise, the Peer responds as specified in [] and [] and ignores the attributes related to this extension. If the Peer has not received attributes related to this extension from the Server, and has a policy that requires it to always use this extension, it behaves as if the AUTN were incorrect and fails the authentication.

The AT_MAC attribute MUST be included and checked as specified in []. In the EAP-Response/AKA'-Challenge, there is no message-specific data covered by the MAC. The AT_PUB_ECDHE attribute MUST be included and carries the Peer's public Diffie-Hellman key.

The AT_RES attribute MUST be included and checked as specified in []. When processing this message, the Server MUST process AT_RES before processing other attributes. The Server derives keys and verifies AT_MAC only when this attribute is verified to be valid.

If the Server has proposed the use of the extension specified in this protocol, but the Peer ignores and continues the basic EAP-AKA' authentication, the Server makes a policy decision of whether this is allowed. If this is allowed, it continues the EAP-AKA' authentication to completion. If it is not allowed, the Server MUST behave as if authentication failed.

The AT_CHECKCODE, AT_RESULT_IND, AT_IV, AT_ENCR_DATA, and other attributes may be included as specified in Section 9.4 of [].

6.5.5. EAP-Request/AKA'-Reauthentication

There are no changes for the EAP-Request/AKA'-Reauthentication, but note that the re-authentication process uses the keys generated in the original EAP-AKA' authentication, which employs key material from the Diffie-Hellman procedure if the extension specified in this document is in use.

6.5.6. EAP-Response/AKA'-Reauthentication

There are no changes for the EAP-Response/AKA'-Reauthentication, but as discussed in Section 6.5.5, re-authentication is based on the key material generated by EAP-AKA' and the extension defined in this document.

6.5.7. EAP-Response/AKA'-Synchronization-Failure

There are no changes for the EAP-Response/AKA'-Synchronization-Failure, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST NOT be added to this message. The appearance of these attributes in a received message MUST be ignored.

6.5.8. EAP-Response/AKA'-Authentication-Reject

There are no changes for the EAP-Response/AKA'-Authentication-Reject, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST NOT be added to this message. The appearance of these attributes in a received message MUST be ignored.

6.5.9. EAP-Response/AKA'-Client-Error

There are no changes for the EAP-Response/AKA'-Client-Error, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST NOT be added to this message. The appearance of these attributes in a received message MUST be ignored.

6.5.10. EAP-Request/AKA'-Notification

There are no changes for the EAP-Request/AKA'-Notification.

6.5.11. EAP-Response/AKA'-Notification

There are no changes for the EAP-Response/AKA'-Notification.

7. Security Considerations

This section deals only with changes to security considerations for EAP-AKA' or new information that has been gathered since the publication of [].

As discussed in Section 1, FS is an important countermeasure against adversaries who gain access to long-term keys. The long-term keys can be best protected with good processes, e.g., restricting access to the key material within a factory or among personnel, etc. Even so, not all attacks can be entirely ruled out. For instance, well-resourced adversaries may be able to coerce insiders to collaborate, despite any technical protection measures. The zero trust principles suggest that we assume that breaches are inevitable or have potentially already occurred and that we need to minimize the impact of these breaches (see [NSA-ZT] and [NIST-ZT]). One type of breach is key compromise or key exfiltration.

If a mechanism without ephemeral key exchange (such as 5G-AKA or EAP-AKA') is used, the effects of key compromise are devastating. There are serious consequences to not properly providing FS for the key establishment, for the control plane and the user plane, and for both directions:

  1. An attacker can decrypt 5G communication that they previously recorded.

  2. A passive attacker can eavesdrop (decrypt) all future 5G communication.

  3. An active attacker can impersonate the User Equipment (UE) or the network and inject messages in an ongoing 5G connection between the real UE and the real network.

At the time of writing, best practice security is to mandate FS (as is done in Wi-Fi Protected Access 3 (WPA3), EAP-TLS 1.3, EAP-TTLS 1.3, Internet Key Exchange Protocol Version 2 (IKEv2), Secure Shell (SSH), QUIC, WireGuard, Signal, etc.). In deployments, it is recommended that EAP-AKA methods without FS be phased out in the long term.

