wdiff rfc9678.original rfc9678.txt

Network Working Group

Internet Engineering Task Force (IETF) J. Arkko
Internet-Draft
Request for Comments: 9678 K. Norrman
Updates: 5448, 9048 (if approved) J. Preuß Mattsson
Intended status:
Category: Standards Track Ericsson
Expires: 22 August 2024 19 February
ISSN: 2070-1721 December 2024

Forward Secrecy for Extension to the Improved Extensible Authentication
Protocol Method for Authentication and Key Agreement (EAP-AKA' FS)
draft-ietf-emu-aka-pfs-12

Abstract

This document updates RFC 9048, the improved "Improved Extensible Authentication
Protocol Method for 3GPP Mobile Network Authentication and Key
Agreement (EAP-AKA'), (EAP-AKA')", and its predecessor RFC 5448 with an optional
extension providing ephemeral key exchange. Similarly, this document also updates the
earlier version of the EAP-AKA' specification in RFC 5448. 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, assuming these have been properly deleted.
In addition, EAP-AKA' FS mitigates passive attacks (e.g., large scale large-scale
pervasive monitoring) against future sessions. This forces attackers
to use active attacks instead.

Status of This Memo

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Internet-Drafts are working documents an Internet Standards Track document.

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Internet-Drafts are draft documents valid the IETF community. It has
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This Internet-Draft will expire on 22 August 2024.
https://www.rfc-editor.org/info/rfc9678.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Design and Deployment Objectives . . . . . . . . . . 4
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. AKA . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. EAP-AKA' Protocol . . . . . . . . . . . . . . . . . . . . 6
4.3. Attacks Against Long-Term Keys in Smart Cards . . . . . . 8
5. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 8
6. Extensions to EAP-AKA' . . . . . . . . . . . . . . . . . . . 11
6.1. AT_PUB_ECDHE . . . . . . . . . . . . . . . . . . . . . . 11
6.2. AT_KDF_FS . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Forward Secrecy Key Derivation Functions . . . . . . . . 14
6.4. ECDHE Groups . . . . . . . . . . . . . . . . . . . . . . 16
6.5. Message Processing . . . . . . . . . . . . . . . . . . . 16
6.5.1. EAP-Request/AKA'-Identity . . . . . . . . . . . . . . 16
6.5.2. EAP-Response/AKA'-Identity . . . . . . . . . . . . . 16
6.5.3. EAP-Request/AKA'-Challenge . . . . . . . . . . . . . 17
6.5.4. EAP-Response/AKA'-Challenge . . . . . . . . . . . . . 17
6.5.5. EAP-Request/AKA'-Reauthentication . . . . . . . . . . 18
6.5.6. EAP-Response/AKA'-Reauthentication . . . . . . . . . 18
6.5.7. EAP-Response/AKA'-Synchronization-Failure . . . . . . 18
6.5.8. EAP-Response/AKA'-Authentication-Reject . . . . . . . 18
6.5.9. EAP-Response/AKA'-Client-Error . . . . . . . . . . . 18
6.5.10. EAP-Request/AKA'-Notification . . . . . . . . . . . . 19
6.5.11. EAP-Response/AKA'-Notification . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7.1. Deployment Considerations . . . . . . . . . . . . . . . . 21
7.2. Security Properties . . . . . . . . . . . . . . . . . . . 21
7.3. Denial-of-Service . . . . . . . . . . . . . . . . . . . . 23 Denial of Service
7.4. Identity Privacy . . . . . . . . . . . . . . . . . . . . 24
7.5. Unprotected Data and Privacy . . . . . . . . . . . . . . 24
7.6. Forward Secrecy within AT_ENCR . . . . . . . . . . . . . 24
7.7. Post-Quantum Considerations . . . . . . . . . . . . . . . 25
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Normative References . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 29
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33

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
occur, for instance, instance:

* during the manufacturing process of USIM cards,

* during the transfer of the cards and associated information to the
operator, and

* when a system is running.

Since the publication of reports about such attacks [Heist2015], (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 [RFC7258].

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 [RFC9048], the Improved "Improved Extensible Authentication
Protocol Method for 3GPP Mobile Network Authentication and Key
Agreement (EAP-AKA') method, (EAP-AKA')", with an optional extension providing ephemeral
key exchange minimizing 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,
exchange forces attackers to be active if they want access to future
session keys keys, even if they have access to the long-term key. This is
beneficial,
beneficial because active attacks demand much many more resources to
launch,
launch and are easier to detect. As with other protocols, an active
attacker with access to the long-term key material will will, of course 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 a 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Protocol Design and Deployment Objectives

The extension specified here re-uses 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 [TrustCom2015], (see [TrustCom2015]) and uses much of the same material, 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, 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 forward secrecy FS to the existing mobile network infrastructure can be
done in multiple different ways, this document specifies a solution
that is relatively easily deployable. 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
forward secrecy purposes, FS
being available for both EAP-TLS [RFC5216] [RFC9190] and EAP-AKA'
ensures that that, for many practical systems
forward secrecy 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 [RFC3748].

