Internet DRAFT - draft-ietf-emu-rfc5448bis
draft-ietf-emu-rfc5448bis
Network Working Group J. Arkko
Internet-Draft V. Lehtovirta
Updates: 5448,4187 (if approved) V. Torvinen
Intended status: Informational Ericsson
Expires: November 11, 2021 P. Eronen
Independent
May 10, 2021
Improved Extensible Authentication Protocol Method for 3GPP Mobile
Network Authentication and Key Agreement (EAP-AKA')
draft-ietf-emu-rfc5448bis-10
Abstract
The 3GPP Mobile Network Authentication and Key Agreement (AKA) is an
authentication mechanism for devices wishing to access mobile
networks. RFC 4187 (EAP-AKA) made the use of this mechanism possible
within the Extensible Authentication Protocol (EAP) framework. RFC
5448 (EAP-AKA') was an improved version of EAP-AKA.
This document is the most recent specification of EAP-AKA',
including, for instance, details and references about related to
operating EAP-AKA' in 5G networks.
EAP-AKA' differs from EAP-AKA by providing a key derivation function
that binds the keys derived within the method to the name of the
access network. The key derivation function has been defined in the
3rd Generation Partnership Project (3GPP). EAP-AKA' allows its use
in EAP in an interoperable manner. EAP-AKA' also updates the
algorithm used in hash functions, as it employs SHA-256 / HMAC-
SHA-256 instead of SHA-1 / HMAC-SHA-1 as in EAP-AKA.
This version of EAP-AKA' specification specifies the protocol
behaviour for both 4G and 5G deployments, whereas the previous
version only did this for 4G.
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/.
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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
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This Internet-Draft will expire on November 11, 2021.
Copyright Notice
Copyright (c) 2021 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
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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 Derivation . . . . . . . . . . . . . . . . . . . . . 13
3.4. Hash Functions . . . . . . . . . . . . . . . . . . . . . 15
3.4.1. PRF' . . . . . . . . . . . . . . . . . . . . . . . . 15
3.4.2. AT_MAC . . . . . . . . . . . . . . . . . . . . . . . 15
3.4.3. AT_CHECKCODE . . . . . . . . . . . . . . . . . . . . 15
3.5. Summary of Attributes for EAP-AKA' . . . . . . . . . . . 16
4. Bidding Down Prevention for EAP-AKA . . . . . . . . . . . . . 18
4.1. Summary of Attributes for EAP-AKA . . . . . . . . . . . . 20
5. Peer Identities . . . . . . . . . . . . . . . . . . . . . . . 20
5.1. Username Types in EAP-AKA' Identities . . . . . . . . . . 20
5.2. Generating Pseudonyms and Fast Re-Authentication
Identities . . . . . . . . . . . . . . . . . . . . . . . 21
5.3. Identifier Usage in 5G . . . . . . . . . . . . . . . . . 22
5.3.1. Key Derivation . . . . . . . . . . . . . . . . . . . 23
5.3.2. EAP Identity Response and EAP-AKA' AT_IDENTITY
Attribute . . . . . . . . . . . . . . . . . . . . . . 24
6. Exported Parameters . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 28
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7.2. Discovered Vulnerabilities . . . . . . . . . . . . . . . 30
7.3. Pervasive Monitoring . . . . . . . . . . . . . . . . . . 32
7.4. Security Properties of Binding Network Names . . . . . . 33
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
8.1. Type Value . . . . . . . . . . . . . . . . . . . . . . . 34
8.2. Attribute Type Values . . . . . . . . . . . . . . . . . . 34
8.3. Key Derivation Function Namespace . . . . . . . . . . . . 34
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.1. Normative References . . . . . . . . . . . . . . . . . . 35
9.2. Informative References . . . . . . . . . . . . . . . . . 37
Appendix A. Changes from RFC 5448 . . . . . . . . . . . . . . . 40
Appendix B. Changes to RFC 4187 . . . . . . . . . . . . . . . . 41
Appendix C. Changes from Previous Version of This Draft . . . . 41
Appendix D. Importance of Explicit Negotiation . . . . . . . . . 45
Appendix E. Test Vectors . . . . . . . . . . . . . . . . . . . . 46
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 51
1. Introduction
The 3GPP Mobile Network Authentication and Key Agreement (AKA) is an
authentication mechanism for devices wishing to access mobile
networks. [RFC4187] (EAP-AKA) made the use of this mechanism
possible within the Extensible Authentication Protocol (EAP)
framework [RFC3748].
[RFC5448] (EAP-AKA') was an improved version of EAP-AKA. EAP-AKA'
was defined in RFC 5448 and updated EAP-AKA RFC 4187.
This document is the most recent specification of EAP-AKA',
including, for instance, details and references about related to
operating EAP-AKA' in 5G networks. RFC 5448 is not obsole, but the
most recent and fully backwards compatible specification is in this
document.
EAP-AKA' is commonly implemented in mobile phones and network
equipment. It can be used for authentication to gain network access
via Wireless LAN networks and, with 5G, also directly to mobile
networks.
EAP-AKA' differs from EAP-AKA by providing a different key derivation
function. 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. EAP-AKA' also updates the
algorithm used for hash functions.
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The EAP-AKA' method employs the derived keys CK' and IK' from the
3GPP specification [TS-3GPP.33.402] and updates the used hash
function to SHA-256 [FIPS.180-4] and HMAC to HMAC-SHA-256.
Otherwise, EAP-AKA' is equivalent to EAP-AKA. 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 that 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. See Appendix D for further information.
Note also that choices in authentication protocols should be
secure against bidding down attacks that attempt to force the
participants to use the least secure function. See Section 4 for
further information.
The changes from RFC 5448 to this specification are as follows:
o Update the reference on how the Network Name field is constructed
in the protocol. This 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
derivation. 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'.
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o Summaries of the attributes have been added.
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: Any further updates in 3GPP specifications that affect, for
instance, key derivation is something that EAP-AKA'
implementations need to take into account. Upon such updates
there will be a need to both update this 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
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 5G
identifiers are used in the EAP-AKA' 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.
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 an EAP method that follows the EAP-AKA specification
[RFC4187] in all respects except the following:
o It uses the Type code 0x32, not 0x17 (which is used by EAP-AKA).
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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.
o It employs SHA-256 / HMAC-SHA-256, not SHA-1 / HMAC-SHA-1
[FIPS.180-4] (Section 3.4 [RFC2104]).
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].
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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
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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. For 4G networks, EAP-AKA' MUST use the leading character
"6" (ASCII 36 hexadecimal) instead of "0" for IMSI-based permanent
usernames. For 5G networks, leading character "6" is not used for
IMSI-based permanent user names. Identifier usage in 5G is specified
in Section 5.3. 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.
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Actual Network Name Length
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 both non-3GPP access networks
and 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, with a greater than zero length in the Actual Network Name
Length field. If AT_KDF_INPUT attribute 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.
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
(:). 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
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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
new AT_KDF values MUST define how this attribute is sent and
processed.
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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, calculated as defined in [RFC4187],
Section 8.1. For AT_KDF, the Length field 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
alternative, the peer behaves as if AUTN had been incorrect and
authentication fails (see Figure 3 of [RFC4187]). The peer fails the
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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 since the peer should have
accepted it without further negotiation. 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. That is, the EAP-Request/AKA'-Challenge and, possibly,
the EAP-Response/AKA'-Challenge message are exchanged first to come
up with a mutually acceptable key derivation function, and only then
the possible AKA'-Synchronization-Failure message is sent. The AKA'-
Synchronization-Failure message is sent as a response to the newly
received EAP-Request/AKA'-Challenge which is the last message of the
AT_KDF negotiation. Note that if the first proposed KDF is
acceptable, then last message is at the same time the first EAP-
Request/AKA'-Challenge message. 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 Derivation
Both the peer and server MUST derive the keys as follows.
AT_KDF parameter has the value 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, including bits
n and 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], and Section 5.3.2 in this document for the 5G cases.
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 / HMAC-SHA-256, not SHA-1 / HMAC-SHA-1 (see
[FIPS.180-4] [RFC2104]) 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
[RFC7296]; this is the same function as was defined [RFC4306] that
RFC 5448 referred to). The function takes two arguments. K is a
256-bit value and S is a byte 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].
