Network Working Group J. Arkko
Internet-Draft K. Norrman
Updates: RFC5448 (if approved) V. Torvinen
Intended status: Informational J. Mattsson
Expires: January 12, 2023 Ericsson
July 11, 2022
Forward Secrecy for the Extensible Authentication Protocol Method for
Authentication and Key Agreement (EAP-AKA' FS)
draft-ietf-emu-aka-pfs-07
Abstract
Many different attacks have been reported as part of revelations
associated with pervasive surveillance. Some of the reported attacks
involved compromising the smart card supply chain, such as attacking
SIM card manufacturers and operators in an effort to compromise
shared secrets stored on these cards. Since the publication of those
reports, manufacturing and provisioning processes have gained much
scrutiny and have improved. However, the danger of resourceful
attackers for these systems is still a concern. Always assuming
breach such as key compromise and minimizing the impact of breach are
essential zero-trust principles.
This specification is an optional extension to the EAP-AKA'
authentication method which was defined in [RFC9048]. The extension,
when negotiated, provides Forward Secrecy 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 pre-
shared secret in a SIM card from being able to decrypt any past
communications. In addition, if the attacker stays merely a passive
eavesdropper, the extension prevents attacks against future sessions.
This forces attackers to use active attacks instead.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Design and Deployment Objectives . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. AKA . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. EAP-AKA' Protocol . . . . . . . . . . . . . . . . . . . . 6
3.3. Attacks Against Long-Term Shared Secrets in Smart Cards . 8
4. Requirements Language . . . . . . . . . . . . . . . . . . . . 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. New Key Derivation Functions . . . . . . . . . . . . . . 14
6.4. ECDHE Groups . . . . . . . . . . . . . . . . . . . . . . 15
6.5. Message Processing . . . . . . . . . . . . . . . . . . . 15
6.5.1. EAP-Request/AKA'-Identity . . . . . . . . . . . . . . 16
6.5.2. EAP-Response/AKA'-Identity . . . . . . . . . . . . . 16
6.5.3. EAP-Request/AKA'-Challenge . . . . . . . . . . . . . 16
6.5.4. EAP-Response/AKA'-Challenge . . . . . . . . . . . . . 16
6.5.5. EAP-Request/AKA'-Reauthentication . . . . . . . . . . 17
6.5.6. EAP-Response/AKA'-Reauthentication . . . . . . . . . 17
6.5.7. EAP-Response/AKA'-Synchronization-Failure . . . . . . 17
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 . . . . . . . . . . . . 18
6.5.11. EAP-Response/AKA'-Notification . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
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7.1. Identity Privacy . . . . . . . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . 23
9.2. Informative References . . . . . . . . . . . . . . . . . 24
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 25
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
Many different attacks have been reported as part of revelations
associated with pervasive surveillance. Some of the reported attacks
involved compromising the smart card supply chain, such as attacking
SIM card manufacturers and operators in an effort to compromise
shared secrets stored on these cards. Such attacks are conceivable,
for instance, during the manufacturing process of cards, or during
the transfer of cards and associated information to the operator.
Since the publication of reports about such attacks, manufacturing
and provisioning processes have gained much scrutiny and have
improved.
However, the danger of resourceful attackers attempting to gain
information about SIM cards is still a concern. They are a high-
value target and concern a large number of people. 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 unlawful
access to key material. While the better protection of manufacturing
and other processes is essential in protecting against this, there is
one question that we as protocol designers can ask. Is there
something that we can do to limit the consequences of attacks, should
they occur?
The authors want to provide a public specification of an extension
that helps defend against one aspect of pervasive surveillance. This
is important, given the large number of users such practices may
affect. It is also a stated goal of the IETF to ensure that we
understand the surveillance concerns related to IETF protocols and
take appropriate countermeasures [RFC7258]. This document does that
for EAP-AKA'.
This specification is an optional extension to the EAP-AKA'
authentication method [RFC9048]. While optional, the use of this
extension is RECOMMENDED.