4.1. AKA

We use the term Authentication "Authentication and Key Agreement (AKA) 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 adds 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 UICC (Universal Universal Integrated
Circuit Card), Card (UICC), an embedded UICC, or integrated in a Trusted
Execution Environment (TEE). In this document document, we use the term "USIM
card" to refer to any Subscriber Identity Module (SIM) capable of
running AKA.

The goal of AKA is to mutually authenticate the USIM and the so-
called home environment, which is the authentication server Server in the
subscribers
subscriber's home operator's network.

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
Authentication Database (AD) generates authentication vectors. The
3GPP authentication server Server takes the role of EAP server. Server. The USIM
combined with the mobile phone takes the role of the client. The
difference between EAP-AKA [RFC4187] and EAP-AKA' [RFC9048] 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 [RFC9048] and [RFC4187]. Note the use of EAP-terminology 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) Identifier (NAI)) |
+----------------------------------------------------------->|
| +-----------------------------------------------------+--+ +-------------------------------------------------------+--+
| | The Server determines the network name and ensures that |
| | the given access network is authorized to use the |
| | claimed name. The server Server then runs the AKA' algorithms EAP-AKA' |
| | algorithms generating RAND and AUTN, and derives session keys from |
| | keys from CK' and IK'. RAND and AUTN are sent as AT_RAND and |
| | AT_RAND and AT_AUTN attributes, whereas the network name is |
| | 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 Peer determines what the network name should be, | |
| based on, e.g., what access technology it is using. | |
| The peer 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 | |
| in running the AKA' EAP-AKA' algorithms, verifying AUTN from | |
| AT_AUTN and MAC Message Authentication Code (MAC) from the | |
| AT_MAC attributes. The peer Peer then | |
| generates RES. The peer | |
| Peer also derives session keys from CK'/IK. The AT_RES | |
| 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 |
| | 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 [RFC9048].

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 fail, at least to some extent 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 to compromise 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 Secrecy (FS) for EAP-AKA' is achieved by using an Elliptic
Curve Diffie-Hellman (ECDH) exchange [RFC7748]. To provide FS, the
exchange must be run in an ephemeral manner, i.e., both sides
generate temporary keys according to the negotiated ciphersuite,
e.g., ciphersuite. For
example, for X25519 X25519, this is done as specified in [RFC7748]. This
method is referred to as ECDHE, "ECDHE", where the last 'E' "E" stands for Ephemeral.
"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 [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 Server and peer, Peer and to establish keying 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 below 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 represent represents the 3GPP
Authentication Database (AD) and server. Server. The details of those
interactions are outside the scope of this document, document; however, and the
reader is referred to the 3GPP specifications. For 5G specifications (for 5G, this is
specified in 3GPP TS 33.501 [TS.33.501] [TS.33.501]).

USIM Peer Server AD
| | | |
| | EAP-Req/Identity | |
| |<---------------------------+ |
| | | |
| | EAP-Resp/Identity | |
| | (Privacy-Friendly) | |
| +--------------------------->| |
| +-------+----------------------------+----------------+--+ +-------+----------------------------+----------------+----+
| | The Server now has an identity for the peer. Peer. The server Server |
| | then asks the help of the AD to run AKA EAP-AKA algorithms, |
| | generating RAND, AUTN, XRES, CK, and IK. Typically, the |
| | AD performs the first part of key derivations so that the |
| | the authentication server Server gets the CK' and IK' keys already |
| | 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 |
| | key of that key pair and the first EAP method message to |
| | the peer. 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 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 Peer checks if it wants to do the FS extension. If | |
| If yes, it will eventually respond with AT_PUB_ECDHE and | |
| 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 Peer needs to query the | |
| auth parameters from the USIM card. | |
+--+--------------+----------------------------+---------+ |
| | | |
| RAND, AUTN | | |
|<-------------+ | |
| | | |
| CK, IK, RES | | |
+------------->| | |
+--+--------------+----------------------------+---------+ |
| The peer Peer now has everything to respond. If it wants to | |
| to participate in the FS extension, it will then generate | |
| generate its key pair, calculate a shared key based on its key | |
| its key pair and the server's Server's public key. Finally, it proceeds | |
| proceeds to derive all EAP-AKA' key values and constructs a | |
| constructs a full response. | |
+--+--------------+----------------------------+---------+ |
| | | |
| | EAP-Resp/AKA'-Challenge | |
| | AT_RES, AT_PUB_ECDHE, | |
| | AT_MAC | |
| +--------------------------->| |
| +-------+----------------------------+----------------+--+
| | The server 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 |
| | who held the long-term key, only an active attacker could |
| | could have determined the generated session keys; in basic |
| | basic EAP-AKA' the generated keys are only based on CK and |
| | 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