3.5. Summary of Attributes for EAP-AKA'
Table 1 provides a guide to which attributes may be found in which
kinds of messages, and in what quantity.
Messages are denoted with numbers in parentheses as follows:
(1) EAP-Request/AKA-Identity,
(2) EAP-Response/AKA-Identity,
(3) EAP-Request/AKA-Challenge,
(4) EAP-Response/AKA-Challenge,
(5) EAP-Request/AKA-Notification,
(6) EAP-Response/AKA-Notification,
(7) EAP-Response/AKA-Client-Error
(8) EAP-Request/AKA-Reauthentication,
(9) EAP-Response/AKA-Reauthentication,
(10) EAP-Response/AKA-Authentication-Reject, and
(11) EAP-Response/AKA-Synchronization-Failure.
The column denoted with "E" indicates whether the attribute is a
nested attribute that MUST be included within AT_ENCR_DATA.
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In addition,the numbered columns indicate the quantity of the
attribute within the message as follows:
"0" indicates that the attribute MUST NOT be included in the
message,
"1" indicates that the attribute MUST be included in the message,
"0-1" indicates that the attribute is sometimes included in the
message,
"0+" indicates that zero or more copies of the attribute MAY be
included in the message,
"1+" indicates that there MUST be at least one attribute in the
message but more than one MAY be included in the message, and
"0*" indicates that the attribute is not included in the message
in cases specified in this document, but MAY be included in the
future versions of the protocol.
The attribute table is shown below. The table is largely the same as
in the EAP-AKA attribute table ([RFC4187] Section 10.1), but changes
how many times AT_MAC may appear in EAP-Response/AKA'-Challenge
message as it does not appear there when AT_KDF has to be sent from
the peer to the server. The table also adds the AT_KDF and
AT_KDF_INPUT attributes.
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Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)(11) E
AT_PERMANENT_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N
AT_ANY_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N
AT_FULLAUTH_ID_REQ 0-1 0 0 0 0 0 0 0 0 0 0 N
AT_IDENTITY 0 0-1 0 0 0 0 0 0 0 0 0 N
AT_RAND 0 0 1 0 0 0 0 0 0 0 0 N
AT_AUTN 0 0 1 0 0 0 0 0 0 0 0 N
AT_RES 0 0 0 1 0 0 0 0 0 0 0 N
AT_AUTS 0 0 0 0 0 0 0 0 0 0 1 N
AT_NEXT_PSEUDONYM 0 0 0-1 0 0 0 0 0 0 0 0 Y
AT_NEXT_REAUTH_ID 0 0 0-1 0 0 0 0 0-1 0 0 0 Y
AT_IV 0 0 0-1 0* 0-1 0-1 0 1 1 0 0 N
AT_ENCR_DATA 0 0 0-1 0* 0-1 0-1 0 1 1 0 0 N
AT_PADDING 0 0 0-1 0* 0-1 0-1 0 0-1 0-1 0 0 Y
AT_CHECKCODE 0 0 0-1 0-1 0 0 0 0-1 0-1 0 0 N
AT_RESULT_IND 0 0 0-1 0-1 0 0 0 0-1 0-1 0 0 N
AT_MAC 0 0 1 0-1 0-1 0-1 0 1 1 0 0 N
AT_COUNTER 0 0 0 0 0-1 0-1 0 1 1 0 0 Y
AT_COUNTER_TOO_SMALL 0 0 0 0 0 0 0 0 0-1 0 0 Y
AT_NONCE_S 0 0 0 0 0 0 0 1 0 0 0 Y
AT_NOTIFICATION 0 0 0 0 1 0 0 0 0 0 0 N
AT_CLIENT_ERROR_CODE 0 0 0 0 0 0 1 0 0 0 0 N
AT_KDF 0 0 1+ 0+ 0 0 0 0 0 0 1+ N
AT_KDF_INPUT 0 0 1 0 0 0 0 0 0 0 0 N
Table 1: The attribute table
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. It
is only included in EAP-AKA messages. (Those messages are protected
with the AT_MAC attribute.) This approach 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.
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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
This is set to 136.
Length
The length of the attribute, calculated as defined in [RFC4187],
Section 8.1. For AT_BIDDING, the Length 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, this specification does not provide protection
against bidding "down" attacks in the other direction, i.e.,
attackers forcing the endpoints to use EAP-AKA'.
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4.1. Summary of Attributes for EAP-AKA
The appearance of the AT_BIDDING attribute in EAP-AKA exchanges is
shown below, using the notation from Section 3.5:
Attribute (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)(11) E
AT_BIDDING 0 0 1 0 0 0 0 0 0 0 0 N
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. Exposing the
IMSI is undesirable, because as a permanent identity it is easily
trackable. In addition, since IMSIs may be used in other contexts as
well, there would be additional opportunities for such tracking.
In EAP-AKA', 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.
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'
peers. 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 GUTI
(5G Globally Unique Temporary Identifier) or 5G privacy
identifiers (see Section 5.3.2), for instance SUCI
(Subscription Concealed Identifier).
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(2) EAP-AKA' pseudonym usernames. For example,
2s7ah6n9q@example.com might be a valid pseudonym identity. In
this example, 2s7ah6n9q is the pseudonym username.
(3) EAP-AKA' fast re-authentication usernames. For example,
43953754@example.com 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
This section provides some additional guidance for implementations
for producing secure pseudonyms and fast re-authentication
identities. It does not impact backwards compatibility, because each
server consumes only the identities it itself generates. However,
adherence to the guidance will provide better security.
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 enhance privacy some additional requirements need to be
applied.
The pseudonym usernames and fast re-authentication identities MUST be
generated in a cryptographically secure way so that that it is
computationally infeasible for an attacker to differentiate two
identities belonging to the same user from two identities belonging
to different users. This can be achieved, for instance, by using
random or pseudo-random identifiers such as random byte strings or
ciphertexts. See also [RFC4086] for guidance on random number
generation.
Note that the pseudonym and fast re-authentication usernames also
MUST NOT include substrings that can be used to relate the username
to a particular entity or a particular permanent identity. For
instance, the usernames can not include any subscriber-identifying
part of an IMSI or other permanent identifier. Similarly, no part of
the username can be formed by a fixed mapping that stays the same
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across multiple different pseudonyms or fast re-authentication
identities for the same subscriber.
When the identifier used to identify a subscriber in an EAP-AKA'
authentication exchange is a privacy-friendly identifier that is used
only once, the EAP-AKA' peer MUST NOT use a pseudonym provided in
that authentication exchange in subsequent exchanges more than once.
To ensure that this does not happen, EAP-AKA' server MAY decline to
provide a pseudonym in such authentication exchanges. An important
case where such privacy-friendly identifiers are used is in 5G
networks (see Section 5.3).
5.3. Identifier Usage in 5G
In EAP-AKA', the peer identity may be communicated to the server in
one of three ways:
o As a part of link layer establishment procedures, externally to
EAP.
o With the EAP-Response/Identity message in the beginning of the EAP
exchange, but before the selection of EAP-AKA'.
o Transmitted from the peer to the server using EAP-AKA' messages
instead of EAP-Response/Identity. In this case, the server
includes an identity requesting attribute (AT_ANY_ID_REQ,
AT_FULLAUTH_ID_REQ or AT_PERMANENT_ID_REQ) in the EAP-Request/AKA-
Identity message; and the peer includes the AT_IDENTITY attribute,
which contains the peer's identity, in the EAP-Response/AKA-
Identity message.
The identity carried above may be a permanent identity, privacy
friendly identity, pseudonym identity, or fast re-authentication
identity as defined in Section 5.1.
5G supports the concept of privacy identifiers, and it is important
for interoperability that the right type of identifier is used.
5G defines the SUbscription Permanent Identifier (SUPI) and
SUbscription Concealed Identifier (SUCI) [TS-3GPP.23.501]
[TS-3GPP.33.501] [TS-3GPP.23.003]. SUPI is globally unique and
allocated to each subscriber. However, it is only used internally in
the 5G network, and is privacy sensitive. The SUCI is a privacy
preserving identifier containing the concealed SUPI, using public key
cryptography to encrypt the SUPI.
