Improved Extensible Authentication Protocol
Method for 3GPP Mobile Network Authentication and Key Agreement
(EAP-AKA')EricssonJorvas02420Finlandjari.arkko@piuha.netEricssonJorvas02420Finlandvesa.lehtovirta@ericsson.comEricssonJorvas02420Finlandvesa.torvinen@ericsson.comIndependentFinlandpe@iki.fiEAPAKAAKA'3GPPThe 3GPP mobile network Authentication and Key Agreement (AKA)
is an authentication mechanism for devices wishing to
access mobile networks. RFC 4187 (EAP-AKA) made the use of this mechanism
possible within the Extensible Authentication Protocol (EAP)
framework. RFC 5448 (EAP-AKA') was an improved version of EAP-AKA.This document is the most recent specification of EAP-AKA',
including, for instance, details about and references related to
operating EAP-AKA' in 5G networks.EAP-AKA' differs from EAP-AKA by providing a key derivation
function that binds the keys derived within the method to the name
of the access network. The key derivation function has been defined
in the 3rd Generation Partnership Project (3GPP). EAP-AKA' allows
its use in EAP in an interoperable manner. EAP-AKA' also
updates the algorithm used in hash functions, as it employs SHA-256 / HMAC-SHA-256 instead of
SHA-1 / HMAC-SHA-1, which is used in EAP-AKA.This version of EAP-AKA' specification defines the protocol
behavior for both 4G and 5G deployments, whereas the previous
version defined protocol behavior for 4G deployments only.IntroductionThe 3GPP mobile network Authentication and Key Agreement (AKA) is
an authentication mechanism for devices wishing to access mobile
networks. (EAP-AKA) made the use of this mechanism
possible within the Extensible Authentication Protocol (EAP)
framework .EAP-AKA' is an improved version of
EAP-AKA. EAP-AKA' was defined in RFC 5448 , and it updated EAP-AKA
. This document is the most recent specification of EAP-AKA',
including, for instance, details about and references related to
operating EAP-AKA' in 5G networks. This document does not obsolete RFC 5448; however, this document is the
most recent and fully backwards-compatible specification.EAP-AKA' is commonly implemented in mobile phones and network
equipment. It can be used for authentication to gain network access via
Wireless LAN networks and, with 5G, also directly to mobile
networks.EAP-AKA' differs from EAP-AKA by providing a different key
derivation function. This function binds the keys derived within the
method to the name of the access network. This limits the effects of
compromised access network nodes and keys. EAP-AKA' also updates
the algorithm used for hash functions.The EAP-AKA' method employs the derived keys CK' and IK' from the
3GPP specification and updates the
hash function that is used to SHA-256 and HMAC
to HMAC-SHA-256. Otherwise, EAP-AKA' is equivalent to EAP-AKA. Given
that a different EAP method Type value is used for EAP-AKA and
EAP-AKA', a mutually supported method may be negotiated using the
standard mechanisms in EAP .
Note that any change of the key derivation must be unambiguous
to both sides in the protocol. That is, it must not be possible to
accidentally connect old equipment to new equipment and get the
key derivation wrong or to attempt to use incorrect keys without getting
a proper error message. See for further
information.Note also that choices in authentication protocols should be
secure against bidding down attacks that attempt to force the
participants to use the least secure function. See for further information.This specification makes the following changes from RFC 5448:
Updates the reference that specifies how the Network Name field is
constructed in the protocol. This update ensures that EAP-AKA' is
compatible with 5G deployments. RFC 5448 referred to the Release 8
version of . This document points
to the first 5G version, Release 15.
Specifies how EAP and EAP-AKA' use identifiers in 5G. Additional
identifiers are introduced in 5G, and for interoperability, it is
necessary that the right identifiers are used as inputs in the key
derivation. In addition, for identity privacy it is important that
when privacy-friendly identifiers in 5G are used, no trackable, permanent
identifiers are passed in EAP-AKA', either.
Specifies session identifiers and other exported parameters, as
those were not specified in despite
requirements set forward in to do so.
Also, while specified session identifiers
for EAP-AKA, it only did so for the full authentication case, not
for the case of fast re-authentication.
Updates the requirements on generating pseudonym usernames and
fast re-authentication identities to ensure identity privacy.
Describes what has been learned about any vulnerabilities
in AKA or EAP-AKA'.
Describes the privacy and pervasive monitoring considerations
related to EAP-AKA'.
Adds summaries of the attributes.
Some of the updates are small. For instance,
the reference update to does not change the 3GPP specification number,
only the version. But this reference is crucial for the correct calculation
of the keys that result from running the EAP-AKA' method, so an
update of the RFC with the newest version pointer may be warranted.
