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draft-ietf-emu-rfc5448bis-03.txt
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draft-ietf-emu-rfc5448bis-03.txt
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Network Working Group J. Arkko
Internet-Draft V. Lehtovirta
Obsoletes: 5448 (if approved) V. Torvinen
Updates: 4187 (if approved) Ericsson
Intended status: Informational P. Eronen
Expires: April 22, 2019 Nokia
October 19, 2018
Improved Extensible Authentication Protocol Method for 3rd Generation
Authentication and Key Agreement (EAP-AKA')
draft-ietf-emu-rfc5448bis-03
Abstract
This specification defines an EAP method, EAP-AKA', a small revision
of the EAP-AKA method. EAP-AKA' provides a key derivation function
that binds the keys derived within the method to the name of the
access network. The key derivation mechanism has been defined in the
3rd Generation Partnership Project (3GPP). This specification allows
its use in EAP in an interoperable manner. In addition, EAP-AKA'
employs SHA-256 instead of SHA-1.
This specification also updates RFC 4187 EAP-AKA to prevent bidding
down attacks from EAP-AKA'.
This version of the EAP-AKA' specification provides updates to
specify the protocol behaviour for 5G deployments as well.
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."
This Internet-Draft will expire on April 22, 2019.
Arkko, et al. Expires April 22, 2019 [Page 1]
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Copyright Notice
Copyright (c) 2018 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
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. EAP-AKA' . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. AT_KDF_INPUT . . . . . . . . . . . . . . . . . . . . . . 8
3.2. AT_KDF . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Key Generation . . . . . . . . . . . . . . . . . . . . . 13
3.4. Hash Functions . . . . . . . . . . . . . . . . . . . . . 15
3.4.1. PRF' . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4.2. AT_MAC . . . . . . . . . . . . . . . . . . . . . . . 15
3.4.3. AT_CHECKCODE . . . . . . . . . . . . . . . . . . . . 15
4. Bidding Down Prevention for EAP-AKA . . . . . . . . . . . . . 16
5. Peer Identities . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Username Types in EAP-AKA' Identities . . . . . . . . . . 18
5.2. Generating Pseudonyms and Fast Re-Authentication
Identities . . . . . . . . . . . . . . . . . . . . . . . 18
5.3. Identifier Usage in 5G . . . . . . . . . . . . . . . . . 19
5.3.1. Key Derivation . . . . . . . . . . . . . . . . . . . 20
5.3.2. EAP Identity Response and EAP-AKA' AT_IDENTITY
Attribute . . . . . . . . . . . . . . . . . . . . . . 21
6. Exported Parameters . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.2. Discovered Vulnerabilities . . . . . . . . . . . . . . . 28
7.3. Pervasive Monitoring . . . . . . . . . . . . . . . . . . 29
7.4. Security Properties of Binding Network Names . . . . . . 30
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
8.1. Type Value . . . . . . . . . . . . . . . . . . . . . . . 31
8.2. Attribute Type Values . . . . . . . . . . . . . . . . . . 31
8.3. Key Derivation Function Namespace . . . . . . . . . . . . 32
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . 32
Arkko, et al. Expires April 22, 2019 [Page 2]
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9.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Changes from RFC 5448 . . . . . . . . . . . . . . . 36
Appendix B. Changes from RFC 4187 to RFC 5448 . . . . . . . . . 37
Appendix C. Changes from Previous Version of This Draft . . . . 37
Appendix D. Importance of Explicit Negotiation . . . . . . . . . 38
Appendix E. Test Vectors . . . . . . . . . . . . . . . . . . . . 39
Appendix F. Contributors . . . . . . . . . . . . . . . . . . . . 43
Appendix G. Acknowledgments . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
This specification defines an Extensible Authentication Protocol
(EAP)[RFC3748] method, EAP-AKA', a small revision of the EAP-AKA
method originally defined in [RFC4187]. EAP-AKA' provides a new key
derivation function, specified in [TS-3GPP.33.402]. This function
binds the keys derived within the method to the name of the access
network. This limits the effects of compromised access network nodes
and keys. This specification defines the EAP encapsulation for AKA
when the new key derivation mechanism is in use.
3GPP has defined a number of applications for the revised AKA
mechanism, some based on native encapsulation of AKA over 3GPP radio
access networks and others based on the use of EAP.