Given the choice between these two types of identifiers, EAP-AKA'
ensures interoperability as follows:
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o Where identifiers are used within EAP-AKA' -- such as key
derivation -- specify what values exactly should be used, to avoid
ambiguity (see Section 5.3.1).
o Where identifiers are carried within EAP-AKA' packets -- such as
in the AT_IDENTITY attribute -- specify which identifiers should
be filled in (see Section 5.3.2).
In 5G, the normal mode of operation is that identifiers are only
transmitted outside EAP. However, in a system involving terminals
from many generations and several connectivity options via 5G and
other mechanisms, implementations and the EAP-AKA' specification need
to prepare for many different situations, including sometimes having
to communicate identities within EAP.
The following sections clarify which identifiers are used and how.
5.3.1. Key Derivation
In EAP-AKA', the peer identity is used in the Section 3.3 key
derivation formula.
The identity needs to be represented in exact correct format for the
key derivation formula to produce correct results.
If the AT_KDF_INPUT parameter contains the prefix "5G:", the AT_KDF
parameter has the value 1, and this authentication is not a fast re-
authentication, then the peer identity used in the key derivation
MUST be as specified in Annex F.3 of [TS-3GPP.33.501] and Clause 2.2
of [TS-3GPP.23.003]. This is in contrast to [RFC5448], which used
the identity as communicated in EAP and represented as a NAI. Also,
in contrast to [RFC5448], in 5G EAP-AKA' does not use the "0" or "6"
prefix in front of the identifier.
For an example of the format of the identity, see Clause 2.2 of
[TS-3GPP.23.003].
In all other cases, the following applies:
The identity used in the key derivation formula MUST be exactly
the one sent in EAP-AKA' AT_IDENTITY attribute, if one was sent,
regardless of the kind of identity that it may have been. If no
AT_IDENTITY was sent, the identity MUST be the exactly the one
sent in the generic EAP Identity exchange, if one was made.
If no identity was communicated inside EAP, then the identity is
the one communicated outside EAP in link layer messaging.
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In this case, the used identity MUST be the identity most recently
communicated by the peer to the network, again regardless of what
type of identity it may have been.
5.3.2. EAP Identity Response and EAP-AKA' AT_IDENTITY Attribute
The EAP authentication option is only available in 5G when the new 5G
core network is also in use. However, in other networks an EAP-AKA'
peer may be connecting to other types of networks and existing
equipment.
When the EAP server is in a 5G network, the 5G procedures for EAP-
AKA' apply. When EAP server is defined to be in a 5G network is
specified in [TS-3GPP.33.501].
Note: Currently, the following conditions are specified: when the
EAP peer uses the 5G Non-Access Stratum (NAS) protocol
[TS-3GPP.24.501] or when the EAP peer attaches to a network that
advertises 5G connectivity without NAS [TS-3GPP.23.501]. Possible
future conditions may also be specified by 3GPP.
When the 5G procedures for EAP-AKA' apply, EAP identity exchanges are
generally not used as the identity is already made available on
previous link layer exchanges.
In this situation, the EAP Identity Response and EAP-AKA' AT_IDENTITY
attribute are handled as specified in Annex F.2 of [TS-3GPP.33.501].
When used in EAP-AKA', the format of the SUCI MUST be as specified in
[TS-3GPP.23.003] Section 28.7.3, with the semantics defined in
[TS-3GPP.23.003] Section 2.2B. Also, in contrast to [RFC5448], in 5G
EAP-AKA' does not use the "0" or "6" prefix in front of the
identifier.
For an example of an IMSI in NAI format, see [TS-3GPP.23.003]
Section 28.7.3.
Otherwise, the peer SHOULD employ IMSI, SUPI, or a NAI as it is
configured to use.
6. Exported Parameters
When not using fast re-authentication, the EAP-AKA' Session-Id is the
concatenation of the EAP Type Code (0x32, one byte) with the contents
of the RAND field from the AT_RAND attribute, followed by the
contents of the AUTN field in the AT_AUTN attribute :
Session-Id = 0x32 || RAND || AUTN
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When using fast re-authentication, the EAP-AKA' Session-Id is the
concatenation of the EAP Type Code (0x32) with the contents of the
NONCE_S field from the AT_NONCE_S attribute, followed by the contents
of the MAC field from the AT_MAC attribute from EAP-Request/AKA-
Reauthentication:
Session-Id = 0x32 || NONCE_S || MAC
The Peer-Id is the contents of the Identity field from the
AT_IDENTITY attribute, using only the Actual Identity Length bytes
from the beginning. Note that the contents are used as they are
transmitted, regardless of whether the transmitted identity was a
permanent, pseudonym, or fast EAP re-authentication identity. If no
AT_IDENTITY attribute was exchanged, the exported Peer-Id is the
identity provided from the EAP Identity Response packet. If no EAP
Identity Response was provided either, the exported Peer-Id is the
null string (zero length).
The Server-Id is the null string (zero length).
7. Security Considerations
A summary of the security properties of EAP-AKA' follows. These
properties are very similar to those in EAP-AKA. We assume that HMAC
SHA-256 is at least as secure as HMAC SHA-1 (see also [RFC6194].
This is called the SHA-256 assumption in the remainder of this
section. Under this assumption, EAP-AKA' is at least as secure as
EAP-AKA.
If the AT_KDF attribute has value 1, then the security properties of
EAP-AKA' are as follows:
Protected ciphersuite negotiation
EAP-AKA' has no ciphersuite negotiation mechanisms. It does have
a negotiation mechanism for selecting the key derivation
functions. This mechanism is secure against bidding down attacks
from EAP-AKA' to EAP-AKA. 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.
Per assumptions in Section 4, there is no protection against
bidding down attacks from EAP-AKA to EAP-AKA', should EAP-AKA'
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somehow be considered less secure some day than EAP-AKA. Such
protection was not provided in RFC 5448 implementations and
consequently neither does this specification provide it. If such
support is needed, it would have to be added as a separate new
feature.
In general, it is expected that the current negotiation
capabilities in EAP-AKA' are sufficient for some types of
extensions, including adding Perfect Forward Secrecy
([I-D.ietf-emu-aka-pfs]) and perhaps others. But as with how EAP-
AKA' itself came about, some larger changes may require a new EAP
method type. One example of such change would be the introduction
of new algorithms.
Mutual authentication
Under the SHA-256 assumption, the properties of EAP-AKA' are at
least as good as those of EAP-AKA in this respect. Refer to
[RFC4187], Section 12 for further details.
Integrity protection
Under the SHA-256 assumption, the properties of EAP-AKA' are at
least as good (most likely better) as those of EAP-AKA in this
respect. Refer to [RFC4187], Section 12 for further details. The
only difference is that a stronger hash algorithm and keyed MAC,
SHA-256 / HMAC-SHA-256, is used instead of SHA-1 / HMAC-SHA-1.
Replay protection
Under the SHA-256 assumption, the properties of EAP-AKA' are at
least as good as those of EAP-AKA in this respect. Refer to
[RFC4187], Section 12 for further details.
Confidentiality
The properties of EAP-AKA' are exactly the same as those of EAP-
AKA in this respect. Refer to [RFC4187], Section 12 for further
details.
Key derivation
EAP-AKA' supports key derivation with an effective key strength
against brute force attacks equal to the minimum of the length of
the derived keys and the length of the AKA base key, i.e., 128
bits or more. The key hierarchy is specified in Section 3.3.
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The Transient EAP Keys used to protect EAP-AKA packets (K_encr,
K_aut, K_re), the MSK, and the EMSK are cryptographically
separate. If we make the assumption that SHA-256 behaves as a
pseudo-random function, an attacker is incapable of deriving any
non-trivial information about any of these keys based on the other
keys. An attacker also cannot calculate the pre-shared secret
from IK, CK, IK', CK', K_encr, K_aut, K_re, MSK, or EMSK by any
practically feasible means.
EAP-AKA' adds an additional layer of key derivation functions
within itself to protect against the use of compromised keys.