AT_PUB_ECDHE:
This is set to TBA1 BY 152 by IANA.

Length
The

Length:
This is the length of the attribute, set as other attributes in
EAP-AKA [RFC4187]. 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

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 [RFC7748] Section 5. 5 of [RFC7748].

* 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 forward secrecy FS key
generation function, if the Forward Secrecy (FS) 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 which that
is still used together with the forward secrecy FS KDF value. The basic KDF value is
also used by those EAP peers 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 forward secrecy FS KDF values: The

* the basic KDF value in AT_KDF is 1, as specified in [RFC5448] and
[RFC9048],
[RFC9048] and

* the forward secrecy 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 KDF KDFs or new
forward secrecy FS
KDF values need to specify how they are treated and what combinations
are allowed. This requirement is an update to how [RFC5448] and
[RFC9048] 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

AT_KDF_FS:
This is set to TBA2 BY 153 by IANA.

Length
The

Length:
This is the length of the attribute, attribute; it MUST be set to 1.

FS Key Derivation Function
An Function:
This is an enumerated value representing the forward secrecy key
derivation function FS Key Derivation
Function (KDF) that the server Server (or peer) Peer) wishes to use. See
Section 6.3 for the functions specified in this document. Note:
This
this field has a different name space than the similar field in
the AT_KDF attribute Key Derivation Function KDF defined in [RFC9048].

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 Server includes a public key value for,
however. So 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 Peer supports and is willing to use the FS Key Derivation
Function KDF indicated by
the first AT_KDF_FS attribute, and is willing and able to use the
extension defined in this document, the function is taken into use will be used
without any further negotiation.

* If the peer Peer does not support this function or is unwilling to use
it, it responds to the server Server with an indication that a different
function is needed. Similarly Similarly, with the negotiation process
defined in [RFC9048] for AT_KDF, the peer Peer sends EAP-Response/AKA'-
Challenge an EAP-Response/
AKA'-Challenge message that contains only one attribute, AT_KDF_FS
AT_KDF_FS, with the value set to the desired alternative function
from among the ones suggested by the server Server earlier. If there is
no suitable alternative, the peer Peer has a choice of either falling
back to EAP-
AKA' EAP-AKA' or behaving as if the AUTN had been incorrect and
failing authentication (see Figure 3 of [RFC4187]). The peer 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 key derivation function; KDF; see below for further
information).

* If the peer 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, Peer, the server 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 Server is not a
valid alternative. If the peer Peer has replied with the first AT_KDF_FS
value, the server 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 Server side, see Section 3 and
Figure 2 in [RFC4187]. Otherwise, the server Server re-sends the EAP-Response/AKA'-Challenge EAP-
Response/AKA'-Challenge message, but adds the selected alternative to
the beginning of the list of AT_KDF_FS attributes, 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 Peer's request to change the FS Key Derivation Function KDF is the only valid situation
where such duplication may occur.

When the peer Peer receives the new EAP-Request/AKA'-Challenge message, it
MUST check that the requested change, and only the requested change change,
occurred in the list of AT_KDF_FS attributes. If yes, so, it continues.
If not, it behaves as if AT_MAC had been were incorrect and fails the
authentication. If the peer Peer receives multiple EAP-Request/AKA'-
Challenge messages with differing AT_KDF_FS attributes without having
requested negotiation, the peer Peer MUST behave as if AT_MAC had been were
incorrect and fail the authentication.

6.3. Forward Secrecy Key Derivation Functions

Two new FS Key Derivation Function 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 key derivation function and KDF and, at the same time
selects time, select an ECDHE group to be used.

