Network Working Group J. Arkko
Request for Comments: 5448 V. Lehtovirta
Updates: 4187 Ericsson
Category: Informational P. Eronen
Nokia
May 2009
Improved Extensible Authentication Protocol Method for3rd Generation Authentication and Key Agreement (EAP-AKA')
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Abstract
This specification defines a new EAP method, EAP-AKA', which is a
small revision of the EAP-AKA (Extensible Authentication Protocol
Method for 3rd Generation Authentication and Key Agreement) method.
The change is a new key derivation function that binds the keys
derived within the method to the name of the access network. The new
key derivation mechanism has been defined in the 3rd Generation
Partnership Project (3GPP). This specification allows its use in EAP
in an interoperable manner. In addition, EAP-AKA' employs SHA-256
instead of SHA-1.
This specification also updates RFC 4187, EAP-AKA, to prevent bidding
down attacks from EAP-AKA'.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 22. Requirements Language . . . . . . . . . . . . . . . . . . . . 33. EAP-AKA' . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.1. AT_KDF_INPUT . . . . . . . . . . . . . . . . . . . . . . . 63.2. AT_KDF . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3. Key Generation . . . . . . . . . . . . . . . . . . . . . . 103.4. Hash Functions . . . . . . . . . . . . . . . . . . . . . . 123.4.1. PRF' . . . . . . . . . . . . . . . . . . . . . . . . . 123.4.2. AT_MAC . . . . . . . . . . . . . . . . . . . . . . . . 133.4.3. AT_CHECKCODE . . . . . . . . . . . . . . . . . . . . . 134. Bidding Down Prevention for EAP-AKA . . . . . . . . . . . . . 145. Security Considerations . . . . . . . . . . . . . . . . . . . 155.1. Security Properties of Binding Network Names . . . . . . . 186. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 196.1. Type Value . . . . . . . . . . . . . . . . . . . . . . . . 196.2. Attribute Type Values . . . . . . . . . . . . . . . . . . 196.3. Key Derivation Function Namespace . . . . . . . . . . . . 197. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 208. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 209. References . . . . . . . . . . . . . . . . . . . . . . . . . . 209.1. Normative References . . . . . . . . . . . . . . . . . . . 209.2. Informative References . . . . . . . . . . . . . . . . . . 21Appendix A. Changes from RFC 4187 . . . . . . . . . . . . . . . . 23Appendix B. Importance of Explicit Negotiation . . . . . . . . . 23Appendix C. Test Vectors . . . . . . . . . . . . . . . . . . . . 241. Introduction
This specification defines a new Extensible Authentication Protocol
(EAP)[RFC3748] method, EAP-AKA', which is a small revision of the
EAP-AKA method originally defined in [RFC4187]. What is new in EAP-
AKA' is that it has a new key derivation function, specified in
[3GPP.33.402]. This function binds the keys derived within the
method to the name of the access network. This limits the effects of
compromised access network nodes and keys. This specification
defines the EAP encapsulation for AKA when the new key derivation
mechanism is in use.
3GPP has defined a number of applications for the revised AKA
mechanism, some based on native encapsulation of AKA over 3GPP radio
access networks and others based on the use of EAP.
For making the new key derivation mechanisms usable in EAP-AKA,
additional protocol mechanisms are necessary. Given that RFC 4187
calls for the use of CK (the encryption key) and IK (the integrity
key) from AKA, existing implementations continue to use these. Any
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change of the key derivation must be unambiguous to both sides in the
protocol. That is, it must not be possible to accidentally connect
old equipment to new equipment and get the key derivation wrong or
attempt to use wrong keys without getting a proper error message.
The change must also be secure against bidding down attacks that
attempt to force the participants to use the least secure mechanism.
This specification therefore introduces a variant of the EAP-AKA
method, called EAP-AKA'. This method can employ the derived keys CK'
and IK' from the 3GPP specification and updates the used hash
function to SHA-256 [FIPS.180-2.2002]. But it is otherwise
equivalent to RFC 4187. Given that a different EAP method type value
is used for EAP-AKA and EAP-AKA', a mutually supported method may be
negotiated using the standard mechanisms in EAP [RFC3748].
Note: Appendix B explains why it is important to be explicit about
the change of semantics for the keys, and why other approaches
would lead to severe interoperability problems.
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 explains the
security differences between EAP-AKA and EAP-AKA'. Section 6
describes the IANA considerations and Appendix A explains what
updates to RFC 4187 EAP-AKA have been made in this specification.