This is discussed further in Section 7.4.
EAP-AKA' uses a pseudo-random function modeled after the one used
in IKEv2 [RFC7296] together with SHA-256.
Key strength
See above.
Dictionary attack resistance
Under the SHA-256 assumption, the properties of EAP-AKA' are at
least as good as those of EAP-AKA in this respect. Refer to
[RFC4187], Section 12 for further details.
Fast reconnect
Under the SHA-256 assumption, the properties of EAP-AKA' are at
least as good as those of EAP-AKA in this respect. Refer to
[RFC4187], Section 12 for further details. Note that
implementations MUST prevent performing a fast reconnect across
method types.
Cryptographic binding
Note that this term refers to a very specific form of binding,
something that is performed between two layers of authentication.
It is not the same as the binding to a particular network name.
The properties of EAP-AKA' are exactly the same as those of EAP-
AKA in this respect, i.e., as it is not a tunnel method, this
property is not applicable to it. Refer to [RFC4187], Section 12
for further details.
Session independence
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The properties of EAP-AKA' are exactly the same as those of EAP-
AKA in this respect. Refer to [RFC4187], Section 12 for further
details.
Fragmentation
The properties of EAP-AKA' are exactly the same as those of EAP-
AKA in this respect. Refer to [RFC4187], Section 12 for further
details.
Channel binding
EAP-AKA', like EAP-AKA, does not provide channel bindings as
they're defined in [RFC3748] and [RFC5247]. New skippable
attributes can be used to add channel binding support in the
future, if required.
However, including the Network Name field in the AKA' algorithms
(which are also used for other purposes than EAP-AKA') provides a
form of cryptographic separation between different network names,
which resembles channel bindings. However, the network name does
not typically identify the EAP (pass-through) authenticator. See
Section 7.4 for more discussion.
7.1. Privacy
[RFC6973] suggests that the privacy considerations of IETF protocols
be documented.
The confidentiality properties of EAP-AKA' itself have been discussed
above under "Confidentiality".
EAP-AKA' uses several different types of identifiers to identify the
authenticating peer. It is strongly RECOMMENDED to use the privacy-
friendly temporary or hidden identifiers, i.e., the 5G GUTI or SUCI,
pseudonym usernames, and fast re-authentication usernames. The use
of permanent identifiers such as the IMSI or SUPI may lead to an
ability to track the peer and/or user associated with the peer. The
use of permanent identifiers such as the IMSI or SUPI is strongly NOT
RECOMMENDED.
As discussed in Section 5.3, when authenticating to a 5G network,
only the SUCI identifier is normally used. The use of EAP-AKA'
pseudonyms in this situation is at best limited, because the SUCI
already provides a stronger mechanism. In fact, the re-use of the
same pseudonym multiple times will result in a tracking opportunity
for observers that see the pseudonym pass by. To avoid this, the
peer and server need to follow the guidelines given in Section 5.2.
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When authenticating to a 5G network, per Section 5.3.1, both the EAP-
AKA' peer and server need to employ the permanent identifier, SUPI,
as an input to key derivation. However, this use of the SUPI is only
internal. As such, the SUPI need not be communicated in EAP
messages. Therefore, SUPI MUST NOT be communicated in EAP-AKA' when
authenticating to a 5G network.
While the use of SUCI in 5G networks generally provides identity
privacy, this is not true if the null-scheme encryption is used to
construct the SUCI (see [TS-3GPP.33.501] Annex C). The use of this
scheme turns the use of SUCI equivalent to the use of SUPI or IMSI.
The use of the null scheme is NOT RECOMMENDED where identity privacy
is important.
The use of fast re-authentication identities when authenticating to a
5G network does not have the same problems as the use of pseudonyms,
as long as the 5G authentication server generates the fast re-
authentication identifiers in a proper manner specified in
Section 5.2.
Outside 5G, the peer can freely choose between the use of permanent,
pseudonym, or fast re-authentication identifiers:
o A peer that has not yet performed any EAP-AKA' exchanges does not
typically have a pseudonym available. If the peer does not have a
pseudonym available, then the privacy mechanism cannot be used,
and the permanent identity will have to be sent in the clear.
The terminal SHOULD store the pseudonym in non-volatile memory so
that it can be maintained across reboots. An active attacker that
impersonates the network may use the AT_PERMANENT_ID_REQ attribute
([RFC4187] Section 4.1.2) to learn the subscriber's IMSI.
However, as discussed in [RFC4187] Section 4.1.2, the terminal can
refuse to send the cleartext permanent identity if it believes
that the network should be able to recognize the pseudonym.
o When pseudonyms and fast re-authentication identities are used,
the peer relies on the properly created identifiers by the server.
It is essential that an attacker cannot link a privacy-friendly
identifier to the user in any way or determine that two
identifiers belong to the same user as outlined in Section 5.2.
The pseudonym usernames and fast re-authentication identities MUST
NOT be used for other purposes (e.g., in other protocols).
If the peer and server cannot guarantee that SUCI can be used or
pseudonyms will be available, generated properly, and maintained
reliably, and identity privacy is required then additional protection
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from an external security mechanism such as tunneled EAP methods such
as TTLS [RFC5281] or TEAP [RFC7170] may be used. The benefits and
the security considerations of using an external security mechanism
with EAP-AKA are beyond the scope of this document.
Finally, as with other EAP methods, even when privacy-friendly
identifiers or EAP tunneling is used, typically the domain part of an
identifier (e.g., the home operator) is visible to external parties.
7.2. Discovered Vulnerabilities
There have been no published attacks that violate the primary secrecy
or authentication properties defined for Authentication and Key
Agreement (AKA) under the originally assumed trust model. The same
is true of EAP-AKA'.
However, there have been attacks when a different trust model is in
use, with characteristics not originally provided by the design, or
when participants in the protocol leak information to outsiders on
purpose, and there have been some privacy-related attacks.
For instance, the original AKA protocol does not prevent supplying
keys by an insider to a third party as done in, e.g., by Mjolsnes and
Tsay in [MT2012] where a serving network lets an authentication run
succeed, but then misuses the session keys to send traffic on the
authenticated user's behalf. This particular attack is not different
from any on-path entity (such as a router) pretending to send
traffic, but the general issue of insider attacks can be a problem,
particularly in a large group of collaborating operators.
Another class of attacks is the use of tunneling of traffic from one
place to another, e.g., as done by Zhang and Fang in [ZF2005] to
leverage security policy differences between different operator
networks, for instance. To gain something in such an attack, the
attacker needs to trick the user into believing it is in another
location. If policies between different locations differ, for
instance, in some location it is not required to encrypt all payload
traffic, the attacker may trick the user into opening a
vulnerability. As an authentication mechanism, EAP-AKA' is not
directly affected by most such attacks. EAP-AKA' network name
binding can also help alleviate some of the attacks. In any case, it
is recommended that EAP-AKA' configuration not be dependent on the
location of where a request comes from, unless the location
information can be cryptographically confirmed, e.g., with the
network name binding.
Zhang and Fang also looked at Denial-of-Service attacks [ZF2005]. A
serving network may request large numbers of authentication runs for
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a particular subscriber from a home network. While resynchronization
process can help recover from this, eventually it is possible to
exhaust the sequence number space and render the subscriber's card
unusable. This attack is possible for both native AKA and EAP-AKA'.
However, it requires the collaboration of a serving network in an
attack. It is recommended that EAP-AKA' implementations provide
means to track, detect, and limit excessive authentication attempts
to combat this problem.
There have also been attacks related to the use of AKA without the
generated session keys (e.g., [BT2013]). Some of those attacks
relate to the use of originally man-in-the-middle vulnerable HTTP
Digest AKAv1 [RFC3310]. This has since then been corrected in
[RFC4169]. The EAP-AKA' protocol uses session keys and provides
channel binding, and as such, is resistant to the above attacks
except where the protocol participants leak information to outsiders.
Basin et al [Basin2018] have performed formal analysis and concluded
that the AKA protocol would have benefited from additional security
requirements, such as key confirmation.