Note: Any further updates in 3GPP specifications that affect,
for instance, key derivation is something that EAP-AKA'
implementations need to take into account. Upon such updates, there
will be a need to update both this specification and the
implementations.It is an explicit non-goal of this specification to include any other
technical modifications, addition of new features, or other
changes. The EAP-AKA' base protocol is stable and needs to stay that
way. If there are any extensions or variants, those need to be
proposed as standalone extensions or even as different authentication
methods.The rest of this specification is structured as follows. defines the EAP-AKA' method. adds support to EAP-AKA to prevent bidding down
attacks from EAP-AKA'. specifies
requirements regarding the use of peer identities, including how
5G identifiers are used in the EAP-AKA' context. specifies which parameters EAP-AKA'
exports out of the method.
explains the security differences between EAP-AKA and EAP-AKA'. describes the IANA considerations, and and explain the updates to
RFC 5448 EAP-AKA' and RFC 4187 EAP-AKA that have been made in this
specification. explains some of the design
rationale for creating EAP-AKA'. Finally,
provides test vectors.Requirements LanguageThe key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 when, and only
when, they appear in all capitals, as shown here.EAP-AKA'EAP-AKA' is an EAP method that follows the EAP-AKA specification
in all respects except the following:
It uses the Type code 0x32, not 0x17 (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 / HMAC-SHA-256 , not SHA-1 / HMAC-SHA-1 (see ).
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 .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 for International Mobile
Subscriber Identifier (IMSI) based usernames. For 4G networks, EAP-AKA' MUST use
the leading character "6" (ASCII 36 hexadecimal) instead of "0" for
IMSI-based permanent usernames.
For 5G networks, the leading character "6" is not used for IMSI-based permanent usernames.
Identifier usage in 5G is specified in . All
other usage and processing of the leading characters, usernames,
and identities is as defined by EAP-AKA .
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 , 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.AT_KDF_INPUTThe format of the AT_KDF_INPUT attribute is shown below.The fields are as follows:
AT_KDF_INPUT
This is set to 23.
Length
The length of the
attribute, calculated as defined in .
Actual Network Name Length
This is
a 2 byte actual length field, needed due to the requirement that
the previous field is expressed in multiples of 4 bytes per the usual
EAP-AKA rules. The Actual Network Name Length field
provides the length of the network name in bytes.
Network Name
This field contains
the network name of the access network for which the authentication is
being performed. The name does not include any terminating null
characters. Because the length of the entire attribute must be a
multiple of 4 bytes, the sender pads the name with 1, 2, or 3
bytes of all zero bits when necessary.
Only the server sends the AT_KDF_INPUT attribute. The value is sent
as specified in for both non-3GPP access
networks and for 5G
access networks. 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, with
a greater than zero length in the Actual Network Name Length
field. If the AT_KDF_INPUT attribute
is empty, the peer behaves as if AUTN had been incorrect and
authentication fails. See Section 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_KDFAT_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:
AT_KDF
This is set to 24.
Length
The length of the
attribute, calculated as defined in .
For AT_KDF, the Length field MUST be set to 1.
Key Derivation Function
An
enumerated value representing the key derivation function that the
server (or peer) wishes to use. Value 1 represents the default key
derivation function for EAP-AKA', i.e., employing CK' and IK' as
defined in .
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
). 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 since the peer should have accepted it without further negotiation. If the peer has replied with
the first AT_KDF value, the server behaves as if AT_MAC of the
response had been incorrect and fails the authentication. For an
overview of the failed authentication process in the server side, see
Section 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 with processing the received EAP-Request/AKA'-Challenge as
specified in and 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 . This happens after
AT_KDF negotiation has already completed. That is, the
EAP-Request/AKA'-Challenge and, possibly, the
EAP-Response/AKA'-Challenge message are exchanged first to determine
a mutually acceptable key derivation function, and only then
is the possible AKA'-Synchronization-Failure message sent. The
AKA'-Synchronization-Failure message is sent as a response to the
newly received EAP-Request/AKA'-Challenge, which is the last message
of the AT_KDF negotiation. Note that if the first proposed KDF is
acceptable, then last message is at the same time the first
EAP-Request/AKA'-Challenge message. The
AKA'-Synchronization-Failure message MUST contain the AUTS
parameter as specified in 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.Key DerivationBoth the peer and server MUST derive the keys as follows.
AT_KDF parameter has the value 1
In this case, MK is derived and used as
follows:Here [n..m] denotes the substring from bit n to m, including
bits n and m. PRF' is a new
pseudo-random function specified in . 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
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
and in of in this document for
the 5G cases.When the server creates an AKA challenge and corresponding AUTN, CK,
CK', IK, and IK' values, it MUST set the Authentication Management
Field (AMF) separation bit to 1 in the AKA algorithm .
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 . 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 .
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 .Hash FunctionsEAP-AKA' uses SHA-256 / HMAC-SHA-256, not SHA-1 / HMAC-SHA-1 (see
) as in
EAP-AKA. This requires a change to the pseudo-random function (PRF) as
well as the AT_MAC and AT_CHECKCODE attributes.PRF'The PRF' construction is the same one IKEv2 uses (see
;
this is the same function as was defined that RFC 5448 referred
to). The function takes two arguments. K is a 256-bit value
and S is a byte string of arbitrary length. PRF' is defined
as follows:PRF' produces as many bits of output as is needed. HMAC-SHA-256 is
the application of HMAC to SHA-256.AT_MACWhen 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
.AT_CHECKCODEWhen 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 .Summary of Attributes for EAP-AKA' identifies which attributes may be found
in which kinds of messages, and in what quantity.Messages are denoted with numbers in parentheses as follows:
(1)
EAP-Request/AKA-Identity
(2)
EAP-Response/AKA-Identity
(3)
EAP-Request/AKA-Challenge
(4)
EAP-Response/AKA-Challenge
(5)
EAP-Request/AKA-Notification
(6)
EAP-Response/AKA-Notification
(7)
EAP-Response/AKA-Client-Error
(8)
EAP-Request/AKA-Reauthentication
(9)
EAP-Response/AKA-Reauthentication
(10)
EAP-Response/AKA-Authentication-Reject
(11)
EAP-Response/AKA-Synchronization-Failure
The column denoted with "E" indicates whether the attribute is a nested attribute that MUST be included within AT_ENCR_DATA.In addition, the numbered columns indicate the quantity of the attribute within the message as follows:
"0"
Indicates that the attribute MUST NOT
be included in the message.