Appendix B explains some of the design rationale for creating EAP-
AKA'. Finally, Appendix C provides test vectors.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. EAP-AKA'
EAP-AKA' is a new EAP method that follows the EAP-AKA specification
[RFC4187] in all respects except the following:
o It uses the Type code 50, not 23 (which is used by EAP-AKA).
o It carries the AT_KDF_INPUT attribute, as defined in Section 3.1,
to ensure that both the peer and server know the name of the
access network.
o It supports key derivation function negotiation via the AT_KDF
attribute (Section 3.2) to allow for future extensions.
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o It calculates keys as defined in Section 3.3, not as defined in
EAP-AKA.
o It employs SHA-256 [FIPS.180-2.2002], not SHA-1 [FIPS.180-1.1995]
(Section 3.4).
Figure 1 shows an example of the authentication process. Each
message AKA'-Challenge and so on represents the corresponding message
from EAP-AKA, but with EAP-AKA' Type code. The definition of these
messages, along with the definition of attributes AT_RAND, AT_AUTN,
AT_MAC, and AT_RES can be found in [RFC4187].
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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. EAP-AKA' MUST use the leading character "6" (ASCII 36
hexadecimal) instead of "0" for IMSI-based permanent usernames. All
other usage and processing of the leading characters, usernames, and
identities is as defined by EAP-AKA [RFC4187]. For instance, the
pseudonym and fast re-authentication usernames need to be constructed
so that the server can recognize them. As an example, a pseudonym
could begin with a leading "7" character (ASCII 37 hexadecimal) and a
fast re-authentication username could begin with "8" (ASCII 38
hexadecimal). Note that a server that implements only EAP-AKA may
not recognize these leading characters. According to Section 4.1.4
of [RFC4187], such a server will re-request the identity via the EAP-
Request/AKA-Identity message, making obvious to the peer that EAP-AKA
and associated identity are expected.
3.1. AT_KDF_INPUT
The format of the AT_KDF_INPUT attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_KDF_INPUT | Length | Actual Network Name Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Network Name .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_KDF_INPUT
This is set to 23.
Length
The length of the attribute, calculated as defined in [RFC4187],
Section 8.1.
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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. Per [3GPP.33.402],
the server always verifies the authorization of a given access
network to use a particular name before sending it to the peer over
EAP-AKA'. The value of the AT_KDF_INPUT attribute from the server
MUST be non-empty. If it is empty, the peer behaves as if AUTN had
been incorrect and authentication fails. See Section 3 and Figure 3
of [RFC4187] for an overview of how authentication failures are
handled.
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 [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
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
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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,
[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 [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.
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.
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The format of the AT_KDF attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_KDF | Length | Key Derivation Function |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_KDF
This is set to 24.
Length
The length of the attribute, MUST be set to 1.
Key Derivation Function
An enumerated value representing the key derivation function that
the server (or peer) wishes to use. Value 1 represents the
default key derivation function for EAP-AKA', i.e., employing CK'
and IK' as defined in Section 3.3.
Servers MUST send one or more AT_KDF attributes in the EAP-Request/
AKA'-Challenge message. These attributes represent the desired
functions ordered by preference, the most preferred function being
the first attribute.
Upon receiving a set of these attributes, if the peer supports and is
willing to use the key derivation function indicated by the first
attribute, the function is taken into use without any further
negotiation. However, if the peer does not support this function or
is unwilling to use it, it does not process the received EAP-Request/
AKA'-Challenge in any way except by responding with the EAP-Response/
AKA'-Challenge message that contains only one attribute, AT_KDF with
the value set to the selected alternative. If there is no suitable
alternative, the peer behaves as if AUTN had been incorrect and
authentication fails (see Figure 3 of [RFC4187]). The peer fails the
authentication also if there are any duplicate values within the list
of AT_KDF attributes (except where the duplication is due to a
request to change the key derivation function; see below for further
information).
Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the
peer, the server checks that the suggested AT_KDF value was one of
the alternatives in its offer. The first AT_KDF value in the message
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from the server is not a valid alternative. If the peer has replied
with the first AT_KDF value, the server behaves as if AT_MAC of the
response had been incorrect and fails the authentication. For an
overview of the failed authentication process in the server side, see
Section 3 and Figure 2 of [RFC4187]. Otherwise, the server re-sends
the EAP-Response/AKA'-Challenge message, but adds the selected
alternative to the beginning of the list of AT_KDF attributes and
retains the entire list following it. Note that this means that the
selected alternative appears twice in the set of AT_KDF values.
Responding to the peer's request to change the key derivation
function is the only legal situation where such duplication may
occur.