In the context of pervasive monitoring revelations, there were also
reports of compromised long term pre-shared keys used in SIM and AKA
[Heist2015]. While no protocol can survive the theft of key material
associated with its credentials, there are some things that alleviate
the impacts in such situations. These are discussed further in
Section 7.3.
Arapinis et al ([Arapinis2012]) describe an attack that uses the AKA
resynchronization protocol to attempt to detect whether a particular
subscriber is on a given area. This attack depends on the ability of
the attacker to have a false base station on the given area, and the
subscriber performing at least one authentication between the time
the attack is set up and run.
Borgaonkar et al discovered that the AKA resynchronization protocol
may also be used to predict the authentication frequency of a
subscribers if non-time-based SQN generation scheme is used
[Borgaonkar2018]. The attacker can force the re-use of the keystream
that is used to protect the SQN in the AKA resynchronization
protocol. The attacker then guesses the authentication frequency
based on the lowest bits of two XORed SQNs. The researchers' concern
was that the authentication frequency would reveal some information
about the phone usage behavior, e.g., number of phone calls made or
number of SMS messages sent. There are a number of possible triggers
for authentication, so such information leak is not direct, but can
be a concern. The impact of the attack is also different depending
on whether time or non-time-based SQN generation scheme is used.
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Similar attacks are possible outside AKA in the cellular paging
protocols where the attacker can simply send application layer data,
short messages or make phone calls to the intended victim and observe
the air-interface (e.g., [Kune2012] and [Shaik2016]). Hussain et.
al. demonstrated a slightly more sophisticated version of the attack
that exploits the fact that 4G paging protocol uses the IMSI to
calculate the paging timeslot [Hussain2019]. As this attack is
outside AKA, it does not impact EAP-AKA'.
Finally, bad implementations of EAP-AKA' may not produce pseudonym
usernames or fast re-authentication identities in a manner that is
sufficiently secure. While it is not a problem with the protocol
itself, following the recommendations in Section 5.2 mitigate this
concern.
7.3. Pervasive Monitoring
As required by [RFC7258], work on IETF protocols needs to consider
the effects of pervasive monitoring and mitigate them when possible.
As described in Section 7.2, after the publication of RFC 5448, new
information has come to light regarding the use of pervasive
monitoring techniques against many security technologies, including
AKA-based authentication.
For AKA, these attacks relate to theft of the long-term shared secret
key material stored on the cards. Such attacks are conceivable, for
instance, during the manufacturing process of cards, through coercion
of the card manufacturers, or during the transfer of cards and
associated information to an operator. Since the publication of
reports about such attacks, manufacturing and provisioning processes
have gained much scrutiny and have improved.
In particular, it is crucial that manufacturers limit access to the
secret information and the cards only to necessary systems and
personnel. It is also crucial that secure mechanisms be used to
store and communicate the secrets between the manufacturer and the
operator that adopts those cards for their customers.
Beyond these operational considerations, there are also technical
means to improve resistance to these attacks. One approach is to
provide Perfect Forward Secrecy (PFS). This would prevent any
passive attacks merely based on the long-term secrets and observation
of traffic. Such a mechanism can be defined as a backwards-
compatible extension of EAP-AKA', and is pursued separately from this
specification [I-D.ietf-emu-aka-pfs]. Alternatively, EAP-AKA'
authentication can be run inside a PFS-capable tunneled
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authentication method. In any case, the use of some PFS-capable
mechanism is recommended.
7.4. Security Properties of Binding Network Names
The ability of EAP-AKA' to bind the network name into the used keys
provides some additional protection against key leakage to
inappropriate parties. The keys used in the protocol are specific to
a particular network name. If key leakage occurs due to an accident,
access node compromise, or another attack, the leaked keys are only
useful when providing access with that name. For instance, a
malicious access point cannot claim to be network Y if it has stolen
keys from network X. Obviously, if an access point is compromised,
the malicious node can still represent the compromised node. As a
result, neither EAP-AKA' nor any other extension can prevent such
attacks; however, the binding to a particular name limits the
attacker's choices, allows better tracking of attacks, makes it
possible to identify compromised networks, and applies good
cryptographic hygiene.
The server receives the EAP transaction from a given access network,
and verifies that the claim from the access network corresponds to
the name that this access network should be using. It becomes
impossible for an access network to claim over AAA that it is another
access network. In addition, if the peer checks that the information
it has received locally over the network-access link layer matches
with the information the server has given it via EAP-AKA', it becomes
impossible for the access network to tell one story to the AAA
network and another one to the peer. These checks prevent some
"lying NAS" (Network Access Server) attacks. For instance, a roaming
partner, R, might claim that it is the home network H in an effort to
lure peers to connect to itself. Such an attack would be beneficial
for the roaming partner if it can attract more users, and damaging
for the users if their access costs in R are higher than those in
other alternative networks, such as H.
Any attacker who gets hold of the keys CK and IK, produced by the AKA
algorithm, can compute the keys CK' and IK' and, hence, the Master
Key (MK) according to the rules in Section 3.3. The attacker could
then act as a lying NAS. In 3GPP systems in general, the keys CK and
IK have been distributed to, for instance, nodes in a visited access
network where they may be vulnerable. In order to reduce this risk,
the AKA algorithm MUST be computed with the AMF separation bit set to
1, and the peer MUST check that this is indeed the case whenever it
runs EAP-AKA'. Furthermore, [TS-3GPP.33.402] requires that no CK or
IK keys computed in this way ever leave the home subscriber system.
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The additional security benefits obtained from the binding depend
obviously on the way names are assigned to different access networks.
This is specified in [TS-3GPP.24.302]. See also [TS-3GPP.23.003].
Ideally, the names allow separating each different access technology,
each different access network, and each different NAS within a
domain. If this is not possible, the full benefits may not be
achieved. For instance, if the names identify just an access
technology, use of compromised keys in a different technology can be
prevented, but it is not possible to prevent their use by other
domains or devices using the same technology.
8. IANA Considerations
IANA should update the Extensible Authentication Protocol (EAP)
Registry and the EAP-AKA and EAP-SIM Parameters so that entries
pointing to RFC 5448 will point to this RFC instead.
8.1. Type Value
EAP-AKA' has the EAP Type value 0x32 in the Extensible Authentication
Protocol (EAP) Registry under Method Types. Per Section 6.2 of
[RFC3748], this allocation can be made with Designated Expert and
Specification Required.
8.2. Attribute Type Values
EAP-AKA' shares its attribute space and subtypes with EAP-SIM
[RFC4186] and EAP-AKA [RFC4187]. No new registries are needed.
However, a new Attribute Type value (23) in the non-skippable range
has been assigned for AT_KDF_INPUT (Section 3.1) in the EAP-AKA and
EAP-SIM Parameters registry under Attribute Types.
Also, a new Attribute Type value (24) in the non-skippable range has
been assigned for AT_KDF (Section 3.2).
Finally, a new Attribute Type value (136) in the skippable range has
been assigned for AT_BIDDING (Section 4).
8.3. Key Derivation Function Namespace
IANA has also created a new namespace for EAP-AKA' AT_KDF Key
Derivation Function Values. This namespace exists under the EAP-AKA
and EAP-SIM Parameters registry. The initial contents of this
namespace are given below; new values can be created through the
Specification Required policy [RFC8126].
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Value Description Reference
--------- ---------------------- -------------------------------
0 Reserved [RFC Editor: Refer to this RFC]
1 EAP-AKA' with CK'/IK' [RFC Editor: Refer to this RFC]
2-65535 Unassigned
9. References
9.1. Normative References
[TS-3GPP.23.003]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Core Network and Terminals; Numbering,
addressing and identification (Release 16)",
3GPP Technical Specification 23.003 version 16.5.0,
December 2020.
[TS-3GPP.23.501]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Services and System Aspects; 3G
Security; Security architecture and procedures for 5G
System; (Release 16)", 3GPP Technical Specification 23.501
version 16.7.0, December 2020.
[TS-3GPP.24.302]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Core Network and Terminals; Access to
the 3GPP Evolved Packet Core (EPC) via non-3GPP access
networks; Stage 3; (Release 16)", 3GPP Technical
Specification 24.302 version 16.4.0, July 2020.
