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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5448
Network Working Group J. Arkko
Internet-Draft V. Lehtovirta
Updates: 4187 (if approved) Ericsson
Intended status: Informational P. Eronen
Expires: February 28, 2009 Nokia
August 27, 2008
Improved Extensible Authentication Protocol Method for 3rd Generation
Authentication and Key Agreement (EAP-AKA')
draft-arkko-eap-aka-kdf-02
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Abstract
This specification defines a new EAP method, EAP-AKA', a small
revision of the EAP-AKA 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 3GPP. This specification allows its use in EAP in an
interoperable manner. In addition, EAP-AKA' employs SHA256 instead
of SHA1.
This specification also updates RFC 4187 EAP-AKA to add support for
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preventing bidding down attacks between itself and EAP-AKA'.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements language . . . . . . . . . . . . . . . . . . . . 4
3. EAP-AKA' . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. AT_KDF_INPUT . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. AT_KDF . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Key Generation . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Hash Functions . . . . . . . . . . . . . . . . . . . . . . 11
3.4.1. PRF' . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.2. AT_MAC . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.3. AT_CHECKCODE . . . . . . . . . . . . . . . . . . . . . 11
4. Bidding Down Prevention for EAP-AKA . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
5.1. Security Properties of Binding Network Names . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Changes from RFC 4187 . . . . . . . . . . . . . . . . 19
Appendix B. Editor's Note: Importance of Explicit Negotiation . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Introduction
This specification defines a new Extensible Authentication Protocol
(EAP)[RFC3748] method, EAP-AKA', 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 directly, existing implementations continue to use
these. Any change of the key derivation must be unambiguous to both
sides in the protocol. That is, it must not be possible to
accidentally connect old equipment to new equipment and get the key
derivation wrong or attempt to use wrong keys without getting a
proper error message. The change must also be secure against bidding
down attacks that attempt to force the participants to use the least
secure mechanism.
This specification therefore introduces a variant of the EAP-AKA
method, called EAP-AKA'. This method can employ the derived keys CK'
and IK' from the 3GPP specification and updates the used hash
function to SHA256. 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].
Editor's Note: A number of other possible ways to use the new key
derivation functions have been proposed. These include
configuration and reliance on a particular domain employing only
the new functions. Appendix B explains why these approaches lead
to severe interoperability problems and why it is important to be
explicit about the change of semantics in protocol design. RFC
Editor: Please delete this and other Editor's Notes upon
publication of this specification as an RFC.
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
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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.
Editor's Note: The publication of this RFC depends on its
normative references [3GPP.33.102] and [3GPP.33.402] from 3GPP
reaching their final Release 8 status at 3GPP. This is expected
to happen shortly. The RFC Editor should check with the 3GPP
liaisons that this has happened.
2. Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHOULD", and "SHOULD NOT", 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 TBD1 BY IANA, 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 calculates keys as defined in Section 3.3, not as defined in
EAP-AKA.
o It employs SHA256, not SHA1 (Section 3.4).
o It has support for possible future key derivation function changes
via the AT_KDF attribute (Section 3.2) .
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 from [RFC4187].
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Peer Server
| EAP-Request/Identity |
|<------------------------------------------------------|
| |
| EAP-Response/Identity |
| (Includes user's NAI) |
|------------------------------------------------------>|
| +-------------------------------+
| | Server determines the |
| | network name, runs AKA' |
| | algorithms generating RAND |
| | and AUTN, derives session |
| | keys from CK'/IK'. RAND and |
| | AUTN are sent as AT_RAND and |
| | AT_AUTN attributes, whereas |
| | network name is transported |
| | in 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)|
|<------------------------------------------------------|
+--------------------------------------+ |
| Peer verifies the network name from | |
| AT_KDF_INPUT, and uses it in running | |
| the AKA' algorithms, verifying AUTN | |
| and generating RES. The peer also | |
| derives session keys from CK'/IK'. | |
| AT_RES and AT_MAC are constructed. | |
+--------------------------------------+ |
| EAP-Response/AKA'-Challenge |
| (AT_RES, AT_MAC) |
|------------------------------------------------------>|
| +--------------------------------+
| | Server checks the RES and MAC |
| | values received in AT_RES and |
| | AT_MAC, respectively. Success |
| | requires both to be found |
| | correct. |
| +--------------------------------+
| EAP-Success |
|<------------------------------------------------------|
Figure 1: EAP-AKA' Authentication Process
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3.1. 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 TBD2 BY IANA.
Length
The length of the attribute, calculated as defined in [RFC4187]
Section 8.1.