For making the new key derivation mechanisms usable in EAP-AKA,
additional protocol mechanisms are necessary. Given that RFC 4187
calls for the use of CK (the encryption key) and IK (the integrity
key) from AKA, existing implementations continue to use these. Any
change of the key derivation must be unambiguous to both sides in the
protocol. That is, it must not be possible to accidentally connect
old equipment to new equipment and get the key derivation wrong or
attempt to use wrong keys without getting a proper error message.
The change must also be secure against bidding down attacks that
attempt to force the participants to use the least secure mechanism.
This specification therefore introduces a variant of the EAP-AKA
method, called EAP-AKA'. This method can employ the derived keys CK'
and IK' from the 3GPP specification and updates the used hash
function to SHA-256 [FIPS.180-4]. But it is otherwise equivalent to
RFC 4187. Given that a different EAP method type value is used for
EAP-AKA and EAP-AKA', a mutually supported method may be negotiated
using the standard mechanisms in EAP [RFC3748].
Note: Appendix D explains why it is important to be explicit about
the change of semantics for the keys, and why other approaches
would lead to severe interoperability problems.
Arkko, et al. Expires April 22, 2019 [Page 3]
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This version of the EAP-AKA' specification obsoletes RFC 5448. The
changes are as follows:
o Update the reference on how the Network Name field is constructed
in the protocol. The update ensures that EAP-AKA' is compatible
with 5G deployments. RFC 5448 referred to the Release 8 version
of [TS-3GPP.24.302] and this update points to the first 5G
version, Release 15.
o Specify how EAP and EAP-AKA' use identifiers in 5G. Additional
identifiers are introduced in 5G, and for interoperability, it is
necessary that the right identifiers are used as inputs in the key
generation. In addition, for identity privacy it is important
that when privacy-friendly identifiers in 5G are used, no
trackable, permanent identifiers are passed in EAP-AKA' either.
o Specify session identifiers and other exported parameters, as
those were not specified in [RFC5448] despite requirements set
forward in [RFC5247] to do so. Also, while [RFC5247] specified
session identifiers for EAP-AKA, it only did so for the full
authentication case, not for the case of fast re-authentication.
o Update the requirements on generating pseudonym usernames and fast
re-authentication identities to ensure identity privacy.
o Describe what has been learned about any vulnerabilities in AKA or
EAP-AKA'.
o Describe the privacy and pervasive monitoring considerations
related to EAP-AKA'.
Some of the updates are small. For instance, for the first update,
the reference update does not change the 3GPP specification number,
only the version. But this reference is crucial in correct
calculation of the keys resulting from running the EAP-AKA' method,
so an update of the RFC with the newest version pointer may be
warranted.
Note: This specification refers only to the 5G specifications.
Any further update that affects, for instance, key generation is
something that EAP-AKA' implementations should take into account.
Upon such updates there will be a need to both update the
specification and the implementations.
It is an explicit non-goal of this draft to include any other
technical modifications, addition of new features or other changes.
The EAP-AKA' base protocol is stable and needs to stay that way. If
Arkko, et al. Expires April 22, 2019 [Page 4]
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there are any extensions or variants, those need to be proposed as
standalone extensions or even as different authentication methods.
The rest of this specification is structured as follows. Section 3
defines the EAP-AKA' method. Section 4 adds support to EAP-AKA to
prevent bidding down attacks from EAP-AKA'. Section 5 specifies
requirements regarding the use of peer identities, including how how
EAP-AKA' identifiers are used in 5G context. Section 6 specifies
what parameters EAP-AKA' exports out of the method. Section 7
explains the security differences between EAP-AKA and EAP-AKA'.
Section 8 describes the IANA considerations and Appendix A and
Appendix B explains what updates to RFC 5448 EAP-AKA' and RFC 4187
EAP-AKA have been made in this specification. Appendix D explains
some of the design rationale for creating EAP-AKA' Finally,
Appendix E provides test vectors.
Editor's Note: The publication of this RFC depends on its
normative references [TS-3GPP.24.302] and [TS-3GPP.33.501]
reaching a stable status for Release 15, as indicated by 3GPP.
This is expected to happen shortly. The RFC Editor should check
with the 3GPP liaisons that this has happened. RFC Editor: Please
delete this note upon publication of this specification as an RFC.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. EAP-AKA'
EAP-AKA' is a new EAP method that follows the EAP-AKA specification
[RFC4187] in all respects except the following:
o It uses the Type code 50, not 23 (which is used by EAP-AKA).
o It carries the AT_KDF_INPUT attribute, as defined in Section 3.1,
to ensure that both the peer and server know the name of the
access network.
o It supports key derivation function negotiation via the AT_KDF
attribute (Section 3.2) to allow for future extensions.
o It calculates keys as defined in Section 3.3, not as defined in
EAP-AKA.