"1"
Indicates that the attribute MUST be
included in the message.
"0-1"
Indicates that the attribute is sometimes included
in the message
"0+"
Indicates that zero or more copies of the attribute
MAY be included in the message.
"1+"
Indicates that there MUST be at least one attribute
in the message but more than one MAY be included in the message.
"0*"
Indicates that the attribute is not included in the
message in cases specified in this document, but MAY be included in
the future versions of the protocol.
The attribute table is shown below. The table is largely the
same as in the EAP-AKA attribute table (),
but changes how many times AT_MAC may appear in an
EAP-Response/AKA'-Challenge message as it does not appear there
when AT_KDF has to be sent from the peer to the server. The table
also adds the AT_KDF and AT_KDF_INPUT attributes.
The Attribute Table
Attribute
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
E
AT_PERMANENT_ID_REQ
0-1
0
0
0
0
0
0
0
0
0
0
N
AT_ANY_ID_REQ
0-1
0
0
0
0
0
0
0
0
0
0
N
AT_FULLAUTH_ID_REQ
0-1
0
0
0
0
0
0
0
0
0
0
N
AT_IDENTITY
0
0-1
0
0
0
0
0
0
0
0
0
N
AT_RAND
0
0
1
0
0
0
0
0
0
0
0
N
AT_AUTN
0
0
1
0
0
0
0
0
0
0
0
N
AT_RES
0
0
0
1
0
0
0
0
0
0
0
N
AT_AUTS
0
0
0
0
0
0
0
0
0
0
1
N
AT_NEXT_PSEUDONYM
0
0
0-1
0
0
0
0
0
0
0
0
Y
AT_NEXT_REAUTH_ID
0
0
0-1
0
0
0
0
0-1
0
0
0
Y
AT_IV
0
0
0-1
0*
0-1
0-1
0
1
1
0
0
N
AT_ENCR_DATA
0
0
0-1
0*
0-1
0-1
0
1
1
0
0
N
AT_PADDING
0
0
0-1
0*
0-1
0-1
0
0-1
0-1
0
0
Y
AT_CHECKCODE
0
0
0-1
0-1
0
0
0
0-1
0-1
0
0
N
AT_RESULT_IND
0
0
0-1
0-1
0
0
0
0-1
0-1
0
0
N
AT_MAC
0
0
1
0-1
0-1
0-1
0
1
1
0
0
N
AT_COUNTER
0
0
0
0
0-1
0-1
0
1
1
0
0
Y
AT_COUNTER_TOO_SMALL
0
0
0
0
0
0
0
0
0-1
0
0
Y
AT_NONCE_S
0
0
0
0
0
0
0
1
0
0
0
Y
AT_NOTIFICATION
0
0
0
0
1
0
0
0
0
0
0
N
AT_CLIENT_ERROR_CODE
0
0
0
0
0
0
1
0
0
0
0
N
AT_KDF
0
0
1+
0+
0
0
0
0
0
0
1+
N
AT_KDF_INPUT
0
0
1
0
0
0
0
0
0
0
0
N
Bidding Down Prevention for EAP-AKAAs 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. It is
only included in EAP-AKA messages, which are protected with
the AT_MAC attribute. This approach 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:
AT_BIDDING
This is set to 136.
Length
The length of the
attribute, calculated as defined in . For AT_BIDDING, the Length MUST be set to 1.
D
This bit is set to 1 if the
sender supports EAP-AKA', is willing to use it, and prefers it
over EAP-AKA. Otherwise, it should be set to zero.
Reserved
This field MUST be set
to zero when sent and ignored on receipt.
The server sends this attribute in the EAP-Request/AKA-Challenge
message. If the peer supports EAP-AKA', it compares the received value
to its own capabilities. If it turns out that both the server and peer
would have been able to use EAP-AKA' and preferred it over EAP-AKA,
the peer behaves as if AUTN had been incorrect and fails the
authentication (see Figure 3 of ). 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, this specification does not provide protection against bidding "down" attacks in the other direction, i.e., attackers forcing
the endpoints to use EAP-AKA'.Summary of Attributes for EAP-AKAThe appearance of the AT_BIDDING attribute in EAP-AKA exchanges
is shown below, using the notation from :
AT_BIDDING Attribute Appearance
Attribute
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
E
AT_BIDDING
0
0
1
0
0
0
0
0
0
0
0
N
Peer IdentitiesEAP-AKA' peer identities are as specified in , with the addition of some
requirements specified in this section.EAP-AKA' includes optional identity privacy support
that can be used to hide the cleartext permanent identity and
thereby make the subscriber's EAP exchanges untraceable to
eavesdroppers. EAP-AKA' can also use the privacy-friendly identifiers
specified for 5G networks.The permanent identity is usually based on the IMSI. Exposing the
IMSI is undesirable because, as a permanent identity, it is easily
trackable. In addition, since IMSIs may be used in other contexts as
well, there would be additional opportunities for such tracking.In EAP-AKA', identity privacy is based on temporary
usernames or pseudonym usernames. These are similar to, but
separate from, the Temporary Mobile Subscriber Identities (TMSI) that
are used on cellular networks.Username Types in EAP-AKA' Identities specifies that there
are three types of usernames: permanent, pseudonym, and fast
re-authentication usernames. This specification extends this
definition as follows. There are four types of usernames:
Regular usernames. These are external names given to
EAP-AKA' peers. The regular usernames are further subdivided into to
categories:
Permanent usernames, for instance, IMSI-based
usernames.