The extension, when negotiated, provides Forward Secrecy for the
session key generated as a part of the authentication run in EAP-
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AKA'. This prevents an attacker who has gained access to the long-
term pre-shared secret in a SIM card from being able to decrypt any
past communications. In addition, if the attacker stays merely a
passive eavesdropper, the extension prevents attacks against future
sessions. This forces attackers to use active attacks instead. This
is beneficial, because active attacks demand much more resources to
launch, and can generally be detected much easier. 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. The attacker is
forced to attempt to exfiltrate key material, if it can, on a
continuous basis, as opposed to learning it once [RFC7624].
Attacks against AKA authentication via compromising the long-term
secrets in the SIM cards have been an active discussion topic in many
contexts. Forward secrecy is on the list of features for the next
release of 3GPP (5G Phase 2), and this document provides a basis for
providing this feature in a particular fashion.
It should also be noted that 5G network architecture 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
potential to improve security for large number of users.
2. Protocol Design and Deployment Objectives
This extension specified here re-uses 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 maintains the interface between the Universal
Subscriber Identification Module (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], and uses much of the same material, but 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 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 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.
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The use of this extension is at the discretion of the authenticating
parties. It should be noted that FS and defenses against passive
attacks are by no means a panacea, but they can provide a partial
defense that increases the cost and risk associated with pervasive
surveillance.
While adding forward secrecy to the existing mobile network
infrastructure can be done in multiple different ways, the authors
believe that the approach chosen here is relatively easily
deployable. In particular:
o As noted above, no new credentials are needed; there is no change
to SIM cards.
o FS property can be incorporated into any current or future system
that supports EAP, without changing any network functions beyond
the EAP endpoints.
o Key generation happens at the endpoints, enabling 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).
o While EAP-AKA' is just one EAP method, for practical purposes
forward secrecy being available for both EAP-TLS [RFC5216]
[RFC9190] and EAP-AKA' ensures that for many practical systems
forward secrecy can be enabled for either all or significant
fraction of users.
3. Background
3.1. AKA
AKA is based on challenge-response mechanisms and symmetric
cryptography. AKA typically runs in a UMTS Subscriber Identity
Module (USIM) or a CDMA2000 (Removable) User Identity Module
((R)UIM). In contrast with its earlier GSM counterparts, AKA
provides long key lengths and mutual authentication.
AKA works in the following manner:
o The identity module and the home environment have agreed on a
secret key beforehand.
o The actual authentication process starts by having the home
environment produce an authentication vector, based on the secret
key and a sequence number. The authentication vector contains a
random part RAND, an authenticator part AUTN used for
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authenticating the network to the identity module, an expected
result part XRES, a 128-bit session key for integrity check IK,
and a 128-bit session key for encryption CK.
o The authentication vector is passed to the serving network, which
uses it to authenticate the device.
o The RAND and the AUTN are delivered to the identity module.
o The identity module verifies the AUTN, again based on the secret
key and the sequence number. If this process is successful (the
AUTN is valid and the sequence number used to generate AUTN is
within the correct range), the identity module produces an
authentication result RES and sends it to the serving network.
o The serving network verifies the correct result from the identity
module. If the result is correct, IK and CK can be used to
protect further communications between the identity module and the
home environment.
3.2. EAP-AKA' Protocol
When AKA are embedded into EAP, the authentication on the network
side is moved to the home environment; the serving network performs
the role of a pass-through authenticator. Figure 1 describes the
basic flow in the EAP-AKA' authentication process. The definition of
the full protocol behaviour, along with the definition of attributes
AT_RAND, AT_AUTN, AT_MAC, and AT_RES can be found in [RFC9048] and
[RFC4187].