Arkko, et al. Expires April 22, 2019 [Page 5]
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o It employs SHA-256, not SHA-1 [FIPS.180-4] (Section 3.4).
Figure 1 shows an example of the authentication process. Each
message AKA'-Challenge and so on represents the corresponding message
from EAP-AKA, but with EAP-AKA' Type code. The definition of these
messages, along with the definition of attributes AT_RAND, AT_AUTN,
AT_MAC, and AT_RES can be found in [RFC4187].
Arkko, et al. Expires April 22, 2019 [Page 6]
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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, and |
| | derives session keys from CK' and IK'. RAND and |
| | AUTN are sent as AT_RAND and AT_AUTN attributes, |
| | whereas the network name is transported in the |
| | AT_KDF_INPUT attribute. AT_KDF signals the used |
| | key derivation function. The session keys are |
| | used in creating the AT_MAC attribute. |
| +--------------------------------------------------+
| EAP-Request/AKA'-Challenge |
| (AT_RAND, AT_AUTN, AT_KDF, AT_KDF_INPUT, AT_MAC)|
|<-------------------------------------------------------|
+------------------------------------------------------+ |
| The peer determines what the network name should be, | |
| based on, e.g., what access technology it is using. | |
| The peer also retrieves the network name sent by | |
| the network from the AT_KDF_INPUT attribute. The | |
| two names are compared for discrepancies, and if | |
| necessary, the authentication is aborted. Otherwise,| |
| the network name from AT_KDF_INPUT attribute is | |
| used in running the AKA' algorithms, verifying AUTN | |
| from AT_AUTN and MAC from AT_MAC attributes. The | |
| peer then generates RES. The peer also derives | |
| session keys from CK'/IK'. The AT_RES and AT_MAC | |
| attributes are constructed. | |
+------------------------------------------------------+ |
| EAP-Response/AKA'-Challenge |
| (AT_RES, AT_MAC) |
|------------------------------------------------------->|
| +--------------------------------------------------+
| | Server checks the RES and MAC values received |
| | in AT_RES and AT_MAC, respectively. Success |
| | requires both to be found correct. |
| +--------------------------------------------------+
| EAP-Success |
|<-------------------------------------------------------|
Figure 1: EAP-AKA' Authentication Process
Arkko, et al. Expires April 22, 2019 [Page 7]
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EAP-AKA' can operate on the same credentials as EAP-AKA and employ
the same identities. However, EAP-AKA' employs different leading
characters than EAP-AKA for the conventions given in Section 4.1.1 of
[RFC4187] for International Mobile Subscriber Identifier (IMSI) based
usernames. EAP-AKA' MUST use the leading character "6" (ASCII 36
hexadecimal) instead of "0" for IMSI-based permanent usernames. All
other usage and processing of the leading characters, usernames, and
identities is as defined by EAP-AKA [RFC4187]. For instance, the
pseudonym and fast re-authentication usernames need to be constructed
so that the server can recognize them. As an example, a pseudonym
could begin with a leading "7" character (ASCII 37 hexadecimal) and a
fast re-authentication username could begin with "8" (ASCII 38
hexadecimal). Note that a server that implements only EAP-AKA may
not recognize these leading characters. According to Section 4.1.4
of [RFC4187], such a server will re-request the identity via the EAP-
Request/AKA-Identity message, making obvious to the peer that EAP-AKA
and associated identity are expected.
3.1. AT_KDF_INPUT
The format of the AT_KDF_INPUT attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_KDF_INPUT | Length | Actual Network Name Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Network Name .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_KDF_INPUT
This is set to 23.
Length
The length of the attribute, calculated as defined in [RFC4187],
Section 8.1.
Actual Network Name Length
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This is a 2 byte actual length field, needed due to the
requirement that the previous field is expressed in multiples of 4
bytes per the usual EAP-AKA rules. The Actual Network Name Length
field provides the length of the network name in bytes.
Network Name
This field contains the network name of the access network for
which the authentication is being performed. The name does not
include any terminating null characters. Because the length of
the entire attribute must be a multiple of 4 bytes, the sender
pads the name with 1, 2, or 3 bytes of all zero bits when
necessary.