Privacy-friendly temporary usernames, for instance, 5G
GUTI (5G Globally Unique Temporary Identifier) or 5G privacy identifiers (see ), for instance, SUCI
(Subscription Concealed Identifier).
EAP-AKA' pseudonym usernames. For example,
2s7ah6n9q@example.com might be a valid pseudonym identity. In
this example, 2s7ah6n9q is the pseudonym username.
EAP-AKA' fast re-authentication usernames. For example,
43953754@example.com might be a valid fast re-authentication
identity and 43953754 the fast re-authentication
username.
The permanent, privacy-friendly temporary, and pseudonym
usernames are only used with full authentication, and fast
re-authentication usernames only with fast re-authentication.
Unlike permanent usernames and pseudonym usernames, privacy-friendly
temporary usernames and fast re-authentication usernames
are one-time identifiers, which are not reused across EAP
exchanges.Generating Pseudonyms and Fast Re-Authentication IdentitiesThis section provides some additional guidance to
implementations for producing secure pseudonyms and fast
re-authentication identities. It does not impact backwards
compatibility because each server consumes only the identities that it
generates itself. However, adherence to the guidance will provide
better security.
As specified by ,
pseudonym usernames and fast re-authentication identities are
generated by the EAP server in an implementation-dependent
manner. RFC 4187 provides some general requirements on how these
identities are transported, how they map to the NAI syntax, how
they are distinguished from each other, and so on.
However, to enhance privacy, some additional requirements need to
be applied.The pseudonym usernames and fast re-authentication identities
MUST be generated in a cryptographically secure way so that
it is computationally infeasible for an attacker to differentiate
two identities belonging to the same user from two identities
belonging to different users. This can be achieved, for instance,
by using random or pseudo-random identifiers such as random byte
strings or ciphertexts. See also for guidance
on random number generation.Note that the pseudonym and fast re-authentication usernames
also MUST NOT include substrings that can be used to relate the
username to a particular entity or a particular permanent
identity. For instance, the usernames cannot include any
subscriber-identifying part of an IMSI or other permanent
identifier. Similarly, no part of the username can be formed by a
fixed mapping that stays the same across multiple different
pseudonyms or fast re-authentication identities for the same
subscriber.When the identifier used to identify a subscriber in an
EAP-AKA' authentication exchange is a privacy-friendly identifier
that is used only once, the EAP-AKA' peer MUST NOT use a pseudonym
provided in that authentication exchange in subsequent exchanges
more than once. To ensure that this does not happen, the EAP-AKA'
server MAY decline to provide a pseudonym in such authentication
exchanges. An important case where such privacy-friendly
identifiers are used is in 5G networks (see ).Identifier Usage in 5GIn EAP-AKA', the peer identity may be communicated to the server
in one of three ways:
As a part of link-layer establishment procedures, externally to
EAP.
With the EAP-Response/Identity message in the beginning of the
EAP exchange, but before the selection of EAP-AKA'.
Transmitted from the peer to the server using EAP-AKA' messages
instead of EAP-Response/Identity. In this case, the server
includes an identity-requesting attribute (AT_ANY_ID_REQ,
AT_FULLAUTH_ID_REQ, or AT_PERMANENT_ID_REQ) in the
EAP-Request/AKA-Identity message, and the peer includes the
AT_IDENTITY attribute, which contains the peer's identity, in the
EAP-Response/AKA-Identity message.
The identity carried above may be a permanent identity, privacy-friendly
identity, pseudonym identity, or fast re-authentication
identity as defined in .5G supports the concept of privacy identifiers, and it is
important for interoperability that the right type of identifier is
used.5G defines the SUbscription Permanent Identifier (SUPI) and
SUbscription Concealed Identifier (SUCI) . SUPI is globally unique and allocated to
each subscriber. However, it is only used internally in the 5G
network and is privacy sensitive. The SUCI is a privacy-preserving
identifier containing the concealed SUPI, using public key
cryptography to encrypt the SUPI.Given the choice between these two types of identifiers,
EAP-AKA' ensures interoperability as follows:
Where identifiers are used within EAP-AKA' -- such as key
derivation -- specify what values exactly should be used,
to avoid ambiguity (see ).
Where identifiers are carried within EAP-AKA' packets --
such as in the AT_IDENTITY attribute -- specify which identifiers
should be filled in (see ).