The FS extension provides assistance against passive attacks from attackers that have compromised the key material on USIM cards. Passive attacks are attractive for attackers performing large-scale pervasive monitoring as they require far fewer resources and are much harder to detect. The extension also provides protection against active attacks as the attacker is forced to be on-path during the AKA run and subsequent communication between the parties. Without FS, an active attacker that has compromised the long-term key can inject messages in a connection between the real Peer and the real Server without being on-path. This extension is most useful when implemented in a context where the MSK or EMSK are used in protocols not providing FS. For instance, if used with IKEv2 [], the session keys produced by IKEv2 will in any case have this property, so the improvements from the use of EAP-AKA' FS are not that useful. However, typical link-layer usage of EAP does not involve running another key exchange with forward secrecy. Therefore, using EAP to authenticate access to a network is one situation where the extension defined in this document can be helpful.

The FS extension generates key material using the ECDHE exchange in order to gain the FS property. This means that once an EAP-AKA' authentication run ends, the session that it was used to protect is closed, and the corresponding keys are destroyed. Even someone who has recorded all of the data from the authentication run and session and gets access to all of the AKA long-term keys cannot reconstruct the keys used to protect the session or any previous session, without doing a brute-force search of the session key space.

Even if a compromise of the long-term keys has occurred, FS is still provided for all future sessions, as long as the attacker does not become an active attacker.

The extension does not provide protection against active attackers that mount an on-path attack on future EAP-AKA' runs and have access to the long-term key. They will be able to eavesdrop on the traffic protected by the resulting session key(s). Still, past sessions where FS was in use remain protected.

Using EAP-AKA' FS once provides FS. FS limits the effect of key leakage in one direction (compromise of a key at time T2 does not compromise some key at time T1 where T1 < T2). Protection in the other direction (compromise at time T1 does not compromise keys at time T2) can be achieved by rerunning ECDHE frequently. If a long-term authentication key has been compromised, rerunning EAP-AKA' FS gives protection against passive attackers. Using the terms in [], FS without rerunning ECDHE does not stop an attacker from doing static key exfiltration. Frequently rerunning EC(DHE) forces an attacker to do dynamic key exfiltration (or content exfiltration).

7.1. Deployment Considerations

Achieving FS requires that, when a connection is closed, each endpoint MUST destroy not only the ephemeral keys used by the connection but also any information that could be used to recompute those keys.

Similarly, other parts of the system matter. For instance, when the keys generated by EAP are transported to a pass-through authenticator, such transport must also provide forward secure encryption with respect to the long-term keys used to establish its security. Otherwise, an adversary may attack the transport connection used to carry keys from EAP, and use this method to gain access to current and past keys from EAP, which, in turn, would lead to the compromise of anything protected by those EAP keys.

Of course, these considerations apply to any EAP method, not only this one.

7.2. Security Properties

The following security properties of EAP-AKA' are impacted through this extension:

Protected ciphersuite negotiation:

EAP-AKA' has a negotiation mechanism for selecting the KDFs, and this mechanism has been extended by the extension specified in this document. The resulting mechanism continues to be secure against bidding-down attacks.

There are two specific needs in the negotiation mechanism:

Negotiating KDFs within the extension:
The negotiation mechanism allows changing the offered KDF, 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.
Negotiating the use of this extension:

This extension is offered by the Server through presenting the AT_KDF_FS and AT_PUB_ECDHE attributes in the EAP-Request/AKA'-Challenge message. These attributes are protected by AT_MAC, so attempts to change or omit them by an adversary will be detected.

These attempts will be detected, except of course, if the adversary holds the long-term key and is willing to engage in an active attack. For instance, such an attack can forge the negotiation process so that no FS will be provided. However, as noted above, an attacker with these capabilities will, in any case, be able to impersonate any party in the protocol and perform on-path attacks. That is not a situation that can be improved by a technical solution. However, as discussed in the Introduction, even an attacker with access to the long-term keys is required to be on-path on each AKA run and subsequent communication, which makes mass surveillance more laborious.

The security properties of the extension also depend on a policy choice. As discussed in Section 6.5.4, both the Peer and the Server make a policy decision of what to do when it was willing to perform the extension specified in this protocol, but the other side does not wish to use the extension. Allowing this has the benefit of allowing backwards compatibility to equipment that did not yet support the extension. When the extension is not supported or negotiated by the parties, no FS can obviously be provided.

If turning off the extension specified in this protocol is not allowed by policy, the use of legacy equipment that does not support this protocol is no longer possible. This may be appropriate when, for instance, support for the extension is sufficiently widespread or required in a particular version of a mobile network.