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Peer Server
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's Network Access Identifier, NAI) |
|------------------------------------------------------>|
| +-------------------------------------------------+
| | Server determines the network name and ensures |
| | that the given access network is authorized to |
| | use the claimed name. The server then runs the |
| | AKA' algorithms generating RAND and AUTN, |
| | derives session keys from CK' and IK'. RAND and |
| | AUTN are sent as AT_RAND and AT_AUTN attributes,|
| | whereas the network name is transported in the |
| | AT_KDF_INPUT attribute. AT_KDF signals the used |
| | key derivation function. The session keys are |
| | used in creating the AT_MAC attribute. |
| +-------------------------------------------------+
| EAP-Request/AKA'-Challenge |
| (AT_RAND, AT_AUTN, AT_KDF, AT_KDF_INPUT, AT_MAC)|
|<------------------------------------------------------|
+-----------------------------------------------------+ |
| The peer determines what the network name should be,| |
| based on, e.g., what access technology it is using.| |
| The peer also retrieves the network name sent by | |
| the network from the AT_KDF_INPUT attribute. The | |
| two names are compared for discrepancies, and if | |
| necessary, the authentication is aborted. Otherwise,| |
| the network name from AT_KDF_INPUT attribute is | |
| used in running the AKA' algorithms, verifying AUTN | |
| from AT_AUTN and MAC from AT_MAC attributes. The | |
| peer then generates RES. The peer also derives | |
| session keys from CK'/IK'. The AT_RES and AT_MAC | |
| attributes are constructed. | |
+-----------------------------------------------------+ |
| EAP-Response/AKA'-Challenge |
| (AT_RES, AT_MAC) |
|------------------------------------------------------>|
| +-------------------------------------------------+
| | Server checks the RES and MAC values received |
| | in AT_RES and AT_MAC, respectively. Success |
| | requires both to be found correct. |
| +-------------------------------------------------+
| EAP-Success |
|<------------------------------------------------------|
Figure 1: EAP-AKA' Authentication Process
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3.3. Attacks Against Long-Term Shared Secrets in Smart Cards
Current 3GPP systems use SIM pre-shared key based protocols and
Authentication and Key Agreement (AKA) to authenticate subscribers.
The general security properties and potential vulnerabilities of AKA
and EAP-AKA' are discussed in [RFC9048].
An important vulnerability in that discussion relates to the recent
reports of compromised long term pre-shared keys used in AKA
[Heist2015]. These attacks are not specific to AKA or EAP-AKA', as
all security systems fail at least to some extent if key material is
stolen. However, the reports indicate a need to look into solutions
that can operate at least to an extent under these types of attacks.
It is noted in [Heist2015] that some security can be retained even in
the face of the attacks by providing Forward Secrecy (FS) [DOW1992]
for the session key. If AKA would have provided FS, compromising the
pre-shared key would not be sufficient to perform passive attacks;
the attacker is, in addition, forced to be a Man-In-The-Middle (MITM)
during the AKA run and subsequent communication between the parties.
4. 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.
5. Protocol Overview
Introducing FS for EAP-AKA' can be achieved by using an Elliptic
Curve Diffie-Hellman (ECDH) exchange [RFC7748]. In EAP-AKA' FS this
exchange is run in an ephemeral manner, i.e., both sides generate
temporary keys as specified in [RFC7748]. This method is referred to
as ECDHE, where the last 'E' stands for Ephemeral. The two intially
registered elliptic curves and their wire format is 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 can ignore.
The purpose of the protocol is to achieve mutual authentication
between the EAP server and peer, and to establish keying material for
secure communication between the two. This document specifies the
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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 our goal has
been to not require new infrastructure or credentials, the flow
diagrams also show the conceptual interaction with the USIM card and
the 3GPP authentication server (HSS). The details of those
interactions are outside the scope of this document, however, and the
reader is referred to the 3GPP specifications .
USIM Peer Server HSS
| | | |
| | EAP-Req/Identity | |
| |<-------------------------| |
| | | |
| | EAP-Resp/Identity | |
| |------------------------->| |
| | | |
| +-------------------------------------------------+
| | Server now has an identity for the peer. |
| | The server then asks the help of |
| | HSS to run AKA algorithms, generating RAND, |
| | AUTN, XRES, CK, IK. Typically, the HSS performs |
| | the first part of key 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' |
| | |<------------|
| | | |
| +-------------------------------------------------+
| | Server now has the needed authentication vector.|
| | It generates an ephemeral key pair, 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 |
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| | 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 5448 (draft-ietf-emu-rfc5448bis) 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 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 specification is used, the keys |
| | generated from EAP-AKA' are based on both |
| | CK/IK as well as the ECDHE value. Even if there |
| | was an attacker who held the long-term secret |
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| | keys, only an active attacker could have |
| | determined the generated session keys; in basic |
| | EAP-AKA' the 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 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 TBA1 BY IANA.