The FS Key Derivation Function KDF type value is only used in the AT_KDF_FS attribute. When
the forward secrecy FS extension is used, the AT_KDF_FS attribute determines how to
derive the keys MK_ECDHE, K_re,
MSK, MK_ECDHE key, K_re key, Master Session Key (MSK), and EMSK.
Extended Master Session Key (EMSK). The AT_KDF_FS attribute should
not be confused with the different range of key derivation functions KDFs that can be
represented in the AT_KDF attribute as defined in [RFC9048]. When
the forward secrecy FS extension is used, the AT_KDF attribute only specifies how to
derive the keys MK, K_encr, Master Key (MK), the K_encr key, and K_aut. the K_aut key.

Key derivation in this extension produces exactly the same keys for
internal use within one authentication run as EAP-AKA' [RFC9048]
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 re-
authentication 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 Key Derivation Function KDF field in the AT_KDF_FS attribute is set to 1 or 2 and
the Key Derivation Function KDF field in the AT_KDF attribute is set to 1, the Master Key (MK) MK and
accompanying keys are derived as follows. follows:

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 [RFC9048].

* 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
[RFC3748].

The CK and IK are produced by the AKA algorithm. The IK' and CK' are
derived as specified in [RFC9048] 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 P-256, the SHARED_SECRET is the shared secret computed as
specified in Section 5.7.1.2 of [SP-800-56A]. Public key validation
requirements are defined in Section 5 of [SP-800-56A]. At least
partial public-key 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 X25519, the SHARED_SECRET is the shared secret computed as
specified in Section 6.1 of [RFC7748]. Both the peer Peer and the server Server
MAY check for the zero-value shared secret as specified in
Section 6.1 of [RFC7748].

| Note: The If performed inappropriately, the way that the shared
| secret is tested for zero can, if
performed inappropriately, can provide an ability for attackers
| to listen to CPU power usage side channels. Refer to [RFC7748]
| for a description of how to perform this check in a way that it
| does not become a problem.

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'
authentication process
starts again from the beginning.

The rest of the computation proceeds as defined in Section 3.3 of
[RFC9048].

For readability, an explanation of the notation used above is copied
here: [n..m] denotes the substring from bit n to m. PRF' is a new
pseudo-random function specified in [RFC9048]. 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, and EMSK is the Extended Master Session Key,
512 bits. MSK and EMSK are outputs from a successful EAP method run
[RFC3748].

CK and IK are produced by the AKA algorithm. IK' and CK' are derived
as specified in [RFC9048] from IK and CK.

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

Identity is the peer identity as specified in Section 7 of [RFC4187].
A privacy-friendly identifier [RFC9048] SHALL be used.

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
deciding the
decision to use of a particular key derivation function KDF in AT_KDF_FS. the AT_KDF_FS attribute.

For "EAP-AKA' with ECDHE and X25519" X25519", the group is the Curve25519
group specified in [RFC7748]. The support for this group is
REQUIRED.

For "EAP-AKA' with ECDHE and P-256" 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 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 [RFC9048] or [RFC4187]
apply.

6.5.1. EAP-Request/AKA'-Identity

No changes,

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

No changes,

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 Peer identifier SHALL comply with the privacy-friendly privacy-
friendly requirements of [RFC9190]. An example of a compliant way of
constructing a privacy-friendly peer Peer identifier is using a non-NULL non-null
SUCI [TS.33.501].

6.5.3. EAP-Request/AKA'-Challenge

The server Server sends the EAP-Request/AKA'-Challenge on full
authentication as specified by [RFC4187] and [RFC9048]. The
attributes AT_RAND, AT_AUTN, and AT_MAC MUST be included and checked
on reception as specified in [RFC4187]. 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 Server's public Diffie-Hellman key. If either AT_KDF_FS
or AT_PUB_ECDHE is missing on reception, the peer Peer MUST treat it as if
neither one was sent, sent and the 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 AT_NEXT_REAUTH_ID, and other attributes may be
included as specified in Section 9.3 of [RFC4187].

When processing this message, the peer Peer MUST process AT_RAND, AT_AUTN,
AT_KDF_FS, and AT_PUB_ECDHE before processing other attributes. Only The
Peer derives keys and verifies AT_MAC only if these attributes are
verified to be valid, the peer derives keys and
verifies AT_MAC. valid. If the peer Peer is unable or unwilling to perform
the extension specified in this document, it proceeds as defined in
[RFC9048]. Finally, if there is an error error, see Section 6.3.1. 6.3.1 of
[RFC4187].