Key derivation:
This extension provides FS. As described in several places in this specification, this can be roughly summarized as follows: an attacker with access to long-term keys is unable to obtain session keys of ended past sessions, assuming these sessions deleted all relevant session key material. This extension does not change the properties related to re-authentication. No new Diffie-Hellman run is performed during the re-authentication allowed by EAP-AKA'. However, if this extension was in use when the original EAP-AKA' authentication was performed, the keys used for re-authentication (K_re) are based on the Diffie-Hellman keys; hence, they continue to be equally safe against exposure of the long-term key as the original authentication.

7.3. Denial of Service

It is worthwhile to discuss Denial-of-Service (DoS) attacks and their impact on this protocol. The calculations involved in public key cryptography require computing power, which could be used in an attack to overpower either the Peer or the Server. While some forms of DoS attacks are always possible, the following factors help mitigate the concerns relating to public key cryptography and EAP-AKA' FS.

  • In a 5G context, other parts of the connection setup involve public key cryptography, so while performing additional operations in EAP-AKA' is an additional concern, it does not change the overall situation. As a result, the relevant system components need to be dimensioned appropriately, and detection and management mechanisms to reduce the effect of attacks need to be in place.

  • This specification is constructed so that it is possible to have a separation between the USIM and Peer on the client side and between the Server and AD on the network side. This ensures that the most sensitive (or legacy) system components cannot be the target of the attack. For instance, EAP-AKA' and public key cryptography both take place in the phone and not the low-power USIM card.

  • EAP-AKA' has been designed so that the first actual message in the authentication process comes from the Server, and that this message will not be sent unless the user has been identified as an active subscriber of the operator in question. While the initial identity can be spoofed before authentication has succeeded, this reduces the efficiency of an attack.

  • Finally, this memo specifies an order in which computations and checks must occur. For instance, when processing the EAP-Request/AKA'-Challenge message, the AKA authentication must be checked and succeed before the Peer proceeds to calculating or processing the FS-related parameters (see Section 6.5.4). The same is true of an EAP-Response/AKA'-Challenge (see Section 6.5.4). This ensures that the parties need to show possession of the long-term key in some way, and only then will the FS calculations become active. This limits the DoS to specific, identified subscribers. While botnets and other forms of malicious parties could take advantage of actual subscribers and their key material, at least such attacks are:

    1. limited in terms of subscribers they control, and
    2. identifiable for the purposes of blocking the affected subscribers.

7.4. Identity Privacy

As specified in Section 6.5, the Peer identity sent in the Identity Response message needs to follow the privacy-friendly requirements in [].

7.5. Unprotected Data and Privacy

Unprotected data and metadata can reveal sensitive information and need to be selected with care. In particular, this applies to AT_KDF, AT_KDF_FS, AT_PUB_ECDHE, and AT_KDF_INPUT. AT_KDF, AT_KDF_FS, and AT_PUB_ECDHE reveal the used cryptographic algorithms; if these depend on the Peer identity, they leak information about the Peer. AT_KDF_INPUT reveals the network name, although that is done on purpose to bind the authentication to a particular context.

An attacker observing network traffic may use the above types of information for traffic flow analysis or to track an endpoint.

7.6. Forward Secrecy within AT_ENCR

The keys K_encr and K_aut are calculated and used before the shared secret from the ephemeral key exchange is available.

K_encr and K_aut are used to encrypt and calculate a MAC in the EAP-Req/AKA'-Challenge message, especially the DH gx ephemeral pub key. At that point, the Server does not yet have the corresponding gy from the Peer and cannot compute the shared secret. K_aut is then used as the authentication key for the shared secret.

However, for K_encr, none of the encrypted data sent in the EAP-Req/AKA'-Challenge message in the AT_ENCR attribute will be a forward secret. That data may include re-authentication pseudonyms, so an adversary compromising the long-term key would be able to link re-authentication protocol runs when pseudonyms are used, within a sequence of runs followed after a full EAP-AKA' authentication. No such linking would be possible across different full authentication runs. If the pseudonym linkage risk is not acceptable, one way to avoid the linkage is to always require full EAP-AKA' authentication.