Actual Network Name Length
This a 2-byte actual length field, needed due to the requirement
that previous field is expressed in multiples of 4 bytes per the
usual EAP-SIM and EAP 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 one, two, or three bytes of all zero bits when
necessary.
Only the server sends the AT_KDF_INPUT attribute. The peer SHOULD
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 which it can
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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. See Section 3 and
Figure 3 of [RFC4187] for an overview of how authentication failures
are handled.
The Network Name field contains an octet string. This string MUST be
constructed as specified in [3GPP.23.003]. This is done in a manner
that is specific to a particular access technology. For access
technologies where the above reference does not provide an
instruction on how to construct the name, the empty (zero length)
octet string SHOULD be used.
Editor's Note: This 3GPP specification is still being worked on.
It is assumed that the specification ensures that conflicts
potentially arising from using the same name in different types of
networks are avoided. It is also assumed that the 3GPP
specification will have 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) should not be used by this algorithm.
The AT_KDF_INPUT attribute MUST be sent when AT_KDF attribute has the
value 2. Otherwise, the AT_KDF_INPUT attribute SHOULD NOT be sent.
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:
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AT_KDF
This is set to TBD3 BY IANA.
Length
The length of the attribute, MUST be set to 1.
Key Derivation Function
An enumerated value representing the key derivation function that
the server (or peer) wishes to use. Value 1 represents RFC 4187
key derivation, i.e., fallback to EAP-AKA functionality. Value 2
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. The first of these attributes represents the
desired function and the other ones are acceptable alternatives, the
most desired alternative being the second attribute.
Upon receiving this attribute, 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 responds 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]).
Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the
peer, the server checks that the alternative was in its offer. If
not, it behaves as if AT_MAC of the response had been incorrect and
fails the authentication. For an overview of the failed
authentication process in the server side, see Section 3 and Figure 2
in [RFC4187]. Otherwise, the server re-sends the EAP-Response/
AKA'-Challenge message, but moves the selected alternative to the
beginning of the list of AT_KDF attributes.
When the peer receives the new EAP-Request/AKA'-Challenge message, it
MUST check that requested change, and only the requested change
occurred in the list of AT_KDF attributes. If yes, it continues. If
not, it behaves as if AT_MAC had been incorrect and fails the
authentication.
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3.3. Key Generation
Both the peer and server MUST derive the keys as follows.
AT_KDF set to 1
The Master Key (MK) is generated and used as defined in [RFC4187].
Similarly, if fast re-authentication is employed, [RFC4187]
procedures for generating the relevant keys are followed.
There are no restrictions on how the parameters of the AKA
algorithm are selected. In particular, implementations MAY set
the so called AMF separation bit to either 0 or 1 in the AKA
algorithm. The specification of this bit can be found from Annex
H in [3GPP.33.102]. Even if this bit is 1, it MUST NOT change the
key derivation procedures when AT_KDF is set to 1. The same rules
apply even to the EAP-AKA method; the new key derivation
procedures MUST NOT be applied.
AT_KDF set to 2
In this case MK is derived and used as follows:
MK = PRF'(IK'|CK',"EAP-AKA'"|Identity)
K_encr|K_aut|K_re|MSK|EMSK = MK[0..1663]
Here [n..m] denotes the substring from bit n to m. PRF' is a new
pseudo random function specified in Section 3.4. The 1664 first
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.
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 (without length or padding) from AT_KDF_INPUT.
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 AMF separation bit to
1 in the AKA algorithm [3GPP.33.102]. Similarly, the peer MUST
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check that the AMF separation bit set is 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|EMSK = MK[0..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. Identity is the fast re-authentication identity,
counter is the value from 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 on
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 knowns 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 identifer had been
unrecognized and fall back to full authentication. The server
observers the change in the name by comparing where the fast re-
authentication and full authentication EAP transactions were
received from 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 fails 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 SHA256 [FIPS.180-2.2002], not SHA1 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 as IKEv2 uses (see Section 2.13
in [RFC4306]). The function takes two arguments. K is a 256 bit
value and S is an octet string of arbitrary length. PRF' is defined
as follows:
PRF'(K,S) = T1 | T2 | T3 | T4 | ...
where:
T1 = HMAC-SHA256 (K, S | 0x01)
T2 = HMAC-SHA256 (K, T1 | S | 0x02)
T3 = HMAC-SHA256 (K, T2 | S | 0x03)
T4 = HMAC-SHA256 (K, T3 | S | 0x04)
...
PRF' produces as many bits of output as is needed. HMAC-SHA256 is
the application of HMAC [RFC2104] to SHA256.