6.5.4. EAP-Response/AKA'-Challenge

The peer Peer sends an EAP-Response/AKA'-Challenge in response to a valid
EAP-Request/AKA'-Challenge message, as specified by [RFC4187] and
[RFC9048]. If the peer Peer supports and is willing to perform the
extension specified in this protocol, and the server Server had made a valid
request involving the attributes specified in Section 6.5.3, the peer Peer
responds per the rules specified below. Otherwise, the peer Peer responds
as specified in [RFC4187] and [RFC9048] and ignores the attributes
related to this extension. If the peer 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 had
been
were incorrect and fails the authentication.

The AT_MAC attribute MUST be included and checked as specified in
[RFC9048]. In the EAP-Response/AKA'-Challenge, there is no message-
specific data covered by the MAC. The AT_PUB_ECDHE attribute MUST be
included,
included and carries the peer's Peer's public Diffie-Hellman key.

The AT_RES attribute MUST be included and checked as specified in
[RFC4187]. 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 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 AT_ENCR_DATA, and other
attributes may be included as specified in Section 9.4 of [RFC4187].

6.5.5. EAP-Request/AKA'-Reauthentication

No changes,

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, which employs key material from
the Diffie-Hellman procedure if the extension specified in this
document is in use, employs key material
from the Diffie-Hellman procedure. use.

6.5.6. EAP-Response/AKA'-Reauthentication

No changes,

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

No changes,

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

No changes,

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

changes, 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

No changes.

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

6.5.11. EAP-Response/AKA'-Notification

No changes.

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

7. Security Considerations

This section deals only with the changes to security considerations
as they differ from EAP-AKA', for
EAP-AKA' or as new information that has been gathered since the
publication of [RFC9048].

As discussed in Section 1, forward secrecy FS is an important countermeasure against
adversaries who gain access to the 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 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, occurred and that we need to minimize the impact
of these breaches (see [NSA-ZT] [NIST-ZT]. and [NIST-ZT]). One type of breach
is key compromise or key exfiltration.

If a mechanism without ephemeral key exchange such (such as (5G-AKA, 5G-AKA or EAP-
AKA') is used used, the effects of key compromise are devastating. There
are serious consequences of to not properly providing forward secrecy FS for the key establishment. For both
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 UE User Equipment (UE) or the Network
network and inject messages in an ongoing 5G connection between
the real UE and the real network.

Best

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

This

The FS extension provide 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 large-scale
pervasive monitoring as they require much less 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 on-path during the AKA
run and subsequent communication between the parties. Without
forward secrecy FS, an
active attacker that has compromised the long-term key can inject
messages in an a connection between the real Peer and the real server Server
without being on path. on-path. This extension is most useful when used
implemented in a context where the MSK/EMSK MSK or EMSK are used in protocols
not providing forward secrecy. FS. For instance, if used with IKEv2 [RFC7296], the
session keys produced by IKEv2 will in any case have this property,
so
better characteristics of the MSK and EMSK is improvements from the use of EAP-AKA' FS are not that useful.
However, typical link layer link-layer usage of EAP does not involve running
another, forward secure,
another key exchange. 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.

This

The FS extension generates keying 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 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 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
with access to the long-term key
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 forward secrecy. Forward secrecy 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 long-
term authentication key has been compromised, rerunning EAP-AKA' FS
gives protection against passive attackers. Using the terms in
[RFC7624], forward secrecy 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 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 which, in turn 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 negotiation:
EAP-AKA' has a negotiation mechanism for selecting the key
derivation functions, KDFs, and
this mechanism has been extended by the extension specified in
this document. The resulting mechanism continues to be secure
against bidding down bidding-down attacks.

There are two specific needs in the negotiation mechanism:

Negotiating key derivation function KDFs within the extension extension:
The negotiation mechanism allows changing the offered key
derivation function, KDF, but
the change is visible in the final
EAP- Request/AKA'-Challenge EAP-Request/AKA'-Challenge
message that the server Server sends to the peer. Peer. This message is
authenticated via the AT_MAC attribute, and carries both the
chosen alternative and the initially offered list. The peer 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 extension:
This extension is offered by the server 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.

Except

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. Such For instance, such an attack can,
for instance, can forge the
negotiation process so that no FS will be provided. However,
as noted above, an attacker with these capabilities will will, in
any case 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,
Introduction, even an attacker with access to the long-term
keys is required to be on path 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 Peer
and the server 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, widespread or required in a particular version of
a mobile network.

Key derivation derivation:
This extension provides forward secrecy. FS. As described in several places in
this specification, this can be roughly summarized as
that 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,
and hence keys; hence, they continue
to be equally safe against expose exposure of the long-
term long-term key as the
original authentication.

