Improved Extensible Authentication Protocol
Method for 3rd Generation Authentication and Key Agreement
(EAP-AKA')EricssonJorvas02420Finlandjari.arkko@piuha.netEricssonJorvas02420Finlandvesa.lehtovirta@ericsson.comNokia Research CenterP.O. Box 407FIN-00045 Nokia GroupFinlandpasi.eronen@nokia.comEAPAKAAKA'3GPPThis 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'.This specification defines a new Extensible Authentication Protocol
(EAP) method, EAP-AKA', which is a small revision of
the EAP-AKA method originally defined in
. What is new in EAP-AKA' is that it has a new
key derivation function, specified in
. This function binds the keys derived
within the method to the name of the access network. This limits the
effects of compromised access network nodes and keys. This
specification defines the EAP encapsulation for AKA when the new key
derivation mechanism is in use.3GPP has defined a number of applications for the revised AKA
mechanism, some based on native encapsulation of AKA over 3GPP radio
access networks and others based on the use of EAP.For making the new key derivation mechanisms usable in EAP-AKA,
additional protocol mechanisms are necessary. Given that RFC 4187
calls for the use of CK (the encryption key) and IK (the integrity
key) from AKA, existing implementations continue to use these. Any
change of the key derivation must be unambiguous to both sides in the
protocol. That is, it must not be possible to accidentally connect old
equipment to new equipment and get the key derivation wrong or attempt
to use wrong keys without getting a proper error message. The change
must also be secure against bidding down attacks that attempt to force
the participants to use the least secure mechanism.This specification therefore introduces a variant of the EAP-AKA
method, called EAP-AKA'. This method can employ the derived keys CK'
and IK' from the 3GPP specification and updates the used hash function
to SHA-256 . 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
.
Note: 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. defines the EAP-AKA'
method. adds support to EAP-AKA
to prevent bidding down attacks from EAP-AKA'.
explains the security differences between EAP-AKA and
EAP-AKA'. describes the IANA considerations and
explains what updates to RFC 4187 EAP-AKA have
been made in this specification. Finally,
explains some of the design rationale for creating EAP-AKA'.
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 .EAP-AKA' is a new EAP method that follows the EAP-AKA specification
in all respects except the following:
It uses the Type code 50, not 23 (which is used by
EAP-AKA).It carries the AT_KDF_INPUT attribute, as defined in
, to ensure that both the peer and server know
the name of the access network.It supports key derivation function negotiation via the AT_KDF
attribute () to allow for future
extensions.It calculates keys as defined in , not as
defined in EAP-AKA.It employs SHA-256 , not SHA-1
().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 .The format of the AT_KDF_INPUT attribute is shown below.The fields are as follows:This is set to 23.The length of the
attribute, calculated as defined in , Section
8.1.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.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
, 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
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 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 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,
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 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.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.The fields are as follows:This is set to 24.The length of the
attribute, MUST be set to 1.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 .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 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
). The peer fails the authentication also if
there are any duplicate values within the list of AT_KDF attributes
(except where the duplication is due to a request to change the key
derivation function; see below for further information).Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the
peer, the server checks that the suggested AT_KDF value was one of the
alternatives in its offer. The first AT_KDF value in the message from
the server is not a valid alternative. If the peer has replied with
the first AT_KDF value, the server behaves as if AT_MAC of the
response had been incorrect and fails the authentication. For an
overview of the failed authentication process in the server side, see
Section 3 and Figure 2 of . 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. 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.Both the peer and server MUST derive the keys as follows.
In this case, MK is derived and used as
follows:
Here [n..m] denotes the substring from bit n to m. PRF' is a new
pseudo-random function specified in . 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 .
IK' and CK' are derived as
specified in . 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.
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 .
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 . 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:
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 . 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.
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 .
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 .EAP-AKA' uses SHA-256 , not SHA-1
as in EAP-AKA. This requires a change
to the pseudo-random function (PRF) as well as the AT_MAC and
AT_CHECKCODE attributes.The PRF' construction is the same one IKEv2 uses (see Section 2.13 of
). 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' produces as many bits of output as is needed. HMAC-SHA-256 is
the application of HMAC to SHA-256.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
.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:Second, the checkcode is a hash value, calculated with SHA-256
, over the data specified in Section
10.13 of .As discussed in , 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
). 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.The fields are as follows:This is set to 136.The length of the
attribute, MUST be set to 1.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.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 ). 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 () 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'.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:
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. 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 ,
Section 12 for further details. 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
, Section 12 for further details. The only
difference is that a stronger hash algorithm, SHA-256, is used instead
of SHA-1. 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 ,
Section 12 for further details. The properties
of EAP-AKA' are exactly the same as those of EAP-AKA in this
respect. Refer to , Section 12 for further
details. 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
. 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
.
EAP-AKA' uses a pseudo-random function modeled after the one used in
IKEv2 together with SHA-256. See above.
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
, Section 12 for further details. 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 , Section
12 for further details. Note that implementations MUST prevent
performing a fast reconnect across method types. 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
, Section 12 for further details. The
properties of EAP-AKA' are exactly the same as those of EAP-AKA in
this respect. Refer to , Section 12 for further
details. The properties of
EAP-AKA' are exactly the same as those of EAP-AKA in this
respect. Refer to , Section 12 for further
details. EAP-AKA', like
EAP-AKA, does not provide channel bindings as they're defined in
and . 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 the
following section for more discussion.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 . 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,
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 . See also
. 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.EAP-AKA' has the EAP Type value 50 in the Extensible
Authentication Protocol (EAP) Registry under Method Types. Per Section
6.2 of
, this allocation can be made with
Designated Expert and Specification Required..
EAP-AKA' shares its attribute space and subtypes with EAP-SIM
and EAP-AKA . No new
registries are needed.However, a new Attribute Type value (23) in the non-skippable
range has been assigned for AT_KDF_INPUT ()
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 ().Finally, a new Attribute Type value (136) in the skippable range
has been assigned for AT_BIDDING ().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 .
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.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)3GPP3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
3G Security;
Security architecture
(Release 8)
3GPP3GPP System Architecture Evolution (SAE); Security aspects of non-3GPP accesses; Release 83GPPSecure Hash StandardNational Institute of Standards and Technology3rd Generation Partnership Project;
Technical Specification Group Core Network and Terminals;
Numbering, addressing and identification
(Release 8)3GPPSecure Hash StandardNational Institute of Standards and TechnologyThe changes to RFC 4187 relate only to the bidding down prevention
support defined in . 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.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 and
. 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
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.