Only the server sends the AT_KDF_INPUT attribute. The value is sent
as specified in [TS-3GPP.24.302] for non-3GPP access networks, and as
specified in [TS-3GPP.33.501] for 5G access networks. Per
[TS-3GPP.33.402], the server always verifies the authorization of a
given access network to use a particular name before sending it to
the peer over EAP-AKA'. The value of the AT_KDF_INPUT attribute from
the server MUST be non-empty. If it is empty, the peer behaves as if
AUTN had been incorrect and authentication fails. See Section 3 and
Figure 3 of [RFC4187] for an overview of how authentication failures
are handled.
Note: Currently, [TS-3GPP.24.302] or [TS-3GPP.33.501] specify
separate values. The former specifies what is called "Access
Network ID" and the latter specifies what is called "Serving
Network Name". However, from an EAP-AKA' perspective both occupy
the same field, and need to be distinguishable from each other.
Currently specified values are distinguishable, but it would be
useful that this be specified explicitly in the 3GPP
specifications.
In addition, the peer MAY check the received value against its own
understanding of the network name. Upon detecting a discrepancy, the
peer either warns the user and continues, or fails the authentication
process. More specifically, the peer SHOULD have a configurable
policy that it can follow under these circumstances. If the policy
indicates that it can continue, the peer SHOULD log a warning message
or display it to the user. If the peer chooses to proceed, it MUST
use the network name as received in the AT_KDF_INPUT attribute. If
the policy indicates that the authentication should fail, the peer
behaves as if AUTN had been incorrect and authentication fails.
The Network Name field contains a UTF-8 string. This string MUST be
constructed as specified in [TS-3GPP.24.302] for "Access Network
Identity". The string is structured as fields separated by colons
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(:). The algorithms and mechanisms to construct the identity string
depend on the used access technology.
On the network side, the network name construction is a configuration
issue in an access network and an authorization check in the
authentication server. On the peer, the network name is constructed
based on the local observations. For instance, the peer knows which
access technology it is using on the link, it can see information in
a link-layer beacon, and so on. The construction rules specify how
this information maps to an access network name. Typically, the
network name consists of the name of the access technology, or the
name of the access technology followed by some operator identifier
that was advertised in a link-layer beacon. In all cases,
[TS-3GPP.24.302] is the normative specification for the construction
in both the network and peer side. If the peer policy allows running
EAP-AKA' over an access technology for which that specification does
not provide network name construction rules, the peer SHOULD rely
only on the information from the AT_KDF_INPUT attribute and not
perform a comparison.
If a comparison of the locally determined network name and the one
received over EAP-AKA' is performed on the peer, it MUST be done as
follows. First, each name is broken down to the fields separated by
colons. If one of the names has more colons and fields than the
other one, the additional fields are ignored. The remaining
sequences of fields are compared, and they match only if they are
equal character by character. This algorithm allows a prefix match
where the peer would be able to match "", "FOO", and "FOO:BAR"
against the value "FOO:BAR" received from the server. This
capability is important in order to allow possible updates to the
specifications that dictate how the network names are constructed.
For instance, if a peer knows that it is running on access technology
"FOO", it can use the string "FOO" even if the server uses an
additional, more accurate description, e.g., "FOO:BAR", that contains
more information.
The allocation procedures in [TS-3GPP.24.302] ensure that conflicts
potentially arising from using the same name in different types of
networks are avoided. The specification also has detailed rules
about how a client can determine these based on information available
to the client, such as the type of protocol used to attach to the
network, beacons sent out by the network, and so on. Information
that the client cannot directly observe (such as the type or version
of the home network) is not used by this algorithm.
The AT_KDF_INPUT attribute MUST be sent and processed as explained
above when AT_KDF attribute has the value 1. Future definitions of
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new AT_KDF values MUST define how this attribute is sent and
processed.
3.2. AT_KDF
AT_KDF is an attribute that the server uses to reference a specific
key derivation function. It offers a negotiation capability that can
be useful for future evolution of the key derivation functions.
The format of the AT_KDF attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_KDF | Length | Key Derivation Function |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_KDF
This is set to 24.
Length
The length of the attribute, MUST be set to 1.
Key Derivation Function
An enumerated value representing the key derivation function that
the server (or peer) wishes to use. Value 1 represents the
default key derivation function for EAP-AKA', i.e., employing CK'
and IK' as defined in Section 3.3.