In 5G, the normal mode of operation is that identifiers are only
transmitted outside EAP. However, in a system involving terminals
from many generations and several connectivity options via 5G and
other mechanisms, implementations and the EAP-AKA' specification
need to prepare for many different situations, including sometimes
having to communicate identities within EAP.The following sections clarify which identifiers are used and
how.Key DerivationIn EAP-AKA', the peer identity is used in the key derivation formula found in .The identity needs to be represented in exactly the correct format
for the key derivation formula to produce correct results.If the AT_KDF_INPUT parameter contains the prefix "5G:", the
AT_KDF parameter has the value 1, and this authentication is not a
fast re-authentication, then the peer identity used in the key
derivation MUST be as specified in Annex F.3 of and Clause 2.2 of . This is in contrast to , which uses the identity as communicated in
EAP and represented as a NAI. Also, in contrast to , in 5G EAP-AKA' does not use the "0" nor the "6"
prefix in front of the identifier.For an example of the format of the identity, see Clause 2.2 of .In all other cases, the following applies:
The identity used in the key derivation formula MUST be
exactly the one sent in EAP-AKA' AT_IDENTITY attribute, if one
was sent, regardless of the kind of identity that it may have
been. If no AT_IDENTITY was sent, the identity MUST be the
exactly the one sent in the generic EAP Identity exchange, if
one was made.If no identity was communicated inside EAP, then the identity
is the one communicated outside EAP in link-layer messaging.In this case, the used identity MUST be the identity most
recently communicated by the peer to the network, again regardless
of what type of identity it may have been.EAP Identity Response and EAP-AKA' AT_IDENTITY AttributeThe EAP authentication option is only available in 5G when the
new 5G core network is also in use. However, in other networks, an
EAP-AKA' peer may be connecting to other types of networks and
existing equipment.When the EAP server is in a 5G network, the 5G procedures for
EAP-AKA' apply. specifies when the EAP server is in a 5G network.
Note: Currently, the following conditions are specified: when the
EAP peer uses the 5G Non-Access Stratum (NAS) protocol or when the EAP peer attaches to a network
that advertises 5G connectivity without NAS . Possible future conditions
may also be specified by 3GPP.When the 5G procedures for EAP-AKA' apply, EAP identity
exchanges are generally not used as the identity is already made
available on previous link-layer exchanges.In this situation, the EAP Identity Response and EAP-AKA'
AT_IDENTITY attribute are handled as specified in Annex F.2 of
.When used in EAP-AKA', the format of the SUCI MUST be as
specified in , Section 28.7.3, with the semantics defined in
, Section 2.2B. Also, in contrast to , in 5G EAP-AKA' does not use the "0" nor the "6"
prefix in front of the identifier.For an example of an IMSI in NAI format, see , Section 28.7.3.Otherwise, the peer SHOULD employ an IMSI, SUPI, or NAI as it is
configured to use.Exported ParametersWhen not using fast re-authentication, the EAP-AKA' Session-Id is the concatenation of the EAP Type value
(0x32, one byte) with the contents of the RAND field from the AT_RAND attribute
followed by the contents of the AUTN field in the AT_AUTN attribute:
When using fast re-authentication, the EAP-AKA' Session-Id is the
concatenation of the EAP Type value (0x32) with the contents of the
NONCE_S field from the AT_NONCE_S attribute followed by the
contents of the MAC field from the AT_MAC attribute from the
EAP-Request/AKA-Reauthentication:
The Peer-Id is the contents of the Identity field from the
AT_IDENTITY attribute, using only the Actual Identity Length bytes
from the beginning. Note that the contents are used as they are
transmitted, regardless of whether the transmitted identity was a
permanent, pseudonym, or fast EAP re-authentication identity. If no
AT_IDENTITY attribute was exchanged, the exported Peer-Id is the
identity provided from the EAP Identity Response packet. If no EAP
Identity Response was provided either, the exported Peer-Id is the null
string (zero length).The Server-Id is the null string (zero length).Security ConsiderationsA summary of the security properties of EAP-AKA' follows. These
properties are very similar to those in EAP-AKA. We assume that
HMAC SHA-256 is at least as secure as HMAC SHA-1 (see also ). This is called the SHA-256
assumption in the remainder of this section. Under this assumption,
EAP-AKA' is at least as secure as EAP-AKA.If the AT_KDF attribute has value 1, then the security properties
of EAP-AKA' are as follows:
Protected ciphersuite negotiation
EAP-AKA' has no ciphersuite
negotiation mechanisms. It does have a negotiation mechanism for
selecting the key derivation functions. This mechanism is secure
against bidding down attacks from EAP-AKA' to EAP-AKA. The negotiation mechanism allows
changing the offered key derivation function, but the change is
visible in the final EAP-Request/AKA'-Challenge message that the
server sends to the peer. This message is authenticated via the AT_MAC
attribute, and carries both the chosen alternative and the initially
offered list. The peer refuses to accept a change it did not initiate.
As a result, both parties are aware that a change is being made and
what the original offer was.
Per assumptions in , there is no protection
against bidding down attacks from EAP-AKA to EAP-AKA' should EAP-AKA'
somehow be considered less secure some day than EAP-AKA. Such
protection was not provided in RFC 5448 implementations and
consequently neither does this specification provide it. If such
support is needed, it would have to be added as a separate new
feature.
In general, it is expected that the current negotiation
capabilities in EAP-AKA' are sufficient for some types of
extensions, including adding Perfect Forward Secrecy
and perhaps others. However, some larger changes may require a new EAP method type, which is how EAP-AKA' itself happened. One example of such change would be the introduction of new algorithms.