7.7. Post-Quantum Considerations

As of the publication of this document, it is unclear when or even if a quantum computer of sufficient size and power to exploit ECC will exist. Deployments that need to consider risks decades into the future should transition to Post-Quantum Cryptography (PQC) in the not-too-distant future. Other systems may employ PQC when the quantum threat is more imminent. Current PQC algorithms have limitations compared to ECC, and the data sizes could be problematic for some constrained systems. If a Cryptographically Relevant Quantum Computer (CRQC) is built, it could recover the SHARED_SECRET from the ECDHE public keys.

However, this would not affect the ability of EAP-AKA', with or without this extension, to authenticate properly. As symmetric key cryptography is safe even if CRQCs are built, an adversary still will not be able to disrupt authentication as it requires computing a correct AT_MAC value. This computation requires the K_aut key, which is based on the MK, CK', and IK', but not SHARED_SECRET.

Other output keys do include SHARED_SECRET via MK_ECDHE, but they still include the CK' and IK', which are entirely based on symmetric cryptography. As a result, an adversary with a quantum computer still cannot compute the other output keys either.

However, if the adversary has also obtained knowledge of the long-term key, they could then compute the CK', IK', SHARED_SECRET, and any derived output keys. This means that the introduction of a powerful enough quantum computer would disable this protocol extension's ability to provide the forward secrecy capability. This would make it necessary to update the current ECC algorithms in this document to PQC algorithms. This document does not add such algorithms, but a future update can do that.

Symmetric algorithms used in EAP-AKA' FS, such as HMAC-SHA-256 and the algorithms used to generate AT_AUTN and AT_RES, are practically secure against even large, robust quantum computers. EAP-AKA' FS is currently only specified for use with ECDHE key exchange algorithms, but use of any Key Encapsulation Method (KEM), including PQC KEMs, can be specified in the future. While the key exchange is specified with terms of the Diffie-Hellman protocol, the key exchange adheres to a KEM interface. AT_PUB_ECDHE would then contain either the ephemeral public key of the Server or the SHARED_SECRET encapsulated with the Server's public key. Note that the use of a KEM might require other changes, such as including the ephemeral public key of the Server in the key derivation to retain the property that both parties contribute randomness to the session key.

8. IANA Considerations

This extension of EAP-AKA' shares its attribute space and subtypes with the following:

  • "Extensible Authentication Protocol Method for Global System for Mobile Communications (GSM) Subscriber Identity Modules (EAP-SIM)" [],
  • "Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA)" [], and
  • "Improved Extensible Authentication Protocol Method for 3GPP Mobile Network Authentication and Key Agreement (EAP-AKA')" [].

IANA has assigned two new values in the "Attribute Types (Skippable Attributes 128-255)" registry under the "EAP-AKA and EAP-SIM Parameters" group as follows:

152:
AT_PUB_ECDHE (Section 6.1)
153:
AT_KDF_FS (Section 6.2)

IANA has also created the "EAP-AKA' AT_KDF_FS Key Derivation Function Values" registry to represent FS KDF types. The "EAP-AKA' with ECDHE and X25519" and "EAP-AKA' with ECDHE and P-256" types (1 and 2, see Section 6.3) have been assigned, along with one reserved value. The initial contents of this registry are illustrated in Table 1; new values can be created through the Specification Required policy []. Expert reviewers should ensure that the referenced specification is clearly identified and stable and that the proposed addition is reasonable for the given category of allocation.

Table 1: EAP-AKA' AT_KDF_FS Key Derivation Function Values Registry Initial Contents
Value Description Reference
0 Reserved RFC 9678
1 EAP-AKA' with ECDHE and X25519 RFC 9678
2 EAP-AKA' with ECDHE and P-256 RFC 9678
3-65535 Unassigned RFC 9678