Length
The length of the attribute, set as other attributes in EAP-AKA
[RFC4187].
Value
This value is the sender's ECDHE public value. It is calculated
as follows:
* For X25519/Curve25519, the length of this value is 32 bytes,
encoded in binary as specified [RFC7748] Section 6.1.
* For P-256, the length of this value is 33 bytes, encoded in
binary as specified in [FIPS186-4], using the compressed form
from Section 2.7.1 of [SEC2].
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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 indicates the used or desired key generation function,
if the Forward Secrecy extension is taken into use. It will also at
the same time indicate the used or desired ECDHE group. A new
attribute is needed to carry this information, as AT_KDF carries the
legacy KDF value for those EAP peers that cannot or do not want to
use this extension.
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 | Key Derivation Function |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_KDF_FS
This is set to TBA2 BY IANA.
Length
The length of the attribute, MUST be set to 1.
Key Derivation Function
An enumerated value representing the key derivation function 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 Key Derivation Function 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, 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:
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o If the peer supports and is willing to use the key derivation
function indicated by the first AT_KDF_FS attribute, and is
willing and able to use the extension defined in this
specification, the function is taken into use without any further
negotiation.
o 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 [RFC9048] for AT_KDF, the peer sends 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 AUTN had been incorrect and failing
authentication (see Figure 3 of [RFC4187]). 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 key derivation function; see below for
further information).
o If the peer does not recognize the extension defined in this
specification or is unwilling to use it, it ignores the AT_KDF_FS
attribute.
Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF_FS 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
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 [RFC4187].
Otherwise, the server re-sends the EAP-Response/AKA'-Challenge
message, but adds the selected alternative to the beginning of the
list of AT_KDF_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 key derivation function is the only legal situation where
such duplication may occur.
When the peer receives the new EAP-Request/AKA'-Challenge message, it
MUST check that the requested change, and only the requested change
occurred in the list of AT_KDF_FS attributes. If yes, it continues.
If not, it behaves as if AT_MAC had been incorrect and fails the
authentication. If the peer receives multiple EAP-Request/AKA'-
Challenge messages with differing AT_KDF_FS attributes without having
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requested negotiation, the peer MUST behave as if AT_MAC had been
incorrect and fail the authentication.
6.3. New Key Derivation Functions
Two new Key Derivation Function 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 represent a particular
choice of key derivation function and at the same time selects an
ECDHE group to be used.
The Key Derivation Function type value is only used in the AT_KDF_FS
attribute, and should not be confused with the different range of key
derivation functions that can be represented in the AT_KDF attribute
as defined in [RFC9048].
Key derivation in this extension produces exactly the same keys for
internal use within one authentication run as [RFC9048] EAP-AKA'
does. For instance, 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 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 Key Derivation Function field in the AT_KDF_FS attribute is
set to 1 and the Key Derivation Function field in the AT_KDF
attribute is also set to 1, the Master Key (MK) is derived as follows
below.
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]
Where SHARED_SECRET is the shared secret computed via ECDHE, as
specified in Section 6.1 of [RFC7748].
Both the peer and the server MAY check for zero-value shared secret
as specified in Section 6.1 of [RFC7748]. If such checking is
performed and the SHARED_SECRET has a zero value, both parties MUST
behave as if the current EAP-AKA' authentication process starts again
from the beginning.
Note: The way that shared secret is tested for zero can, if
performed inappropriately, provide an ability for attackers to
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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.
The rest of 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].
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 to use of particular key derivation function in AT_KDF_FS.
For "EAP-AKA' with ECDHE and 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" the group is the NIST P-256 group
(SEC group secp256r1), specified in [FIPS186-4]. The support for
this group is OPTIONAL.
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
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otherwise specified here, the rules from [RFC9048] or [RFC4187]
apply.