7.3. Denial-of-Service

In addition, it 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 Peer or the server. Server. While some forms
of Denial-of-Service DoS attacks are always possible, the following factors help
mitigate the concerns relating to public key cryptography and EAP-AKA' 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
is possible. 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 takes 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. When For instance, when processing the EAP-Request/AKA'-Challenge EAP-Request/
AKA'-Challenge message, for instance, the AKA authentication must be checked and
succeed before the peer Peer proceeds to calculating or processing the
FS related
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 Denial-of-Service 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 (a) are:

a. limited in terms of subscribers they control, and (b)

b. identifiable for the purposes of blocking the affected
subscribers.

7.4. Identity Privacy

As specified in Section 6.5, the peer Peer identity sent in the Identity
Response message needs to follow the privacy-friendly requirements in
[RFC9190].

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, algorithms;
if these depend on the peer identity Peer identity, they leak information about the
peer.
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

They

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 data in the EAP-Req/
AKA'-Challenge EAP-
Req/AKA'-Challenge message, especially the DH g^x ephemeral pub key.
At that point point, the server Server does not yet have the corresponding g^y
from the peer Peer and cannot compute the shared secret. K_aut is then
used as the authentication key for the shared secret.

For K_encr though,

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 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 authentaction authentication
runs. If the pseudonum 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 elliptic
curve cryptography ECC will
exist. Deployments that need to consider risks decades into the
future should transition to Post- Quantum 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 Elliptic Curve Cryptography
(ECC) ECC, and the data sizes could be problematic
for some constrained systems. If a Cryptographically Relevant
Quantum Computer (CRQC) is
built built, it could recover the SHARED_SECRET
from the ECDHE public keys.

This

However, this would not affect the ability of EAP-AKA' - EAP-AKA', with or
without this
extension - extension, to authenticate properly, however. 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 key, which
is based on MK and, ultimately, CK' the MK, CK', and IK', but not SHARED_SECRET.

Other output keys do include SHARED_SECRET via MK_ECDHE, but they
still include also the CK' and IK' 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', and 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 security 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 FS, such as HMAC-SHA-256 and
the algorithms use used to generate AT_AUTN and AT_RES AT_RES, are practically
secure against even large 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 Post-Quantum
Cryptography (PQC) 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 Server or the SHARED_SECRET encapsulated
with the server's Server's public key. Note that the use of a KEM might
require other changes changes, such as including the ephemeral public key of
the server 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 Extensible the following:

* "Extensible Authentication Protocol Method for Global System for
Mobile Communications (GSM) Subscriber Identity Modules (EAP-SIM) (EAP-SIM)"
[RFC4186], EAP-AKA

* "Extensible Authentication Protocol Method for 3rd Generation
Authentication and Key Agreement (EAP-AKA)" [RFC4187], and EAP-AKA'

* "Improved Extensible Authentication Protocol Method for 3GPP
Mobile Network Authentication and Key Agreement (EAP-AKA')"
[RFC9048].

Two

IANA has assigned two new values (TBA1, TBA2) in the skippable range need to be
assigned for AT_PUB_ECDHE (Section 6.1) and AT_KDF_FS (Section 6.2) in the "Attribute Types" Types (Skippable
Attributes 128-255)" registry under the "EAP-AKA and EAP-SIM
Parameters" group.

Also, group as follows:

152: AT_PUB_ECDHE (Section 6.1)

153: AT_KDF_FS (Section 6.2)

IANA is requested to create a new registry has also created the "EAP-AKA' AT_KDF_FS Key Derivation Function
Values" registry to represent FS Key Derivation
Function 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) need to be have been assigned, along with one reserved value. The
initial contents of this registry is are illustrated in Table 1; new
values can be created through the Specification Required policy
[RFC8126]. Expert reviewers should ensure that the referenced
specification is clearly identified and stable, stable and that the proposed
addition is reasonable for the given category of allocation.

+=========+==================+=========================+

+=========+================================+===========+
| Value | Description | Reference |
+=========+==================+=========================+
+=========+================================+===========+
| 0 | Reserved | [TBD BY IANA: THIS RFC] RFC 9678 |
+---------+------------------+-------------------------+
+---------+--------------------------------+-----------+
| 1 | EAP-AKA' with | [TBD BY IANA: THIS RFC] |
| | ECDHE and X25519 | RFC 9678 |
+---------+------------------+-------------------------+
+---------+--------------------------------+-----------+
| 2 | EAP-AKA' with | [TBD BY IANA: THIS RFC] |
| | ECDHE and P-256 | RFC 9678 |
+---------+------------------+-------------------------+
+---------+--------------------------------+-----------+
| 3-65535 | Unassigned | [TBD BY IANA: THIS RFC] RFC 9678 |
+---------+------------------+-------------------------+
+---------+--------------------------------+-----------+

Table 1: Initial Content of the EAP-AKA' AT_KDF_FS Key Derivation Function
Values Registry Initial Contents

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, 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>.