[TS-3GPP.24.501]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Core Network and Terminals; Access to
the 3GPP Evolved Packet Core (EPC) via non-3GPP access
networks; Stage 3; (Release 16)", 3GPP Draft Technical
Specification 24.501 version 16.7.0, December 2020.
[TS-3GPP.33.102]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Services and System Aspects; 3G
Security; Security architecture (Release 16)",
3GPP Technical Specification 33.102 version 16.0.0, July
2020.
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[TS-3GPP.33.402]
3GPP, "3GPP System Architecture Evolution (SAE); Security
aspects of non-3GPP accesses (Release 16)", 3GPP Technical
Specification 33.402 version 16.0.0, July 2020.
[TS-3GPP.33.501]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Services and System Aspects; 3G
Security; Security architecture and procedures for 5G
System (Release 16)", 3GPP Technical Specification 33.501
version 16.5.0, December 2020.
[FIPS.180-4]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-4, August 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997, <https://www.rfc-
editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<https://www.rfc-editor.org/info/rfc3748>.
[RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication
Protocol Method for 3rd Generation Authentication and Key
Agreement (EAP-AKA)", RFC 4187, DOI 10.17487/RFC4187,
January 2006, <https://www.rfc-editor.org/info/rfc4187>.
[RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542,
DOI 10.17487/RFC7542, May 2015, <https://www.rfc-
editor.org/info/rfc7542>.
[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>.
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[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>.
9.2. Informative References
[TS-3GPP.35.208]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Services and System Aspects; 3G
Security; Specification of the MILENAGE Algorithm Set: An
example algorithm set for the 3GPP authentication and key
generation functions f1, f1*, f2, f3, f4, f5 and f5*;
Document 4: Design Conformance Test Data (Release 14)",
3GPP Technical Specification 35.208 version 15.0.0,
October 2018.
[FIPS.180-1]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-1, April 1995,
<http://www.itl.nist.gov/fipspubs/fip180-1.htm>.
[FIPS.180-2]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002,
<http://csrc.nist.gov/publications/fips/fips180-2/
fips180-2.pdf>.
[RFC3310] Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer
Protocol (HTTP) Digest Authentication Using Authentication
and Key Agreement (AKA)", RFC 3310, DOI 10.17487/RFC3310,
September 2002, <https://www.rfc-editor.org/info/rfc3310>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, <https://www.rfc-
editor.org/info/rfc4086>.
[RFC4169] Torvinen, V., Arkko, J., and M. Naslund, "Hypertext
Transfer Protocol (HTTP) Digest Authentication Using
Authentication and Key Agreement (AKA) Version-2",
RFC 4169, DOI 10.17487/RFC4169, November 2005,
<https://www.rfc-editor.org/info/rfc4169>.
[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>.
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[RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity
Selection Hints for the Extensible Authentication Protocol
(EAP)", RFC 4284, DOI 10.17487/RFC4284, January 2006,
<https://www.rfc-editor.org/info/rfc4284>.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005,
<https://www.rfc-editor.org/info/rfc4306>.
[RFC5113] Arkko, J., Aboba, B., Korhonen, J., Ed., and F. Bari,
"Network Discovery and Selection Problem", RFC 5113,
DOI 10.17487/RFC5113, January 2008, <https://www.rfc-
editor.org/info/rfc5113>.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, DOI 10.17487/RFC5247, August 2008,
<https://www.rfc-editor.org/info/rfc5247>.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281,
DOI 10.17487/RFC5281, August 2008, <https://www.rfc-
editor.org/info/rfc5281>.
[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>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013, <https://www.rfc-
editor.org/info/rfc6973>.
[RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
"Tunnel Extensible Authentication Protocol (TEAP) Version
1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
<https://www.rfc-editor.org/info/rfc7170>.
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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.ietf-emu-aka-pfs]
Arkko, J., Norrman, K., and V. Torvinen,"Perfect-Forward Secrecy
for the Extensible Authentication Protocol Method for
Authentication and Key Agreement (EAP-AKA' PFS)", draft-
ietf-emu-aka-pfs-05 (work in progress), October 2020.
[Heist2015]
Scahill, J. and J. Begley, "The great SIM heist", February
2015, in https://firstlook.org/theintercept/2015/02/19/
great-sim-heist/ .
[MT2012] Mjolsnes, S. and J-K. Tsay, "A vulnerability in the UMTS
and LTE authentication and key agreement protocols",
October 2012, in Proceedings of the 6th international
conference on Mathematical Methods, Models and
Architectures for Computer Network Security: computer
network security.
[BT2013] Beekman, J. and C. Thompson, "Breaking Cell Phone
Authentication: Vulnerabilities in AKA, IMS and Android",
August 2013, in 7th USENIX Workshop on Offensive
Technologies, WOOT '13.
[ZF2005] Zhang, M. and Y. Fang, "Breaking Cell Phone
Authentication: Vulnerabilities in AKA, IMS and Android",
March 2005, IEEE Transactions on Wireless Communications,
Vol. 4, No. 2.
[Basin2018]
Basin, D., Dreier, J., Hirsch, L., Radomirovic, S., Sasse,
R., and V. Stettle, "A Formal Analysis of 5G
Authentication", August 2018, arXiv:1806.10360.
[Arapinis2012]
Arapinis, M., Mancini, L., Ritter, E., Ryan, M., Golde,
N., and R. Borgaonkar, "New Privacy Issues in Mobile
Telephony: Fix and Verification", October 2012, CCS'12,
Raleigh, North Carolina, USA.
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[Borgaonkar2018]
Borgaonkar, R., Hirschi, L., Park, S., and A. Shaik, "New
Privacy Threat on 3G, 4G, and Upcoming 5G AKA Protocols",
2018 in IACR Cryptology ePrint Archive.
[Kune2012]
Kune, D., Koelndorfer, J., and Y. Kim, "Location leaks on
the GSM air interface", 2012 in the proceedings of NDSS
'12 held 5-8 February, 2012 in San Diego, California.
[Shaik2016]
Shaik, A., Seifert, J., Borgaonkar, R., Asokan, N., and V.
Niemi, "Practical attacks against privacy and availability
in 4G/LTE mobile communication systems", 2012 in the
proceedings of NDSS '16 held 21-24 February, 2016 in San
Diego, California.
[Hussain2019]
Hussain, S., Echeverria, M., Chowdhury, O., Li, N., and E.
Bertino, "Privacy Attacks to the 4G and 5G Cellular Paging
Protocols Using Side Channel Information", in the
Proceedings of NDSS '19, held 24-27 February, 2019, in San
Diego, California.
Appendix A. Changes from RFC 5448
The changes consist first of all, referring to a newer version of
[TS-3GPP.24.302]. The new version includes an updated definition of
the Network Name field, to include 5G.
Secondly, identifier usage for 5G has been specified in Section 5.3.
Also, the requirements on generating pseudonym usernames and fast re-
authentication identities have been updated from the original
definition in RFC 5448, which referenced RFC 4187. See Section 5.
Thirdly, exported parameters for EAP-AKA' have been defined in
Section 6, as required by [RFC5247], including the definition of
those parameters for both full authentication and fast re-
authentication.
The security, privacy, and pervasive monitoring considerations have
been updated or added. See Section 7.
The references to [RFC2119], [RFC7542], [RFC7296], [RFC8126],
[FIPS.180-1] and [FIPS.180-2] have been updated to their most recent
versions and language in this document changed accordingly. However,
this is merely an update to a newer RFC but the actual protocol
functions are the same as defined in the earlier RFCs.
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Similarly, references to all 3GPP technical specifications have been
updated to their 5G (Release 16) versions or otherwise most recent
version when there has not been a 5G-related update.
Finally, a number of clarifications have been made, including a
summary of where attributes may appear.
Appendix B. Changes to RFC 4187
In addition to specifying EAP-AKA', this document mandates also a
change to another EAP method, EAP-AKA that was defined in RFC 4187.
This change was mandated already in RFC 5448 but repeated here to
ensure that the latest EAP-AKA' specification contains the
instructions about the necessary bidding down feature in EAP-AKA as
well.