Servers MUST send one or more AT_KDF attributes in the EAP-Request/
AKA'-Challenge message. These attributes represent the desired
functions ordered by preference, the most preferred function being
the first attribute.
Upon receiving a set of these attributes, if the peer supports and is
willing to use the key derivation function indicated by the first
attribute, the function is taken into use without any further
negotiation. However, if the peer does not support this function or
is unwilling to use it, it does not process the received EAP-Request/
AKA'-Challenge in any way except by responding with the EAP-Response/
AKA'-Challenge message that contains only one attribute, AT_KDF with
the value set to the selected alternative. If there is no suitable
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alternative, the peer behaves as if AUTN had been incorrect and
authentication fails (see Figure 3 of [RFC4187]). The peer fails the
authentication also if there are any duplicate values within the list
of AT_KDF attributes (except where the duplication is due to a
request to change the key derivation function; see below for further
information).
Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the
peer, the server checks that the suggested AT_KDF value was one of
the alternatives in its offer. The first AT_KDF value in the message
from the server is not a valid alternative. If the peer has replied
with the first AT_KDF value, the server behaves as if AT_MAC of the
response had been incorrect and fails the authentication. For an
overview of the failed authentication process in the server side, see
Section 3 and Figure 2 of [RFC4187]. Otherwise, the server re-sends
the EAP-Response/AKA'-Challenge message, but adds the selected
alternative to the beginning of the list of AT_KDF attributes and
retains the entire list following it. Note that this means that the
selected alternative appears twice in the set of AT_KDF values.
Responding to the peer's request to change the key derivation
function is the only legal situation where such duplication may
occur.
When the peer receives the new EAP-Request/AKA'-Challenge message, it
MUST check that the requested change, and only the requested change,
occurred in the list of AT_KDF attributes. If so, it continues with
processing the received EAP-Request/AKA'-Challenge as specified in
[RFC4187] and Section 3.1 of this document. If not, it behaves as if
AT_MAC had been incorrect and fails the authentication. If the peer
receives multiple EAP-Request/AKA'-Challenge messages with differing
AT_KDF attributes without having requested negotiation, the peer MUST
behave as if AT_MAC had been incorrect and fail the authentication.
Note that the peer may also request sequence number resynchronization
[RFC4187]. This happens after AT_KDF negotiation has already
completed. An AKA'-Synchronization-Failure message is sent as a
response to the newly received EAP-Request/AKA'-Challenge (the last
message of the AT_KDF negotiation). The AKA'-Synchronization-Failure
message MUST contain the AUTS parameter as specified in [RFC4187] and
a copy the AT_KDF attributes as they appeared in the last message of
the AT_KDF negotiation. If the AT_KDF attributes are found to differ
from their earlier values, the peer and server MUST behave as if
AT_MAC had been incorrect and fail the authentication.
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3.3. Key Generation
Both the peer and server MUST derive the keys as follows.
AT_KDF set to 1
In this case, MK is derived and used as follows:
MK = PRF'(IK'|CK',"EAP-AKA'"|Identity)
K_encr = MK[0..127]
K_aut = MK[128..383]
K_re = MK[384..639]
MSK = MK[640..1151]
EMSK = MK[1152..1663]
Here [n..m] denotes the substring from bit n to m. PRF' is a new
pseudo-random function specified in Section 3.4. The first 1664
bits from its output are used for K_encr (encryption key, 128
bits), K_aut (authentication key, 256 bits), K_re (re-
authentication key, 256 bits), MSK (Master Session Key, 512 bits),
and EMSK (Extended Master Session Key, 512 bits). These keys are
used by the subsequent EAP-AKA' process. K_encr is used by the
AT_ENCR_DATA attribute, and K_aut by the AT_MAC attribute. K_re
is used later in this section. MSK and EMSK are outputs from a
successful EAP method run [RFC3748].
IK' and CK' are derived as specified in [TS-3GPP.33.402]. The
functions that derive IK' and CK' take the following parameters:
CK and IK produced by the AKA algorithm, and value of the Network
Name field comes from the AT_KDF_INPUT attribute (without length
or padding) .
The value "EAP-AKA'" is an eight-characters-long ASCII string. It
is used as is, without any trailing NUL characters.
Identity is the peer identity as specified in Section 7 of
[RFC4187].