[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>.

[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, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.

[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/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, 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>.

[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, October 2021,
<https://www.rfc-editor.org/info/rfc9048>.

[SP-800-186]
NIST, "Recommendations for Discrete Logarithm-based
Cryptography: Elliptic Curve Domain Parameters",
NIST Special Publication 800-186, February 2023,
<https://doi.org/10.6028/NIST.SP.800-186>.

[SEC1] Certicom Research, Standards for Efficient Cryptography, "SEC 1: Elliptic
Curve Cryptography",
Standards for Efficient Cryptography 1 (SEC 1) Version 2.0, May 2009,
<https://www.secg.org/sec1-v2.pdf>.

[SEC2] Certicom Research, Standards for Efficient Cryptography, "SEC 2: Recommended
Elliptic Curve Domain Parameters", Standards for Efficient Cryptography 2
(SEC 2) Version 2.0, January
2010, <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, February 2023,
<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 Special Publication 800-56A Revision 3, SP
800-56A, DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
<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, June 1992,
<https://doi.org/10.1007/BF00124891>.

[Heist2015]
Scahill, J. and J. Begley, "The Great SIM Heist", February
2015,
<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,
July 2024, <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", February 2021, <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, January 2006,
<https://www.rfc-editor.org/info/rfc4186>.

[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <https://www.rfc-editor.org/info/rfc5216>.

[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>.

[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, February 2022,
<https://www.rfc-editor.org/info/rfc9190>.

[TrustCom2015]
Arkko, J., Norrman, K., Näslund, M., and B. Sahlin, "A
USIM compatible Compatible 5G AKA protocol Protocol with perfect forward
secrecy", Proceedings of Perfect Forward
Secrecy", IEEE International Conference on Trust, Security
and Privacy in Computing and Communications (TrustCom) 2015, (TrustCom),
DOI 10.1109/Trustcom.2015.506, August 2015,
<https://doi.org/10.1109/Trustcom.2015.506>.

[Heist2015]
Scahill, J. and J. Begley, "The Great SIM Heist", February
2015,
<https://theintercept.com/2015/02/19/great-sim-heist/>.

[DOW1992] Diffie, W., Van Oorschot, P., and M. Wiener,
"Authentication and Authenticated Key Exchanges", Designs,
Codes and Cryptography 2 pp. 107-125, June 1992,
<https://doi.org/10.1007/BF00124891>.

[TS.33.501]
3GPP, "Security architecture and procedures for 5G
System", Version 18.1.0, 3GPP TS 33.501 18.1.0, 33.501, March 2023.

[NIST-ZT] National Institute of Standards and Technology,
"Implementing a Zero Trust Architecture", December 2022,
<https://www.nccoe.nist.gov/sites/default/files/2022-12/
zta-nist-sp-1800-35b-preliminary-draft-2.pdf>.

Appendix A. Change Log

RFC Editor: Please remove this appendix.

The -12 version of the WG draft has the following changes, most due
to IESG review comments in January 2023:

* Update the draft track to Standards Track.

* Clarified the calculation of the Length field in the AT_ECDHE
attribute, along with padding requirements.

* Avoided the use of keywords in operational recommendations, e.g.,
about deployment.

* Changed the definition of what "supported" means to focus on
feature being implemented, but not require that it is usable
during a protocol run, because configuration, new security
information, etc. might imply that a particular feature is
implemented but disabled for policy reasons.

* Changed the MITM terminology to be on-path attacks.

* Corrected a reference typo in the IANA considerations section.

* Shortened the abstract and introduction to the key aspects and
removed duplication.

* Several editorial changes.

The -11 version of the WG draft has the following changes:

* Addressed IETF Last Call comments from directorates, Security AD,
Meiling Cheng, and a detailed review from the author Karl. In
particular:

* Replaced the reference to the deprecated FIPS 186-4 with SP
800-186.

* Changed HSS (Home Subscriber Server) to Authentication Database
(AD) as HSS is a 4G term.

* Explained difference between EAP-AKA and EAP-AKA'

* Explained that the emphemeral key exhange provide more that
forward secrecy and how this is important to mitigate pervasive
monitoring.

* Included links for the zero trust principles.

* Explained why K_encr and K_auth not being protected by the ECDHE
addition.

* Added that a future introduction of KEM might require additional
changes.