6.5.1. EAP-Request/AKA'-Identity
No changes, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST
NOT be added to this message. The appearance of these messages in a
received message MUST be ignored.
6.5.2. EAP-Response/AKA'-Identity
No changes, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST
NOT be added to this message. The appearance of these messages in a
received message MUST be ignored.
6.5.3. EAP-Request/AKA'-Challenge
The 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 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 them as
if neither one was sent, and the assume that the extension defined in
this specification 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 [RFC4187].
When processing this message, the peer MUST process AT_RAND, AT_AUTN,
AT_KDF_FS, AT_PUB_ECDHE before processing other attributes. Only if
these attributes are verified to be valid, the peer derives keys and
verifies AT_MAC. If the peer is unable or unwilling to perform the
extension specified in this document, it proceeds as defined in
[RFC9048]. Finally, the operation in case an error occurs is
specified in Section 6.3.1. of [RFC4187].
6.5.4. EAP-Response/AKA'-Challenge
The peer sends EAP-Response/AKA'-Challenge in response to a valid
EAP-Request/AKA'-Challenge message, as specified by [RFC4187] and
[RFC9048]. If the peer supports and is willing to perform the
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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 [RFC4187] and [RFC9048] 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 AUTN had
been incorrect and fails the authentication.
The AT_MAC attribute MUST be included and checked as specified in
[RFC9048]. In 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
[RFC4187]. When processing this message, the Server MUST process
AT_RES before processing other attributes. Only if these attribute
is verified to be valid, the Server derives keys and verifies AT_MAC.
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 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 [RFC4187].
6.5.5. EAP-Request/AKA'-Reauthentication
No changes, but note that the re-authentication process uses the keys
generated in the original EAP-AKA' authentication, which, if the
extension specified in this documents is in use, employs key material
from the Diffie-Hellman procedure.
6.5.6. EAP-Response/AKA'-Reauthentication
No changes, 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, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST
NOT be added to this message. The appearance of these messages in a
received message MUST be ignored.
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6.5.8. EAP-Response/AKA'-Authentication-Reject
No changes, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST
NOT be added to this message. The appearance of these messages in a
received message MUST be ignored.
6.5.9. EAP-Response/AKA'-Client-Error
No changes, except that the AT_KDF_FS or AT_PUB_ECDHE attributes MUST
NOT be added to this message. The appearance of these messages in a
received message MUST be ignored.
6.5.10. EAP-Request/AKA'-Notification
No changes.
6.5.11. EAP-Response/AKA'-Notification
No changes.
7. Security Considerations
This section deals only with the changes to security considerations
as they differ from EAP-AKA', or as new information has been gathered
since the publication of [RFC9048].
The possibility of attacks against key storage offered in SIM or
other smart cards has been a known threat. But as the discussion in
Section 3.3 shows, the likelihood of practically feasible attacks
such as breaches in the smart card supply chain has increased. Many
of these attacks can be best dealt with improved processes, e.g.,
limiting the access to the key material within the factory or
personnel, etc. But not all attacks can be entirely ruled out for
well-resourced adversaries, irrespective of what the technical
algorithms and protection measures are. Always assuming breach such
as key compromise and minimizing the impact of breach are essential
zero-trust principles.
If a mechanism without forward secrecy such as (5G-AKA, EAP-AKA') is
used the effects of key compromise are devastating. The serious
consequences of breach somewhere in the supply chain or after
delivery that are possible when 5G-AKA or EAP-AKA' is used but not
when something with forward secrecy like EAP-AKA-FS is used are: 1.
A passive attacker can eavesdrop (decrypt) all future 5G
communication (control and user plane both directions), 2. A passive
attacker can decrypt 5G communication that they previously recorded
in the past (control and user plane both directions), and 3. An
active attacker can impersonate UE and Network and inject messages in
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an ongoing 5G connection between the real UE and the real network
(control and user plane both directions).
Best practice security today is to mandate forward secrecy (as is
done in WPA3, EAP-TLS 1.3, EAP-TTLS 1.3, etc). It is RECOMMENDED to
long term completely phase out AKA without forward secrecy.