When the server creates an AKA challenge and corresponding AUTN,
CK, CK', IK, and IK' values, it MUST set the Authentication
Management Field (AMF) separation bit to 1 in the AKA algorithm
[TS-3GPP.33.102]. Similarly, the peer MUST check that the AMF
separation bit is set to 1. If the bit is not set to 1, the peer
behaves as if the AUTN had been incorrect and fails the
authentication.
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On fast re-authentication, the following keys are calculated:
MK = PRF'(K_re,"EAP-AKA' re-auth"|Identity|counter|NONCE_S)
MSK = MK[0..511]
EMSK = MK[512..1023]
MSK and EMSK are the resulting 512-bit keys, taking the first 1024
bits from the result of PRF'. Note that K_encr and K_aut are not
re-derived on fast re-authentication. K_re is the re-
authentication key from the preceding full authentication and
stays unchanged over any fast re-authentication(s) that may happen
based on it. The value "EAP-AKA' re-auth" is a sixteen-
characters-long ASCII string, again represented without any
trailing NUL characters. Identity is the fast re-authentication
identity, counter is the value from the AT_COUNTER attribute,
NONCE_S is the nonce value from the AT_NONCE_S attribute, all as
specified in Section 7 of [RFC4187]. To prevent the use of
compromised keys in other places, it is forbidden to change the
network name when going from the full to the fast re-
authentication process. The peer SHOULD NOT attempt fast re-
authentication when it knows that the network name in the current
access network is different from the one in the initial, full
authentication. Upon seeing a re-authentication request with a
changed network name, the server SHOULD behave as if the re-
authentication identifier had been unrecognized, and fall back to
full authentication. The server observes the change in the name
by comparing where the fast re-authentication and full
authentication EAP transactions were received at the
Authentication, Authorization, and Accounting (AAA) protocol
level.
AT_KDF has any other value
Future variations of key derivation functions may be defined, and
they will be represented by new values of AT_KDF. If the peer
does not recognize the value, it cannot calculate the keys and
behaves as explained in Section 3.2.
AT_KDF is missing
The peer behaves as if the AUTN had been incorrect and MUST fail
the authentication.
If the peer supports a given key derivation function but is unwilling
to perform it for policy reasons, it refuses to calculate the keys
and behaves as explained in Section 3.2.
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3.4. Hash Functions
EAP-AKA' uses SHA-256, not SHA-1 (see [FIPS.180-4]) as in EAP-AKA.
This requires a change to the pseudo-random function (PRF) as well as
the AT_MAC and AT_CHECKCODE attributes.
3.4.1. PRF'
The PRF' construction is the same one IKEv2 uses (see Section 2.13 of
[RFC4306]). The function takes two arguments. K is a 256-bit value
and S is an octet string of arbitrary length. PRF' is defined as
follows:
PRF'(K,S) = T1 | T2 | T3 | T4 | ...
where:
T1 = HMAC-SHA-256 (K, S | 0x01)
T2 = HMAC-SHA-256 (K, T1 | S | 0x02)
T3 = HMAC-SHA-256 (K, T2 | S | 0x03)
T4 = HMAC-SHA-256 (K, T3 | S | 0x04)
...
PRF' produces as many bits of output as is needed. HMAC-SHA-256 is
the application of HMAC [RFC2104] to SHA-256.
3.4.2. AT_MAC
When used within EAP-AKA', the AT_MAC attribute is changed as
follows. The MAC algorithm is HMAC-SHA-256-128, a keyed hash value.
The HMAC-SHA-256-128 value is obtained from the 32-byte HMAC-SHA-256
value by truncating the output to the first 16 bytes. Hence, the
length of the MAC is 16 bytes.
Otherwise, the use of AT_MAC in EAP-AKA' follows Section 10.15 of
[RFC4187].
3.4.3. AT_CHECKCODE
When used within EAP-AKA', the AT_CHECKCODE attribute is changed as
follows. First, a 32-byte value is needed to accommodate a 256-bit
hash output:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_CHECKCODE | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Checkcode (0 or 32 bytes) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second, the checkcode is a hash value, calculated with SHA-256
[FIPS.180-4], over the data specified in Section 10.13 of [RFC4187].
4. Bidding Down Prevention for EAP-AKA
As discussed in [RFC3748], negotiation of methods within EAP is
insecure. That is, a man-in-the-middle attacker may force the
endpoints to use a method that is not the strongest that they both
support. This is a problem, as we expect EAP-AKA and EAP-AKA' to be
negotiated via EAP.