* Explained how ephemeral key exchange is linked to pervasive
monitoring.

* Changed SIM to USIM everywhere. A USIM is required for AKA.

* Changed to long-term key instead of long-term secret or long-term
shared secret.

* Reference updates.

* Various editorial improvements.

The -10 version of the WG draft has the following changes:

* Various nits found by Peter Yee.

The -09 version of the WG draft has the following changes:

* Scalable Vector Graphics (SVG) versions for all figures has been
added and the figures has been slightly modified to render nicely
with aasvg.

* A reference has been added to the Section in SEC1 describing how
to do decompression.

* The strengthened identity protection requirements are now
mentioned in the introduction.

* Corrections and clarifications were made in the IANA
considerations. The table in the IANA section has been made into
a proper xml table.

* Reference updates.

* Various editorial improvements.

The -08 version of the WG draft has the following changes:

* Further clarification of key calculation in Section 6.3.

* Support for the NIST P-256 group has been made mandatory in
Section 6.4, in order to align the requirements with 3GPP SUCI
encryption requirements.

* The interaction between AT_KDF and AT_KDF_FS has been specified
more clearly, including specifying how future specifications need
to specify the treatment of new combinations.

* Addition of a discussion about the impacts of potential future
quantum computing attacks with specific impacts to this extension.

* Addition of a discussion about metadata/unprotected data in
Section 7.5.

* Reference updates.

* Various editorial improvements.

The -07 version of the WG draft has the following changes:

* The impact of forward secrecy explanation has been improved in the
abstract and security considerations.

* The draft now more forcefully explains why the authors believe it
is important to migrate existing systems to use forward secrecy,
and makes a recommendation for this migration.

* The draft does no longer refer to issues within the smart cards
but rather the smart card supply chain.

* The rationale for chosen algorithms is explained.

* Also, the authors have checked the language relating to the public
value encoding, and believe it is exactly according to the
references ([RFC7748] Section 6.1 and [SEC2] Section 2.7.1)

The -06 version of the WG draft is a refresh and a reference update.
However, the following should be noted:

* The draft now uses "forward secrecy" terminology and references
RFC 7624 per recommendations on mailing list discussion.

* There's been mailing list discussion about the encoding of the
public values; the current text requires confirmation from the
working group that it is sufficient.

The -05 version of the WG draft takes into account feedback from the
working group list, about the number of bytes needed to encode P-256
values.

The -04 version of the WG draft takes into account feedback from the
May 2020 WG interim meeting, correcting the reference to the NIST
P-256 specification.

The -03 version of the WG draft is first of all a refresh; there are
no issues that we think need addressing, beyond the one for which
there is a suggestion in -03: The document now suggests an alternate
group/curve as an optional one besides X25519. The specific choice
of particular groups and algorithms is still up to the working group.

The -02 version of the WG draft took into account additional reviews,
and changed the document to update RFC 5448 (or rather, its
successor, [RFC9048]), changed the wording of the recommendation with
regards to the use of this extension, clarified the references to the
definition of X25519 and Curve25519, clarified the distinction to
ECDH methods that use partially static keys, and simplified the use
of AKA and USIM card terminology. Some editorial changes were also
made.

The -00 and -01 versions of the WG draft made no major changes, only
updates to some references.

The -05 version is merely a refresh while the draft was waiting for
WG adoption.

The -04 version of this draft made only editorial changes.

The -03 version of this draft changed the naming of various protocol
components, values, and notation to match with the use of ECDH in
ephemeral mode. The AT_KDF_FS negotiation process was clarified in
that exactly one key is ever sent in AT_KDF_ECDHE. The option of
checking for zero key values IN ECDHE was added. The format of the
actual key in AT_PUB_ECDHE was specified. Denial-of-service
considerations for the FS process have been updated. Bidding down
attacks against this extension itself are discussed extensively.
This version also addressed comments from reviewers, including the
August review from Mohit Sethi, and comments made during IETF-102
discussion.

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
uses
also uses a lot of material from [RFC4187] by J. Arkko and
H. Haverinen Haverinen, as well as [RFC5448] 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 GSMA,
and 3GPP groups for interesting discussions in this problem space.

Authors' Addresses

Jari Arkko
Ericsson
FI-02420 Jorvas
Finland
Email: jari.arkko@piuha.net

Karl Norrman
Ericsson
SE-16483 Stockholm
Sweden
Email: karl.norrman@ericsson.com

John Preuß Mattsson
Ericsson
SE-164 40 Kista
Sweden
Email: john.mattsson@ericsson.com