When the peer receives the new EAP-Request/AKA'-Challenge message, it
MUST check that the requested change, and only the requested change,
occurred in the list of AT_KDF attributes. If so, it continues with
processing the received EAP-Request/AKA'-Challenge as specified in
[RFC4187] and Section 3.1 of this document. If not, it behaves as if
AT_MAC had been incorrect and fails the authentication. If the peer
receives multiple EAP-Request/AKA'-Challenge messages with differing
AT_KDF attributes without having requested negotiation, the peer MUST
behave as if AT_MAC had been incorrect and fail the authentication.
Note that the peer may also request sequence number resynchronization
[RFC4187]. This happens after AT_KDF negotiation has already
completed. An AKA'-Synchronization-Failure message is sent as a
response to the newly received EAP-Request/AKA'-Challenge (the last
message of the AT_KDF negotiation). The AKA'-Synchronization-Failure
message MUST contain the AUTS parameter as specified in [RFC4187] and
a copy the AT_KDF attributes as they appeared in the last message of
the AT_KDF negotiation. If the AT_KDF attributes are found to differ
from their earlier values, the peer and server MUST behave as if
AT_MAC had been incorrect and fail the authentication.
3.3. Key Generation
Both the peer and server MUST derive the keys as follows.
AT_KDF set to 1
In this case, MK is derived and used as follows:
MK = PRF'(IK'|CK',"EAP-AKA'"|Identity)
K_encr = MK[0..127]
K_aut = MK[128..383]
K_re = MK[384..639]
MSK = MK[640..1151]
EMSK = MK[1152..1663]
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Here [n..m] denotes the substring from bit n to m. PRF' is a new
pseudo-random function specified in Section 3.4. The first 1664 bits
from its output are used for K_encr (encryption key, 128 bits), K_aut
(authentication key, 256 bits), K_re (re-authentication key, 256
bits), MSK (Master Session Key, 512 bits), and EMSK (Extended Master
Session Key, 512 bits). These keys are used by the subsequent
EAP-AKA' process. K_encr is used by the AT_ENCR_DATA attribute, and
K_aut by the AT_MAC attribute. K_re is used later in this section.
MSK and EMSK are outputs from a successful EAP method run [RFC3748].
IK' and CK' are derived as specified in [3GPP.33.402]. The functions
that derive IK' and CK' take the following parameters: CK and IK
produced by the AKA algorithm, and value of the Network Name field
comes from the AT_KDF_INPUT attribute (without length or padding) .
The value "EAP-AKA'" is an eight-characters-long ASCII string. It is
used as is, without any trailing NUL characters.
Identity is the peer identity as specified in Section 7 of [RFC4187].
When the server creates an AKA challenge and corresponding AUTN, CK,
CK', IK, and IK' values, it MUST set the Authentication Management
Field (AMF) separation bit to 1 in the AKA algorithm [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.
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,
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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.
3.4. Hash Functions
EAP-AKA' uses SHA-256 [FIPS.180-2.2002], not SHA-1 [FIPS.180-1.1995]
as in EAP-AKA. This requires a change to the pseudo-random function
(PRF) as well as the AT_MAC and AT_CHECKCODE attributes.
3.4.1. PRF'
The PRF' construction is the same one IKEv2 uses (see Section 2.13 of
[RFC4306]). The function takes two arguments. K is a 256-bit value
and S is an octet string of arbitrary length. PRF' is defined as
follows:
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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:
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.180-2.2002], over the data specified in Section 10.13 of
[RFC4187].
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RFC 5448 EAP-AKA' May 20094. Bidding Down Prevention for EAP-AKA
As discussed in [RFC3748], negotiation of methods within EAP is
insecure. That is, a man-in-the-middle attacker may force the
endpoints to use a method that is not the strongest that they both
support. This is a problem, as we expect EAP-AKA and EAP-AKA' to be
negotiated via EAP.
In order to prevent such attacks, this RFC specifies a new mechanism
for EAP-AKA that allows the endpoints to securely discover the
capabilities of each other. This mechanism comes in the form of the
AT_BIDDING attribute. This allows both endpoints to communicate
their desire and support for EAP-AKA' when exchanging EAP-AKA
messages. This attribute is not included in EAP-AKA' messages as
defined in this RFC. It is only included in EAP-AKA messages. This
is based on the assumption that EAP-AKA' is always preferable (see
Section 5). If during the EAP-AKA authentication process it is
discovered that both endpoints would have been able to use EAP-AKA',
the authentication process SHOULD be aborted, as a bidding down
attack may have happened.
The format of the AT_BIDDING attribute is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_BIDDING | Length |D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
AT_BIDDING
This is set to 136.
Length
The length of the attribute, MUST be set to 1.