This extension can provide assistance in situations where there is a
danger of attacks against the key material on SIM cards by
adversaries that cannot or who are unwilling to mount active attacks
against large number of sessions. The extension also provides
protection against active attacks as they are forced to be a Man-In-
The-Middle (MITM) during the AKA run and subsequent communication
between the parties. Without forward secrecy an an active attacker
that has compromised the long-term key can inject messages in an
connection betwen the real Peer and the real server without being a
man-in-the-middle. This extension is most useful when used in a
context where EAP keys are used without further mixing that can
provide Forward Secrecy. For instance, when used with IKEv2
[RFC7296], the session keys produced by IKEv2 have this property, so
better characteristics of EAP keys is not that useful. However,
typical link layer usage of EAP does not involve running Diffie-
Hellman, so using EAP to authenticate access to a network is one
situation where the extension defined in this document can be
helpful.
This extension generates keying 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 forgotten, 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. Of course, as with other protocols, if
the attacker has learned the keys and does become an active attacker,
there is no protection that that can be provided for future sessions.
Among other things, such an active attacker can impersonate any
legitimate endpoint in EAP-AKA', become a MITM in EAP-AKA' or the
extension defined in this document, retrieve all keys, or turn off
FS. Still, past sessions where FS was in use remain protected.
Achieving FS requires that when a connection is closed, each endpoint
MUST forget not only the ephemeral keys used by the connection but
also any information that could be used to recompute those keys.
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Using EAP-AKA' FS once provides forward secrecy. Forward secrecy
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 [RFC7624], forward secrecy 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).
The following security properties of EAP-AKA' are impacted through
this extension:
Protected ciphersuite negotiation
EAP-AKA' has a negotiation mechanism for selecting the key
derivation functions, 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 key derivation function within the extension
The negotiation mechanism allows changing the offered key
derivation function, but the change is visible in the final
EAP- Request/AKA'-Challenge message that the server sends to
the peer. This message is authenticated via the AT_MAC
attribute, and carries both the chosen alternative and the
initially offered list. The peer refuses to accept a change it
did not initiate. As a result, both parties are aware that a
change is being made and what the original offer was.
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.
Except of course, if the adversary holds the long-term shared
secret and is willing to engage in an active attack. Such an
attack can, for instance, 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
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any party in the protocol and perform MITM 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 a MITM on
each AKA run and subsequent communication, which makes mass
surveillance more laborous.
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 peform 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 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 key material that is based on the Diffie-
Hellman keys, yet bound to the authentication through the SIM
card. This means that subsequent payload communications between
the parties are protected with keys that are not solely based on
information in the clear (such as the RAND) and information
derivable from the long-term shared secrets on the SIM card. As a
result, if anyone successfully recovers shared secret information,
they are unable to decrypt communications protected by the keys
generated through this extension. Note that the recovery of
shared secret information could occur either before or after the
time that the protected communications are used. When this
extension is used, communications at time t0 can be protected if
at some later time t1 an adversary learns of long-term shared
secret and has access to a recording of the encrypted
communications.
Obviously, this extension is still vulnerable to attackers that
are willing to perform an active attack and who at the time of the
attack have access to the long-term shared secret.
This extension does not change the properties related to re-
authentication. No new Diffie-Hellman run is performed during the
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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 continue to be equally safe
against expose of the long-term secrets as the original
authentication.
In addition, it is worthwhile to discuss Denial-of-Service 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 Denial-of-Service attacks are always possible, the following
factors help mitigate the concerns relating to public key
cryptography and EAP-AKA' FS.
o In 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.
o This specification is constructed so that a separation between the
USIM and Peer on client side and the Server and HSS on network
side is possible. This ensures that the most sensitive (or
legacy) system components can not be the target of the attack.
For instance, EAP-AKA' and public key cryptography takes place in
the phone and not the low-power SIM card.
o 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.
o Finally, this memo specifies an order in which computations and
checks must occur. When processing the EAP-Request/AKA'-Challenge
message, for instance, 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
EAP-Response/AKA'-Challenge (see Section 6.5.4). This ensures
that the parties need to show possession of the long-term secret
in some way, and only then will the FS calculations become active.