Mutual authentication
Under the
SHA-256 assumption, the properties of EAP-AKA' are at least as good as
those of EAP-AKA in this respect. Refer to , 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
, for further details. The only
difference is that a stronger hash algorithm and keyed MAC, SHA-256 /
HMAC-SHA-256, is used instead of SHA-1 / HMAC-SHA-1.
Replay protection
Under the
SHA-256 assumption, the properties of EAP-AKA' are at least as good as
those of EAP-AKA in this respect. Refer to , for further details.
Confidentiality
The properties
of EAP-AKA' are exactly the same as those of EAP-AKA in this
respect. Refer to , 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
. 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.
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
, 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 , 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
, for further details.
Session independence
The
properties of EAP-AKA' are exactly the same as those of EAP-AKA in
this respect. Refer to , for further
details.
Fragmentation
The properties of
EAP-AKA' are exactly the same as those of EAP-AKA in this
respect. Refer to , for further
details.
Channel binding
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 for more discussion.
Privacy suggests that the privacy considerations
of IETF protocols be documented.The confidentiality properties of EAP-AKA' itself have been
discussed above under "Confidentiality".EAP-AKA' uses several different types of identifiers to identify
the authenticating peer. It is strongly RECOMMENDED to use the
privacy-friendly temporary or hidden identifiers, i.e., the 5G GUTI or SUCI,
pseudonym usernames, and fast re-authentication usernames. The use
of permanent identifiers such as the IMSI or SUPI may lead to an ability to
track the peer and/or user associated with the peer. The use of
permanent identifiers such as the IMSI or SUPI is strongly NOT
RECOMMENDED.As discussed in , when
authenticating to a 5G network, only the SUCI identifier is normally
used. The use of EAP-AKA' pseudonyms in this situation is at best
limited because the SUCI already provides a stronger mechanism.
In fact, reusing the same pseudonym multiple times
will result in a tracking opportunity for observers that see the
pseudonym pass by. To avoid this, the peer and server need to follow
the guidelines given in .When authenticating to a 5G network, per ,
both the EAP-AKA' peer and server need to employ the permanent
identifier SUPI as an input to key derivation. However, this use
of the SUPI is only internal. As such, the SUPI need not be
communicated in EAP messages. Therefore, SUPI MUST NOT be
communicated in EAP-AKA' when authenticating to a 5G network.While the use of SUCI in 5G networks generally provides identity
privacy, this is not true if the null-scheme encryption is used to
construct the SUCI (see , Annex C). The use
of this scheme makes the use of SUCI equivalent to the use of SUPI
or IMSI. The use of the null scheme is NOT RECOMMENDED where
identity privacy is important.The use of fast re-authentication identities when authenticating
to a 5G network does not have the same problems as the use of
pseudonyms, as long as the 5G authentication server generates the
fast re-authentication identifiers in a proper manner specified in
.Outside 5G, the peer can freely choose between the use of permanent, pseudonym, or fast
re-authentication identifiers:
A peer that has not yet performed any EAP-AKA' exchanges does
not typically have a pseudonym available. If the peer does not
have a pseudonym available, then the privacy mechanism cannot be
used, and the permanent identity will have to be sent in the
clear.
The terminal SHOULD store the pseudonym in nonvolatile
memory so that it can be maintained across reboots. An active
attacker that impersonates the network may use the
AT_PERMANENT_ID_REQ attribute () to learn the subscriber's IMSI. However, as discussed in
, the terminal can refuse to
send the cleartext permanent identity if it believes that the
network should be able to recognize the pseudonym.
When pseudonyms and fast re-authentication identities are used,
the peer relies on the properly created identifiers by the server.
It is essential that an attacker cannot link a
privacy-friendly identifier to the user in any way or determine that
two identifiers belong to the same user as outlined in . The pseudonym usernames and fast
re-authentication identities MUST NOT be used for other
purposes (e.g., in other protocols).