The changes to RFC 4187 relate only to the bidding down prevention
support defined in Section 4. In particular, this document does not
change how the Master Key (MK) is calculated or any other aspect of
EAP-AKA. The provisions in this specification for EAP-AKA' do not
apply to EAP-AKA, outside Section 4.
Appendix C. Changes from Previous Version of This Draft
RFC Editor: Please delete this section at the time of publication.
The -00 version of the working group draft is merely a republication
of an earlier individual draft.
The -01 version of the working group draft clarifies updates
relationship to RFC 4187, clarifies language relating to obsoleting
RFC 5448, clarifies when the 3GPP references are expected to be
stable, updates several past references to their more recently
published versions, specifies what identifiers should be used in key
derivation formula for 5G, specifies how to construct the network
name in manner that is compatible with both 5G and previous versions,
and has some minor editorial changes.
The -02 version of the working group draft added specification of
peer identity usage in EAP-AKA', added requirements on the generation
of pseudonym and fast re-authentication identifiers, specified the
format of 5G-identifiers when they are used within EAP-AKA', defined
privacy and pervasive surveillance considerations, clarified when 5G-
related procedures apply, specified what Peer-Id value is exported
when no AT_IDENTITY is exchanged within EAP-AKA', and made a number
of other clarifications and editorial improvements. The security
considerations section also includes a summary of vulnerabilities
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brought up in the context of AKA or EAP-AKA', and discusses their
applicability and impacts in EAP-AKA'.
The -03 version of the working group draft corrected some typos,
referred to the 3GPP specifications for the SUPI and SUCI formats,
updated some of the references to newer versions, and reduced the
strength of some of the recommendations in the security
considerations section from keyword level to normal language (as they
are just deployment recommendations).
The -04 version of the working group draft rewrote the abstract and
some of the introduction, corrected some typos, added sentence to the
abstract about obsoleting RFC 5448, clarified the use of the language
when referring to AT_KDF values vs. AT_KDF attribute number, provided
guidance on random number generation, clarified the dangers relating
to the use of permanent user identities such as IMSIs, aligned the
key derivation function/mechanism terminology, aligned the key
derivation/generation terminology, aligned the octet/byte
terminology, clarified the text regarding strength of SHA-256, added
some cross references between sections, instructed IANA to change
registries to point to this RFC rather than RFC 5448, and changed
Pasi's listed affiliation.
The -05 version of the draft corrected the Section 7.1 statement that
SUCI must not be communicated in EAP-AKA'; this statement was meant
to say SUPI must not be communicated. That was a major bug, but
hopefully one that previous readers understood was a mistake!
The -05 version also changed keyword strengths for identifier
requests in different cases in a 5G network, to match the 3GPP
specifications (see Section 5.3.2.
Tables of where attributes may appear has been added to the -05
version of the document, see Section 3.5 and Section 4.1. The tables
are based on the original table in RFC 4187.
Other changes in the -05 version included the following:
o The attribute appearance table entry for AT_MAC in EAP-Response/
AKA-Challenge has been specified to be 0-1 because it does not
appear when AT_KDF has to be sent; this was based on implementor
feedback.
o Added information about attacks against the re-synchronization
protocol and other attacks recently discussed in academic
conferences.
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o Clarified length field calculations and the AT_KDF negotiation
procedure.
o The treatment of AT_KDF attribute copy in the EAP-Response/AKA'-
Synchronization-Failure message was clarified in Section 3.2.
o Updated and added several references
o Switched to use of hexadecimal for EAP Type Values for consistency
with other documents.
o Made editorial clarifications to a number places in the document.
The version -06 included changes to updates of references to newer
versions on IANA considerations guidelines, NAIs, and IKEv2.
The version -07 includes the following changes, per AD and last call
review comments:
o The use of pseudonyms has been clarified in Section 7.1.
o The document now clarifies that it specifies behaviour both for 4G
and 5G.
o The implications of collisions between "Access Network ID" (4G)
and "Serving Network Name" (5G) have been explained in
Section 3.1.
o The ability of the bidding down protection to protect bidding down
only in the direction from EAP-AKA' to EAP-AKA but the other way
around has been noted in Section 7.
o The implications of the attack described by [Borgaonkar2018] have
been updated.
o Section 3.1 now specifies more clearly that zero-length network
name is not allowed.
o Section 3.1 refers to the network name that is today specified in
[TS-3GPP.24.302] for both 4G (non-3GPP access) and 5G.
o Section 7 now discusses cryptographic agility.
o The document now is clear that any change to key aspects of 3GPP
specifications, such as key derivation for AKA, would affect this
specification and implementations.
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o References have been updated to the latest Release 15 versions,
that are now stable.
o Tables have been numbered.
o Adopted a number of other editorial corrections.
The version -08 includes the following changes:
o Alignment of the 3GPP TS Annex and this draft, so that each
individual part of the specification is stated in only one place.
This has lead to this draft referring to bigger parts of the 3GPP
specification, instead of spelling out the details within this
document. Note that this alignment change is a proposal at this
stage, and will be discussed in the upcoming 3GPP meeting.
o Relaxed the language on using only SUCI in 5G. While that is the
mode of operation expected to be used, [TS-3GPP.33.501] does not
prohibit other types of identifiers.
The version -09 includes the following changes:
o Updated the language relating to obsoleting/updating RFC 5448;
there was an interest to ensure that RFC 5448 stays a valid
specification also in the future, owing to existing
implementations.
o Clarified that the leading digit "6" is not used in 5G networks.
o Updated the language relating to when 5G-specific procedures are
in effect, to support new use cases 3GPP has defined.
o Updated the reference in Section 3.3, as the identities are
different in the 5G case.
o Clarified that the use of the newer reference to IKEv2 RFC did not
change the actual PRF' function from RFC 5448.
o Clarified that the Section 5.2 text does not impact backwards
compatibility.
o Corrected the characterization of the attack from [ZF2005].
o Mentioned 5G GUTIs as one possible 5G-identifier in Section 5.1.
o Updated the references to Release 16. These specifications are
stable in 3GPP.
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Version -10 is the final version and made changes per IESG and
directorate review comments. These changes were editorial. One
duplicate requirement in Section 5.3.1 was removed, and some
references were added for tunnel methods discussion in Section 7.1.
The language about exported parameters was clarified in Section 6.
Appendix D. Importance of Explicit Negotiation
Choosing between the traditional and revised AKA key derivation
functions is easy when their use is unambiguously tied to a
particular radio access network, e.g., Long Term Evolution (LTE) as
defined by 3GPP or evolved High Rate Packet Data (eHRPD) as defined
by 3GPP2. There is no possibility for interoperability problems if
this radio access network is always used in conjunction with new
protocols that cannot be mixed with the old ones; clients will always
know whether they are connecting to the old or new system.
However, using the new key derivation functions over EAP introduces
several degrees of separation, making the choice of the correct key
derivation functions much harder. Many different types of networks
employ EAP. Most of these networks have no means to carry any
information about what is expected from the authentication process.
EAP itself is severely limited in carrying any additional
information, as noted in [RFC4284] and [RFC5113]. Even if these
networks or EAP were extended to carry additional information, it
would not affect millions of deployed access networks and clients
attaching to them.
Simply changing the key derivation functions that EAP-AKA [RFC4187]
uses would cause interoperability problems with all of the existing
implementations. Perhaps it would be possible to employ strict
separation into domain names that should be used by the new clients
and networks. Only these new devices would then employ the new key
derivation function. While this can be made to work for specific
cases, it would be an extremely brittle mechanism, ripe to result in
problems whenever client configuration, routing of authentication
requests, or server configuration does not match expectations. It
also does not help to assume that the EAP client and server are
running a particular release of 3GPP network specifications. Network
vendors often provide features from future releases early or do not
provide all features of the current release. And obviously, there
are many EAP and even some EAP-AKA implementations that are not
bundled with the 3GPP network offerings. In general, these
approaches are expected to lead to hard-to-diagnose problems and
increased support calls.
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Appendix E. Test Vectors
Test vectors are provided below for four different cases. The test
vectors may be useful for testing implementations. In the first two
cases, we employ the MILENAGE algorithm and the algorithm
configuration parameters (the subscriber key K and operator algorithm
variant configuration value OP) from test set 19 in [TS-3GPP.35.208].