This limits the Denial-of-Service 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) limited in terms of subscribers they
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control, and (b) identifiable for the purposes of blocking the
affected subscribers.
7.1. Identity Privacy
Best practice privacy today is to mandate client identity protection
as is done in EAP-TLS 1.3, EAP-TTLS 1.3, etc. A client supporting
EAP-AKA' FS MUST NOT send its username (or any other permanent
identifiers) in cleartext in the Identity Response (or any message
used instead of the Identity Response).
8. IANA Considerations
This extension of EAP-AKA' shares its attribute space and subtypes
with EAP-SIM [RFC4186], EAP-AKA [RFC4186], and EAP-AKA' [RFC9048].
Two new Attribute Type value (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 EAP-AKA and EAP-SIM Parameters registry under
Attribute Types.
Also, a new registry should be created to represent Diffie-Hellman
Key Derivation Function 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 assigned, along with one reserved value. The initial
contents of this namespace are therefore as below; new values can be
created through the Specification Required policy [RFC8126].
Value Description Reference
------ ------------------------------------ ---------------
0 Reserved [TBD BY IANA: THIS RFC]
1 EAP-AKA' with ECDHE and X25519 [TBD BY IANA: THIS RFC]
2 EAP-AKA' with ECDHE and P-256 [TBD BY IANA: THIS RFC]
3-65535 Unassigned [TBD BY IANA: THIS RFC]
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>.
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[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>.
[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>.
[FIPS186-4]
NIST, , "Digital Signature Standard (DSS)", July 2013.
[SEC2] Certicom Research, , "SEC 2: Recommended Elliptic Curve
Domain Parameters", September 2000.
9.2. Informative References
[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>.
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[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>.
[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] PreuĂŸ 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., Naslund, M., and B. Sahlin, "A
USIM compatible 5G AKA protocol with perfect forward
secrecy", August 2015 in Proceedings of the TrustCom 2015,
IEEE.
[Heist2015]
Scahill, J. and J. Begley, "The great SIM heist", February
2015, in https://firstlook.org/theintercept/2015/02/19/
great-sim-heist/ .
[DOW1992] Diffie, W., vanOorschot, P., and M. Wiener,
"Authentication and Authenticated Key Exchanges", June
1992, in Designs, Codes and Cryptography 2 (2): pp.
107-125.
Appendix A. Change Log
The -07 version of the WG draft has the following changes:
o The impact of forward secrecy explanation has been improved in the
abstract and security considerations.
o 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.
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o The draft does no longer refer to issues within the smart cards
but rather the smart card supply chain.
o The rationale for chosen algorithms is explained.
o 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:
o The draft now uses "forward secrecy" terminology and references
RFC 7624 per recommendations on mailing list discussion.
o There's been mailing list disccussion 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 specification 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 SIM 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.
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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.
Appendix B. 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. Naslund, and B. Sahlin. This
document uses also a lot of material from [RFC4187] by J. Arkko and
H. Haverinen as well as [RFC5448] by J. Arkko, V. Lehtovirta, and
P. Eronen.
The authors would also like to thank Tero Kivinen, John Mattsson,
Mohit Sethi, Vesa Lehtovirta, Russ Housley, Sean Turner, Eliot Lear,
Joseph Salowey, Kathleen Moriarty, Zhang Fu, Bengt Sahlin, Ben
Campbell, Prajwol Kumar Nakarmi, Goran Rune, Tim Evans, Helena Vahidi
Mazinani, Anand R. Prasad, Rene Struik, and many other people at the
IETF, GSMA and 3GPP groups for interesting discussions in this
problem space.
Authors' Addresses
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
Karl Norrman
Ericsson
Stockholm 16483
Sweden
Email: karl.norrman@ericsson.com
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Vesa Torvinen
Ericsson
Jorvas 02420
Finland
Email: vesa.torvinen@ericsson.com
John Mattsson
Ericsson
Kista 164 40
Sweden
Email: john.mattsson@ericsson.com
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