9. References

9.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
[RFC3748]
Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC 3748, DOI 10.17487/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, , <>.
[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, , <>.
[RFC7624]
Barnes, R., Schneier, B., Jennings, C., Hardie, T., Trammell, B., Huitema, C., and D. Borkmann, "Confidentiality in the Face of Pervasive Surveillance: A Threat Model and Problem Statement", RFC 7624, DOI 10.17487/RFC7624, , <>.
[RFC7748]
Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, , <>.
[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, , <>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
[RFC9048]
Arkko, J., Lehtovirta, V., Torvinen, V., and P. Eronen, "Improved Extensible Authentication Protocol Method for 3GPP Mobile Network Authentication and Key Agreement (EAP-AKA')", RFC 9048, DOI 10.17487/RFC9048, , <>.
[SEC1]
Standards for Efficient Cryptography, "SEC 1: Elliptic Curve Cryptography", Version 2.0, , <https://www.secg.org/sec1-v2.pdf>.
[SEC2]
Standards for Efficient Cryptography, "SEC 2: Recommended Elliptic Curve Domain Parameters", Version 2.0, , <https://www.secg.org/sec2-v2.pdf>.
[SP-800-186]
Chen, L., Moody, D., Randall, K., Regenscheid, A., and A. Robinson, "Recommendations for Discrete Logarithm-based Cryptography: Elliptic Curve Domain Parameters", NIST SP 800-186, DOI 10.6028/NIST.SP.800-186, , <https://doi.org/10.6028/NIST.SP.800-186>.
[SP-800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. Davis, "Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography", NIST SP 800-56A, DOI 10.6028/NIST.SP.800-56Ar3, , <https://doi.org/10.6028/NIST.SP.800-56Ar3>.

9.2. Informative References

[DOW1992]
Diffie, W., Van Oorschot, P. C., and M. J. Wiener, "Authentication and authenticated key exchanges", Designs, Codes and Cryptography, vol. 2, pp. 107-125, DOI 10.1007/BF00124891, , <https://doi.org/10.1007/BF00124891>.
[Heist2015]
Scahill, J. and J. Begley, "The Great SIM Heist", , <https://theintercept.com/2015/02/19/great-sim-heist/>.
[NIST-ZT]
National Institute of Standards and Technology, "Implementing a Zero Trust Architecture", NIST SP 1800-35, , <https://www.nccoe.nist.gov/sites/default/files/2024-07/zta-nist-sp-1800-35-preliminary-draft-4.pdf>.
[NSA-ZT]
National Security Agency, "Embracing a Zero Trust Security Model", , <https://media.defense.gov/2021/Feb/25/2002588479/-1/-1/0/CSI_EMBRACING_ZT_SECURITY_MODEL_UOO115131-21.PDF>.
[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, , <>.
[RFC5216]
Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216, , <>.
[RFC7258]
Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/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, , <>.
[RFC9190]
Preuß Mattsson, J. and M. Sethi, "EAP-TLS 1.3: Using the Extensible Authentication Protocol with TLS 1.3", RFC 9190, DOI 10.17487/RFC9190, , <>.
[TrustCom2015]
Arkko, J., Norrman, K., Näslund, M., and B. Sahlin, "A USIM Compatible 5G AKA Protocol with Perfect Forward Secrecy", IEEE International Conference on Trust, Security and Privacy in Computing and Communications (TrustCom), DOI 10.1109/Trustcom.2015.506, , <https://doi.org/10.1109/Trustcom.2015.506>.
[TS.33.501]
3GPP, "Security architecture and procedures for 5G System", Version 19.0.0, 3GPP TS 33.501, .

Acknowledgments

The authors would like to note that the technical solution in this document came out of the TrustCom paper [TrustCom2015], whose authors were J. Arkko, K. Norrman, M. Näslund, and B. Sahlin. This document also uses a lot of material from [] by J. Arkko and H. Haverinen, as well as [] by J. Arkko, V. Lehtovirta, and P. Eronen.

The authors would also like to thank Ben Campbell, Meiling Chen, Roman Danyliw, Linda Dunbar, Tim Evans, Zhang Fu, Russ Housley, Tero Kivinen, Murray Kucherawy, Warren Kumari, Eliot Lear, Vesa Lehtovirta, Kathleen Moriarty, Prajwol Kumar Nakarmi, Francesca Palombini, Anand R. Prasad, Michael Richardson, Göran Rune, Bengt Sahlin, Joseph Salowey, Mohit Sethi, Orie Steele, Rene Struik, Vesa Torvinen, Sean Turner, Helena Vahidi Mazinani, Robert Wilton, Paul Wouters, Bo Wu, Peter Yee, and many other people at the IETF, GSMA, and 3GPP groups for interesting discussions in this problem space.

Authors' Addresses

Jari Arkko
Ericsson
FI-02420 Jorvas
Finland
Karl Norrman
Ericsson
SE-16483 Stockholm
Sweden
John Preuß Mattsson
Ericsson
SE-164 40 Kista
Sweden

RFC 9678: Forward Secrecy Extension to the Improved Extensible Authentication Protocol Method for Authentication and Key Agreement (EAP-AKA' FS)

Proposed Standard