The last two cases use artificial values as the output of AKA, and is
useful only for testing the computation of values within EAP-AKA',
not AKA itself.
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Case 1
The parameters for the AKA run are as follows:
Identity: "0555444333222111"
Network name: "WLAN"
RAND: 81e9 2b6c 0ee0 e12e bceb a8d9 2a99 dfa5
AUTN: bb52 e91c 747a c3ab 2a5c 23d1 5ee3 51d5
IK: 9744 871a d32b f9bb d1dd 5ce5 4e3e 2e5a
CK: 5349 fbe0 9864 9f94 8f5d 2e97 3a81 c00f
RES: 28d7 b0f2 a2ec 3de5
Then the derived keys are generated as follows:
CK': 0093 962d 0dd8 4aa5 684b 045c 9edf fa04
IK': ccfc 230c a74f cc96 c0a5 d611 64f5 a76c
K_encr: 766f a0a6 c317 174b 812d 52fb cd11 a179
K_aut: 0842 ea72 2ff6 835b fa20 3249 9fc3 ec23
c2f0 e388 b4f0 7543 ffc6 77f1 696d 71ea
K_re: cf83 aa8b c7e0 aced 892a cc98 e76a 9b20
95b5 58c7 795c 7094 715c b339 3aa7 d17a
MSK: 67c4 2d9a a56c 1b79 e295 e345 9fc3 d187
d42b e0bf 818d 3070 e362 c5e9 67a4 d544
e8ec fe19 358a b303 9aff 03b7 c930 588c
055b abee 58a0 2650 b067 ec4e 9347 c75a
EMSK: f861 703c d775 590e 16c7 679e a387 4ada
8663 11de 2907 64d7 60cf 76df 647e a01c
313f 6992 4bdd 7650 ca9b ac14 1ea0 75c4
ef9e 8029 c0e2 90cd bad5 638b 63bc 23fb
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Case 2
The parameters for the AKA run are as follows:
Identity: "0555444333222111"
Network name: "HRPD"
RAND: 81e9 2b6c 0ee0 e12e bceb a8d9 2a99 dfa5
AUTN: bb52 e91c 747a c3ab 2a5c 23d1 5ee3 51d5
IK: 9744 871a d32b f9bb d1dd 5ce5 4e3e 2e5a
CK: 5349 fbe0 9864 9f94 8f5d 2e97 3a81 c00f
RES: 28d7 b0f2 a2ec 3de5
Then the derived keys are generated as follows:
CK': 3820 f027 7fa5 f777 32b1 fb1d 90c1 a0da
IK': db94 a0ab 557e f6c9 ab48 619c a05b 9a9f
K_encr: 05ad 73ac 915f ce89 ac77 e152 0d82 187b
K_aut: 5b4a caef 62c6 ebb8 882b 2f3d 534c 4b35
2773 37a0 0184 f20f f25d 224c 04be 2afd
K_re: 3f90 bf5c 6e5e f325 ff04 eb5e f653 9fa8
cca8 3981 94fb d00b e425 b3f4 0dba 10ac
MSK: 87b3 2157 0117 cd6c 95ab 6c43 6fb5 073f
f15c f855 05d2 bc5b b735 5fc2 1ea8 a757
57e8 f86a 2b13 8002 e057 5291 3bb4 3b82
f868 a961 17e9 1a2d 95f5 2667 7d57 2900
EMSK: c891 d5f2 0f14 8a10 0755 3e2d ea55 5c9c
b672 e967 5f4a 66b4 bafa 0273 79f9 3aee
539a 5979 d0a0 042b 9d2a e28b ed3b 17a3
1dc8 ab75 072b 80bd 0c1d a612 466e 402c
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Case 3
The parameters for the AKA run are as follows:
Identity: "0555444333222111"
Network name: "WLAN"
RAND: e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0
AUTN: a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0
IK: b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0
CK: c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0
RES: d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0
Then the derived keys are generated as follows:
CK': cd4c 8e5c 68f5 7dd1 d7d7 dfd0 c538 e577
IK': 3ece 6b70 5dbb f7df c459 a112 80c6 5524
K_encr: 897d 302f a284 7416 488c 28e2 0dcb 7be4
K_aut: c407 00e7 7224 83ae 3dc7 139e b0b8 8bb5
58cb 3081 eccd 057f 9207 d128 6ee7 dd53
K_re: 0a59 1a22 dd8b 5b1c f29e 3d50 8c91 dbbd
b4ae e230 5189 2c42 b6a2 de66 ea50 4473
MSK: 9f7d ca9e 37bb 2202 9ed9 86e7 cd09 d4a7
0d1a c76d 9553 5c5c ac40 a750 4699 bb89
61a2 9ef6 f3e9 0f18 3de5 861a d1be dc81
ce99 1639 1b40 1aa0 06c9 8785 a575 6df7
EMSK: 724d e00b db9e 5681 87be 3fe7 4611 4557
d501 8779 537e e37f 4d3c 6c73 8cb9 7b9d
c651 bc19 bfad c344 ffe2 b52c a78b d831
6b51 dacc 5f2b 1440 cb95 1552 1cc7 ba23
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Case 4
The parameters for the AKA run are as follows:
Identity: "0555444333222111"
Network name: "HRPD"
RAND: e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0 e0e0
AUTN: a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0 a0a0
IK: b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0 b0b0
CK: c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0 c0c0
RES: d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0 d0d0
Then the derived keys are generated as follows:
CK': 8310 a71c e6f7 5488 9613 da8f 64d5 fb46
IK': 5adf 1436 0ae8 3819 2db2 3f6f cb7f 8c76
K_encr: 745e 7439 ba23 8f50 fcac 4d15 d47c d1d9
K_aut: 3e1d 2aa4 e677 025c fd86 2a4b e183 61a1
3a64 5765 5714 63df 833a 9759 e809 9879
K_re: 99da 835e 2ae8 2462 576f e651 6fad 1f80
2f0f a119 1655 dd0a 273d a96d 04e0 fcd3
MSK: c6d3 a6e0 ceea 951e b20d 74f3 2c30 61d0
680a 04b0 b086 ee87 00ac e3e0 b95f a026
83c2 87be ee44 4322 94ff 98af 26d2 cc78
3bac e75c 4b0a f7fd feb5 511b a8e4 cbd0
EMSK: 7fb5 6813 838a dafa 99d1 40c2 f198 f6da
cebf b6af ee44 4961 1054 02b5 08c7 f363
352c b291 9644 b504 63e6 a693 5415 0147
ae09 cbc5 4b8a 651d 8787 a689 3ed8 536d
Contributors
The test vectors in Appendix C were provided by Yogendra Pal and
Jouni Malinen, based on two independent implementations of this
specification.
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Jouni Malinen provided suggested text for Section 6. John Mattsson
provided much of the text for Section 7.1. Karl Norrman was the
source of much of the information in Section 7.2.
Acknowledgments
The authors would like to thank Guenther Horn, Joe Salowey, Mats
Naslund, Adrian Escott, Brian Rosenberg, Laksminath Dondeti, Ahmad
Muhanna, Stefan Rommer, Miguel Garcia, Jan Kall, Ankur Agarwal, Jouni
Malinen, John Mattsson, Jesus De Gregorio, Brian Weis, Russ Housley,
Alfred Hoenes, Anand Palanigounder, Michael Richardsson, Roman
Danyliw, Dan Romascanu, Kyle Rose, Benjamin Kaduk, Alissa Cooper,
Erik Kline, Murray Kucherawy, Robert Wilton, Warren Kumari, Andreas
Kunz, Marcus Wong, Kalle Jarvinen, Daniel Migault, and Mohit Sethi
for their in-depth reviews and interesting discussions in this
problem space.
Authors' Addresses
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
Vesa Lehtovirta
Ericsson
Jorvas 02420
Finland
Email: vesa.lehtovirta@ericsson.com
Vesa Torvinen
Ericsson
Jorvas 02420
Finland
Email: vesa.torvinen@ericsson.com
Pasi Eronen
Independent
Finland
Email: pe@iki.fi
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