D
This bit is set to 1 if the sender supports EAP-AKA', is willing
to use it, and prefers it over EAP-AKA. Otherwise, it should be
set to zero.
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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 5) that EAP-AKA' is always stronger than
EAP-AKA. As a result, there is no need to prevent bidding "down"
attacks in the other direction, i.e., attackers forcing the endpoints
to use EAP-AKA'.
5. 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 SHA-
256 is at least as secure as SHA-1. 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.
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.
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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, SHA-256, is
used instead of 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.
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 5.1.
EAP-AKA' uses a pseudo-random function modeled after the one used
in IKEv2 [RFC4306] together with SHA-256.
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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
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.
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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
the following section for more discussion.
5.1. 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
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runs EAP-AKA'. Furthermore, [3GPP.33.402] requires that no CK or IK
keys computed in this way ever leave the home subscriber system.
The additional security benefits obtained from the binding depend
obviously on the way names are assigned to different access networks.
This is specified in [3GPP.24.302]. See also [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.
6. IANA Considerations6.1. Type Value
EAP-AKA' has the EAP Type value 50 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.
6.2. 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).
6.3. 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 [RFC5226].
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Value Description Reference
--------- ---------------------- ---------------
0 Reserved [RFC5448]
1 EAP-AKA' with CK'/IK' [RFC5448]
2-65535 Unassigned
7. Contributors
The test vectors in Appendix C were provided by Yogendra Pal and
Jouni Malinen, based on two independent implementations of this
specification.
8. 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, Brian Weis, Russ Housley, and Alfred Hoenes for their in-
depth reviews and interesting discussions in this problem space.
9. References9.1. Normative References
[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 8)", 3GPP Technical Specification 24.302,
December 2008.
[3GPP.33.102] 3GPP, "3rd Generation Partnership Project;
Technical Specification Group Services and System
Aspects; 3G Security; Security architecture
(Release 8)", 3GPP Technical Specification 33.102,
December 2008.
[3GPP.33.402] 3GPP, "3GPP System Architecture Evolution (SAE);
Security aspects of non-3GPP accesses; Release 8",
3GPP Technical Specification 33.402,
December 2008.
[FIPS.180-2.2002] 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>.
Arkko, et al. Informational [Page 20]
RFC 5448 EAP-AKA' May 2009
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication",
RFC 2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J.,
and H. Levkowetz, "Extensible Authentication
Protocol (EAP)", RFC 3748, June 2004.
[RFC4187] Arkko, J. and H. Haverinen, "Extensible
Authentication Protocol Method for 3rd Generation
Authentication and Key Agreement (EAP-AKA)",
RFC 4187, January 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 5226, May 2008.
9.2. Informative References
[3GPP.23.003] 3GPP, "3rd Generation Partnership Project;
Technical Specification Group Core Network and
Terminals; Numbering, addressing and
identification (Release 8)", 3GPP Draft Technical
Specification 23.003, December 2008.
[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
8)", 3GPP Technical Specification 35.208,
December 2008.
[FIPS.180-1.1995] National Institute of Standards and Technology,
"Secure Hash Standard", FIPS PUB 180-1,
April 1995,
<http://www.itl.nist.gov/fipspubs/fip180-1.htm>.
Arkko, et al. Informational [Page 21]
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[RFC4186] Haverinen, H. and J. Salowey, "Extensible
Authentication Protocol Method for Global System
for Mobile Communications (GSM) Subscriber
Identity Modules (EAP-SIM)", RFC 4186,
January 2006.
[RFC4284] Adrangi, F., Lortz, V., Bari, F., and P. Eronen,
"Identity Selection Hints for the Extensible
Authentication Protocol (EAP)", RFC 4284,
January 2006.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.
[RFC5113] Arkko, J., Aboba, B., Korhonen, J., and F. Bari,
"Network Discovery and Selection Problem",
RFC 5113, January 2008.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management
Framework", RFC 5247, August 2008.
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RFC 5448 EAP-AKA' May 2009Appendix A. Changes from RFC 4187
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 in RFC 4187 (it uses CK
and IK, not CK' and IK'); neither is any processing of the AMF bit
added to RFC 4187.
Appendix B. 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 mechanism. 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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RFC 5448 EAP-AKA' May 2009Appendix C. 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 [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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Authors' Addresses
Jari Arkko
Ericsson
Jorvas 02420
Finland
EMail: jari.arkko@piuha.net
Vesa Lehtovirta
Ericsson
Jorvas 02420
Finland
EMail: vesa.lehtovirta@ericsson.com
Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland
EMail: pasi.eronen@nokia.com
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