If the peer and server cannot guarantee that
SUCI can be used or that pseudonyms will be available,
generated properly, and maintained reliably, and identity privacy
is required, then additional protection from an external security
mechanism such as tunneled EAP methods like Tunneled Transport Layer Security (TTLS) or Tunnel Extensible Authentication Protocol (TEAP)
may be used. The benefits and the security considerations of using an external security
mechanism with EAP-AKA are beyond the scope of this document.Finally, as with other EAP methods, even when privacy-friendly
identifiers or EAP tunneling is used, typically the domain part of
an identifier (e.g., the home operator) is visible to external
parties.Discovered VulnerabilitiesThere have been no published attacks that violate the primary
secrecy or authentication properties defined for Authentication and
Key Agreement (AKA) under the originally assumed trust model. The
same is true of EAP-AKA'.However, there have been attacks when a different trust model is
in use, with characteristics not originally provided by the design,
or when participants in the protocol leak information to outsiders
on purpose, and there have been some privacy-related attacks.For instance, the original AKA protocol does not prevent an insider
supplying keys to a third party, e.g., as described by
and Tsay in where a serving network
lets an authentication run succeed, but then it misuses the session
keys to send traffic on the authenticated user's behalf. This
particular attack is not different from any on-path entity (such as
a router) pretending to send traffic, but the general issue of
insider attacks can be a problem, particularly in a large group of
collaborating operators.Another class of attacks is the use of tunneling of traffic from
one place to another, e.g., as done by Zhang and Fang in to leverage security policy differences between
different operator networks, for instance. To gain something in such
an attack, the attacker needs to trick the user into believing it is
in another location. If policies between locations differ,
for instance, if payload traffic is not required to be encrypted in some location,
the attacker may trick the user into
opening a vulnerability. As an authentication
mechanism, EAP-AKA' is not directly affected by most of these
attacks. EAP-AKA' network name binding can also help alleviate some
of the attacks. In any case, it is recommended that EAP-AKA'
configuration not be dependent on the location of request origin,
unless the location information can be cryptographically
confirmed, e.g., with the network name binding.Zhang and Fang also looked at denial-of-service attacks . A serving network may request large numbers of
authentication runs for a particular subscriber from a home
network. While the resynchronization process can help recover from this,
eventually it is possible to exhaust the sequence number space and
render the subscriber's card unusable. This attack is possible for
both original AKA and EAP-AKA'. However, it requires the collaboration
of a serving network in an attack. It is recommended that EAP-AKA'
implementations provide the means to track, detect, and limit excessive
authentication attempts to combat this problem.There have also been attacks related to the use of AKA without the
generated session keys (e.g., ). Some of
those attacks relate to the use of
HTTP Digest AKAv1 , which was originally vulnerable to man-in-the-middle attacks. This has
since been corrected in . The EAP-AKA'
protocol uses session keys and provides channel binding, and as
such, it is resistant to the above attacks except where the protocol
participants leak information to outsiders.Basin, et al. have performed formal
analysis and concluded that the AKA protocol would have benefited
from additional security requirements such as key confirmation.In the context of pervasive monitoring revelations, there were
also reports of compromised long-term pre-shared keys used in SIM
and AKA . While no protocol can survive
the theft of key material associated with its credentials, there
are some things that alleviate the impacts in such situations.
These are discussed further in .Arapinis, et al. describe an attack
that uses the AKA resynchronization protocol to attempt to detect
whether a particular subscriber is in a given area. This attack
depends on the attacker setting up a false base station
in the given area and on the subscriber performing at least one
authentication between the time the attack is set up and run.Borgaonkar, et al. discovered that the AKA resynchronization
protocol may also be used to predict the authentication frequency of
a subscriber if a non-time-based sequence number (SQN) generation scheme is used . The attacker can force the reuse of the
keystream that is used to protect the SQN in the AKA
resynchronization protocol. The attacker then guesses the
authentication frequency based on the lowest bits of two XORed
SQNs. The researchers' concern was that the authentication frequency
would reveal some information about the phone usage behavior, e.g.,
number of phone calls made or number of SMS messages sent.
There are a number of possible triggers for authentication, so
such an information leak is not direct, but it can be a concern.
The impact
of the attack differs depending on whether the
SQN generation scheme that is used is time-based or not.Similar attacks are possible outside AKA in the cellular paging
protocols where the attacker can simply send application-layer data, send
short messages, or make phone calls to the intended victim and
observe the air interface (e.g., and ). Hussain,
et al. demonstrated a slightly more sophisticated version of the
attack that exploits the fact that the 4G paging protocol uses the IMSI
to calculate the paging timeslot . As this attack is
outside AKA, it does not impact EAP-AKA'.Finally, bad implementations of EAP-AKA' may not produce pseudonym
usernames or fast re-authentication identities in a manner that is
sufficiently secure. While it is not a problem with the protocol
itself, following the recommendations in can mitigate this concern.Pervasive MonitoringAs required by , work on IETF protocols
needs to consider the effects of pervasive monitoring and mitigate
them when possible.As described in , after the publication of RFC 5448, new
information has come to light regarding the use of pervasive
monitoring techniques against many security technologies, including
AKA-based authentication.For AKA, these attacks relate to theft of the long-term, shared-secret
key material stored on the cards. Such attacks are
conceivable, for instance, during the manufacturing process of
cards, through coercion of the card manufacturers, or during the
transfer of cards and associated information to an operator. Since
the publication of reports about such attacks, manufacturing and
provisioning processes have gained much scrutiny and have
improved.In particular, it is crucial that manufacturers limit access to
the secret information and the cards only to necessary systems and
personnel. It is also crucial that secure mechanisms be used to store and
communicate the secrets between the manufacturer and the operator
that adopts those cards for their customers.Beyond these operational considerations, there are also technical
means to improve resistance to these attacks. One approach is to
provide Perfect Forward Secrecy (PFS). This would prevent any
passive attacks merely based on the long-term secrets and
observation of traffic. Such a mechanism can be defined as a
backwards-compatible extension of EAP-AKA' and is pursued
separately from this specification . Alternatively, EAP-AKA'
authentication can be run inside a PFS-capable, tunneled
authentication method. In any case, the use of some PFS-capable
mechanism is recommended. Security Properties of Binding Network NamesThe 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.IANA ConsiderationsIANA has updated the "Extensible Authentication Protocol (EAP)
Registry" and the "EAP-AKA and EAP-SIM Parameters" registry so that entries
that pointed to RFC 5448 now point to this RFC instead.Type ValueEAP-AKA' has the EAP Type value 0x32 in the "Extensible
Authentication Protocol (EAP) Registry" under "Method Types" subregistry. Per
, this allocation can be made with
Designated Expert and Specification Required.Attribute Type ValuesEAP-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 ().Key Derivation Function NamespaceIANA 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 .
