Internet DRAFT - draft-arkko-eap-aka-pfs
draft-arkko-eap-aka-pfs
Network Working Group J. Arkko
Internet-Draft K. Norrman
Intended status: Informational V. Torvinen
Expires: July 25, 2019 Ericsson
January 21, 2019
Perfect-Forward Secrecy for the Extensible Authentication Protocol
Method for Authentication and Key Agreement (EAP-AKA' PFS)
draft-arkko-eap-aka-pfs-04
Abstract
Many different attacks have been reported as part of revelations
associated with pervasive surveillance. Some of the reported attacks
involved compromising smart cards, 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.
This specification is an optional extension to the EAP-AKA'
authentication method which was defined in RFC 5448 (to be superseded
by draft-ietf-emu-rfc5448bis). The extension, when negotiated,
provides Perfect 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 all 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
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on July 25, 2019.
Copyright Notice
Copyright (c) 2019 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
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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_PFS . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. New Key Derivation Function . . . . . . . . . . . . . . . 14
6.4. ECDHE Groups . . . . . . . . . . . . . . . . . . . . . . 15
6.5. Message Processing . . . . . . . . . . . . . . . . . . . 15
6.5.1. EAP-Request/AKA'-Identity . . . . . . . . . . . . . . 15
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 . . . . . . . 17
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
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . 22
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9.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 24
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
Many different attacks have been reported as part of revelations
associated with pervasive surveillance. Some of the reported attacks
involved compromising smart cards, 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 [RFC5448] (to be superseded by
[I-D.ietf-emu-rfc5448bis]). The extension, when negotiated, provides
Perfect 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 all 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. As with other protocols, an active
attacker with access to the long-term key material will of course be
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able to attack all future communications, but risks detection,
particularly if done at scale.
Attacks against AKA authentication via compromising the long-term
secrets in the SIM cards have been an active discussion topic in many
contexts. Perfect 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.
The use of this extension is at the discretion of the authenticating
parties. It should be noted that PFS 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 perfect 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:
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o As noted above, no new credentials are needed; there is no change
to SIM cards.
o PFS 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
perfect forward secrecy being available for both EAP-TLS [RFC5216]
[I-D.mattsson-eap-tls13] and EAP-AKA' ensures that for many
practical systems perfect 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, 3rd
generation 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
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
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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 (and 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
[I-D.ietf-emu-rfc5448bis] 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 (U)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 [I-D.ietf-emu-rfc5448bis].
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 Perfect Forward Security (PFS)
[DOW1992] for the session key. If AKA would have provided PFS,
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 PFS for EAP-AKA' can be achieved by using an Elliptic
Curve Diffie-Hellman (ECDH) exchange [RFC7748]. In EAP-AKA' PFS this
exchange is run in an ephemeral manner, i.e., using temporary keys as
specified in [RFC8031] Section 2. This method is referred to as
ECDHE, where the last 'E' stands for Ephemeral.
The enhancements in the EAP-AKA' PFS 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
calculation of key material, providing new properties that are not
present in key material provided by EAP-AKA' in its original form.
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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_PFS attribute carries other PFS- |
| | related parameters. Both of these are skippable |
| | attributes that can be ignored if the peer does |
| | not support this extension. |
| +-------------------------------------------------+
| | | |
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| | EAP-Req/AKA'-Challenge | |
| | AT_RAND, AT_AUTN, AT_KDF,| |
| | AT_KDF_PFS, AT_KDF_INPUT,| |
| | AT_PUB_ECDHE, AT_MAC | |
| |<-------------------------| |
+-----------------------------------------------------+ |
| The peer checks if it wants to do the PFS 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_PFS 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 PFS 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 |
| | 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. |
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| +-------------------------------------------------+
| | | |
| | EAP-Success | |
| |<-------------------------| |
Figure 2: EAP-AKA' PFS 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. For Curve25519,
the length of this value is 32 bytes, encoded in binary as
specified [RFC7748] Section 6.1.
To retain the security of the keys, the sender SHALL generate a
fresh value for each run of the protocol.
6.2. AT_KDF_PFS
The AT_KDF_PFS indicates the used or desired key generation function,
if the Perfect 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
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the legacy KDF value for those EAP peers that cannot or do not want
to use this extension.
The format of the AT_KDF_PFS 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_PFS | Length | Key Derivation Function |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_KDF_PFS
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
[I-D.ietf-emu-rfc5448bis].
Servers MUST send one or more AT_KDF_PFS 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_PFS attributes, there is always only one AT_PUB_ECDHE
attribute.
Upon receiving a set of these attributes:
o If the peer supports and is willing to use the key derivation
function indicated by the first AT_KDF_PFS attribute, and is
willing and able to use the extension defined in this
specification, the function is taken into use without any further
negotiation.
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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 [I-D.ietf-emu-rfc5448bis] for AT_KDF, the peer sends
EAP-Response/AKA'-Challenge message that contains only one
attribute, AT_KDF_PFS 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_PFS 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_PFS
attribute.
Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF_PFS from
the peer, the server checks that the suggested AT_KDF_PFS value was
one of the alternatives in its offer. The first AT_KDF_PFS value in
the message from the server is not a valid alternative. If the peer
has replied with the first AT_KDF_PFS 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_PFS 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_PFS 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_PFS attributes without
having requested negotiation, the peer MUST behave as if AT_MAC had
been incorrect and fail the authentication.
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6.3. New Key Derivation Function
A new Key Derivation Function type is defined for "EAP-AKA' with
ECDHE and Curve25519", represented by value 1. It represents 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_PFS
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 [I-D.ietf-emu-rfc5448bis].
Key derivation in this extension produces exactly the same keys for
internal use within one authentication run as
[I-D.ietf-emu-rfc5448bis] 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_PFS 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' PFS"|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 2 of [RFC8031] and 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
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.
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The rest of computation proceeds as defined in Section 3.3 of
[I-D.ietf-emu-rfc5448bis].
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 [I-D.ietf-emu-rfc5448bis].
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 [I-D.ietf-emu-rfc5448bis] 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' PFS" is a twelve-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_PFS.
For "EAP-AKA' with ECDHE and Curve25519" the group is the Curve25519
group specified in [RFC8031].
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 [I-D.ietf-emu-rfc5448bis] or
[RFC4187] apply.
6.5.1. EAP-Request/AKA'-Identity
No changes, except that the AT_KDF_PFS 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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6.5.2. EAP-Response/AKA'-Identity
No changes, except that the AT_KDF_PFS 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
[I-D.ietf-emu-rfc5448bis]. 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_PFS 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_PFS
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_PFS, 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
[I-D.ietf-emu-rfc5448bis]. 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
[I-D.ietf-emu-rfc5448bis]. If the peer supports and is willing to
perform the extension specified in this protocol, and the server had
made a valid request involving the attributes specified in
Section 6.5.3, the peer responds per the rules specified below.
Otherwise, the peer responds as specified in [RFC4187] and
[I-D.ietf-emu-rfc5448bis] 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.
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The AT_MAC attribute MUST be included and checked as specified in
[I-D.ietf-emu-rfc5448bis]. 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_PFS or AT_PUB_ECDHE attributes
MUST NOT be added to this message. The appearance of these messages
in a received message MUST be ignored.
6.5.8. EAP-Response/AKA'-Authentication-Reject
No changes, except that the AT_KDF_PFS 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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6.5.9. EAP-Response/AKA'-Client-Error
No changes, except that the AT_KDF_PFS 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 [I-D.ietf-emu-rfc5448bis].
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 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.
This extension can provide assistance in situations where there is a
danger of attacks against the key material on SIM cards by
adversaries that can not or who are unwilling to mount active attacks
against large number of sessions. This extension is most useful when
used in a context where EAP keys are used without further mixing that
can provide Perfect 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 PFS 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
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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, PFS 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
PFS. Still, past sessions where PFS was in use remain protected.
Achieving PFS 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.
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_PFS 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.
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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 PFS will be provided. However, as noted above, an attacker
with these capabilities will in any case be able to impersonate
any party in the protocol and perform 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 PFS
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 (U)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 (U)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.
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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
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' PFS.
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
PFS related parameters (see Section 6.5.4). The same is true of
EAP-Response/AKA'-Challenge (see Section 6.5.4). This ensures
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that the parties need to show possession of the long-term secret
in some way, and only then will the PFS 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
control, and (b) identifiable for the purposes of blocking the
affected subscribers.
8. IANA Considerations
This extension of EAP-AKA' shares its attribute space and subtypes
with EAP-SIM [RFC4186], EAP-AKA [RFC4186], and EAP-AKA'
[I-D.ietf-emu-rfc5448bis].
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_PFS
(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
Curve25519" type (1, see Section 6.3) needs 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 Curve25519 [TBD BY IANA: THIS RFC]
2-65535 Unassigned
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>.
[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>.
[RFC8031] Nir, Y. and S. Josefsson, "Curve25519 and Curve448 for the
Internet Key Exchange Protocol Version 2 (IKEv2) Key
Agreement", RFC 8031, DOI 10.17487/RFC8031, December 2016,
<https://www.rfc-editor.org/info/rfc8031>.
[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>.
[I-D.ietf-emu-rfc5448bis]
Arkko, J., Lehtovirta, V., Torvinen, V., and P. Eronen,
"Improved Extensible Authentication Protocol Method for
3rd Generation Authentication and Key Agreement (EAP-
AKA')", draft-ietf-emu-rfc5448bis-04 (work in progress),
January 2019.
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>.
[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>.
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[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>.
[I-D.mattsson-eap-tls13]
Mattsson, J. and M. Sethi, "Using EAP-TLS with TLS 1.3",
draft-mattsson-eap-tls13-02 (work in progress), March
2018.
[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 -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_PFS 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 PFS 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.
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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 Mattson,
Mohit Sethi, Vesa Lehtovirta, Joseph Salowey, Kathleen Moriarty,
Zhang Fu, Bengt Sahlin, Ben Campbell, Prajwol Kumar Nakarmi, Goran
Rune, Tim Evans, Helena Vahidi Mazinani, Anand R. Prasad, and many
other people at the 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
Vesa Torvinen
Ericsson
Jorvas 02420
Finland
Email: vesa.torvinen@ericsson.com
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