In order to prevent such attacks, this RFC specifies a new mechanism
for EAP-AKA that allows the endpoints to securely discover the
capabilities of each other. This mechanism comes in the form of the
AT_BIDDING attribute. This allows both endpoints to communicate
their desire and support for EAP-AKA' when exchanging EAP-AKA
messages. This attribute is not included in EAP-AKA' messages as
defined in this RFC. It is only included in EAP-AKA messages. This
is based on the assumption that EAP-AKA' is always preferable (see
Section 7). If during the EAP-AKA authentication process it is
discovered that both endpoints would have been able to use EAP-AKA',
the authentication process SHOULD be aborted, as a bidding down
attack may have happened.
The format of the AT_BIDDING attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_BIDDING | Length |D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_BIDDING
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This is set to 136.
Length
The length of the attribute, MUST be set to 1.
D
This bit is set to 1 if the sender supports EAP-AKA', is willing
to use it, and prefers it over EAP-AKA. Otherwise, it should be
set to zero.
Reserved
This field MUST be set to zero when sent and ignored on receipt.
The server sends this attribute in the EAP-Request/AKA-Challenge
message. If the peer supports EAP-AKA', it compares the received
value to its own capabilities. If it turns out that both the server
and peer would have been able to use EAP-AKA' and preferred it over
EAP-AKA, the peer behaves as if AUTN had been incorrect and fails the
authentication (see Figure 3 of [RFC4187]). A peer not supporting
EAP-AKA' will simply ignore this attribute. In all cases, the
attribute is protected by the integrity mechanisms of EAP-AKA, so it
cannot be removed by a man-in-the-middle attacker.
Note that we assume (Section 7) that EAP-AKA' is always stronger than
EAP-AKA. As a result, there is no need to prevent bidding "down"
attacks in the other direction, i.e., attackers forcing the endpoints
to use EAP-AKA'.
5. Peer Identities
EAP-AKA' peer identities are as specified in [RFC4187] Section 4.1,
with the addition of some requirements specified in this section.
EAP-AKA' includes optional identity privacy support that can be used
to hide the cleartext permanent identity and thereby make the
subscriber's EAP exchanges untraceable to eavesdroppers. EAP-AKA'
can also use the privacy friendly identifiers specified for 5G
networks.
The permanent identity is usually based on the IMSI, which may
further help the tracking, because the same identifier may be used in
other contexts as well. Identity privacy is based on temporary
usernames, or pseudonym usernames. These are similar to but separate
from the Temporary Mobile Subscriber Identities (TMSI) that are used
on cellular networks.
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5.1. Username Types in EAP-AKA' Identities
Section 4.1.1.3 of [RFC4187] specified that there are three types of
usernames: permanent, pseudonym, and fast re-authentication
usernames. This specification extends this definition as follows.
There are four types of usernames:
(1) Regular usernames. These are external names given to EAP-
AKA'. The regular usernames are further subdivided into to
categories:
(a) Permanent usernames, for instance IMSI-based usernames.
(b) Privacy-friendly temporary usernames, for instance 5G
privacy identifiers (see Section 5.3.2 and Section 5.3.2.1.
(2) EAP-AKA' pseudonym usernames. For example,
[email protected] might be a valid pseudonym identity. In
this example, 2s7ah6n9q is the pseudonym username.
(3) EAP-AKA' fast re-authentication usernames. For example,
[email protected] might be a valid fast re-authentication
identity and 43953754 the fast re-authentication username.
The permanent, privacy-friendly temporary, and pseudonym usernames
are only used on full authentication, and fast re-authentication
usernames only on fast re-authentication. Unlike permanent usernames
and pseudonym usernames, privacy friendly temporary usernames and
fast re-authentication usernames are one-time identifiers, which are
not re-used across EAP exchanges.
5.2. Generating Pseudonyms and Fast Re-Authentication Identities
As specified by [RFC4187] Section 4.1.1.7, pseudonym usernames and
fast re-authentication identities are generated by the EAP server, in
an implementation-dependent manner. RFC 4187 provides some general
requirements on how these identities are transported, how they map to
the NAI syntax, how they are distinguished from each other, and so
on.
However, to ensure privacy some additional requirements need to be
applied.
The pseudonym usernames and fast re-authentication identities MUST be