3.4.2. AT_MAC
The AT_MAC attribute is changed as follows. The MAC algorithm is
HMAC-SHA256-128, a keyed hash value. The HMAC-SHA256-128 value is
obtained from the 32-byte HMAC-SHA256 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
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 SHA256
[FIPS.180-2.2002], over the data specified in Section 10.13 of
[RFC4187].
4. Bidding Down Prevention for EAP-AKA
As discussed in [RFC3748], negotiation of methods within EAP is
insecure. That is, a man-in-the-middle attacker may force the
endpoints to use a method that is not the strongest one 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 specification 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 preferrable (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:
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AT_BIDDING
This is set to TBD4 BY IANA.
Length
The length of the attribute, MUST be set to 1.
D
This bit is set to 1 if the sender does support EAP-AKA', is
willing to use it, and prefers it over EAP-AKA. Otherwise it
should be set to 0.
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.
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
SHA256 is at least as secure as SHA1. This is called the SHA256
assumption in the remainder of this section. Under this assumption
EAP-AKA' is at least as secure as EAP-AKA.
If AT_KDF has value 1, the security properties of EAP-AKA' are
equivalent to those of EAP-AKA [RFC4187]. If AT_KDF has value 2,
then the security properties 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.
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Mutual authentication
Under the SHA256 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 SHA256 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, SHA256 is used
instead of SHA1.
Replay protection
Under the SHA256 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. An attacker can thus be assumed to be incapable to
derive 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
the EMSK by any non-trivial 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 SHA256.
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Key strength
See above.
Dictionary attack resistance
Under the SHA256 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 SHA256 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 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.
However, including the network name field in the AKA' algorithms
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(which are also used for other purposes than EAP-AKA') does
provide 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 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' or any other extension can prevent such
attacks, but 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 peer verifies that its own observations about the access network
name are consistent with the server's observations. The server
receives the EAP transaction from a given access network, and can
either trust the name claim the access network made over AAA
protocols, or it may additionally verify that this corresponds to the
name that this access network should be using. Where such
verification is implemented, it becomes impossible for an access
network to claim to the peer that it is another access network. This
prevents 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, IK produced by the AKA
algorithm can compute the keys CK', 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 visited access network
where they may be vulnerable. In order to reduce this risk this
specification mandates that the AKA algorithm must be computed with
the AMF separation bit set to 1, and that the peer checks that this
is indeed the case whenever it runs EAP-AKA'. Furthermore,
[3GPP.33.402] requires that no keys computed in this way ever leave
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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.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 Considerations
EAP-AKA' has the EAP Type value TBD1 BY IANA. Per [RFC3748] Section
6.2, this allocation can be made with Designated Expert and
Specification Required.
EAP-AKA' shares its attribute space and message Subtypes, with EAP-
SIM [RFC4186] and EAP-AKA [RFC4186]. No new registries are needed.
However, a new Attribute Type value (TBD2) in the non-skippable range
needs to be assigned for AT_KDF_INPUT (Section 3.1).
Also, a new Attribute Type value (TBD3) in the non-skippable range
needs to be assigned for AT_KDF (Section 3.2). IANA also needs to
create a namespace for EAP-AKA' KDF Type values. The initial
contents of this namespace are given below; new values can be created
through Specification Required policy [RFC5226].
Value Description Reference
--------- ---------------------- ---------------
0 Reserved
1 EAP-AKA with CK/IK [this document]
2 EAP-AKA' with CK'/IK' [this document]
3-65535 Unassigned
Finally, a new Attribute Type value (TBD4) in the skippable range
needs to be assigned for AT_BIDDING (Section 4).
7. Acknowledgments
The authors would like to thank Guenther Horn, Joe Salowey, Mats
Naslund, Adrian Escott, Brian Rosenberg, Ahmad Muhanna, Stefan
Rommer, and Russ Housley for their in-depth reviews and interesting
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discussions in this problem space.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 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.
[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 v 8.0.0, June 2008.
[3GPP.33.102]
3GPP, "3rd Generation Partnership Project; Technical
Specification Group Services and System Aspects; 3G
Security; Security architecture (Release 8)", 3GPP Draft
Technical Specification 33.102 v 8.0.0, June 2008.
[3GPP.33.402]
3GPP, "3GPP System Architecture Evolution (SAE); Security
aspects of non-3GPP accesses; Release 8", 3GPP Draft
Technical Specification 33.402 v 8.0.0, June 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>.
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8.2. Informative References
[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.
Appendix A. Changes from RFC 4187
The changes to RFC 4187 relate only to the bidding down prevention
support defined Section 4.
Appendix B. Editor's Note: 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 LTE as defined by 3GPP or 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] [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
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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 the 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.
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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