EAP-AKA' AT_KDF Key Derivation Function Values
Value
Description
Reference
0
Reserved
RFC 0000
1
EAP-AKA' with CK'/IK'
RFC 0000
2-65535
Unassigned
ReferencesNormative References3rd Generation Partnership Project;
Technical Specification Group Core Network and Terminals;
Numbering, addressing and identification
(Release 16)3GPPVersion 16.5.03rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
3G Security; Security architecture and procedures for 5G System;
(Release 16)3GPPVersion 16.7.03rd Generation Partnership Project;
Technical Specification Group Core Network and Terminals;
Access to the 3GPP Evolved Packet Core (EPC)
via non-3GPP access networks;
Stage 3;
(Release 16)3GPPVersion 16.4.03rd Generation Partnership Project;
Technical Specification Group Core Network and Terminals;
Access to the 3GPP Evolved Packet Core (EPC)
via non-3GPP access networks;
Stage 3;
(Release 16)3GPPVersion 16.7.03rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
3G Security;
Security architecture
(Release 16)
3GPPVersion 16.0.03GPP System Architecture Evolution (SAE); Security aspects of non-3GPP accesses (Release 16)3GPPVersion 16.0.0
3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
3G Security; Security architecture and procedures for 5G System
(Release 16)3GPPVersion 16.5.0Secure Hash StandardNational Institute of Standards and TechnologyInformative References3rd Generation Partnership Project;
Technical Specification Group Services and System
Aspects; 3G Security; Specification of the
MILENAGE Algorithm Set: An example algorithm set
for the 3GPP authentication and key generation
functions f1, f1*, f2, f3, f4, f5 and f5*;
Document 4: Design Conformance Test Data (Release
14)3GPPVersion 15.0.0Secure Hash StandardNational Institute of Standards and TechnologySecure Hash StandardNational Institute of Standards and TechnologyHow Spies Stole the Keys to the Encryption CastleA Vulnerability in the UMTS and LTE Authentication and Key Agreement Protocolsin Computer Network Security, Proceedings of the 6th International Conference on Mathematical Methods, Models and Architectures for Computer Network Security, Lecture Notes in Computer Science, Vol. 7531, pp. 65-76Breaking Cell Phone Authentication: Vulnerabilities in AKA, IMS and Androidin 7th USENIX Workshop on Offensive Technologies, WOOT '13Breaking Cell Phone Authentication: Vulnerabilities in AKA, IMS and AndroidIEEE Transactions on Wireless Communications, Vol. 4, No. 2A Formal Analysis of 5G AuthenticationarXiv:1806.10360New Privacy Issues in Mobile Telephony: Fix and Verificationin CCS '12: Proceedings of the 2012 ACM Conference on Computer and Communications Security, Raleigh, North Carolina, USANew Privacy Threat on 3G, 4G, and Upcoming 5G AKA Protocolsin IACR Cryptology ePrint ArchiveLocation Leaks on the GSM Air Interfacein the proceedings of NDSS '12, held 5-8 February, 2012, San Diego, CaliforniaPractical attacks against Privacy and Availability in 4G/LTE Mobile Communication Systemsin the proceedings of NDSS '16 held 21-24 February, 2016, San Diego, CaliforniaPrivacy Attacks to the 4G and 5G Cellular Paging Protocols Using Side Channel Informationin the proceedings of NDSS '19, held 24-27 February, 2019, San Diego, CaliforniaChanges from RFC 5448The first change from RFC 5448 was to refer to a newer version
of . The new version
includes an updated definition of the Network Name field to
include 5G.Secondly, identifier usage for 5G has been specified in . Also, the requirements for generating
pseudonym usernames and fast re-authentication identities have been
updated from the original definition in RFC 5448, which referenced
RFC 4187. See .Thirdly, exported parameters for EAP-AKA' have been defined in
, as required by , including the definition of those parameters for
both full authentication and fast re-authentication.The security, privacy, and pervasive monitoring considerations
have been updated or added. See .The references to , , , , and have been updated to their most recent
versions, and language in this document has been changed
accordingly. However, this is merely an update to a newer RFC, but
the actual protocol functions are the same as defined in the earlier RFCs. Similarly, references to all 3GPP technical specifications have been
updated to their 5G versions (Release 16) or otherwise most recent
version when there has not been a 5G-related update.Finally, a number of clarifications have been made, including a
summary of where attributes may appear.Changes to RFC 4187In addition to specifying EAP-AKA', this document also mandates a
change to another EAP method -- EAP-AKA that was defined in RFC 4187.
This change was already mandated in RFC 5448 but repeated here to
ensure that the latest EAP-AKA' specification contains the instructions
about the necessary bidding down feature in EAP-AKA as well.The changes to RFC 4187 relate only to the bidding down
prevention support defined in . In
particular, this document does not change how the Master Key (MK) is
calculated or any other aspect of EAP-AKA. The provisions in this
specification for EAP-AKA' do not apply to EAP-AKA, outside of .Importance of Explicit NegotiationChoosing 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 function. While this can be made to work for
specific cases, it would be an extremely brittle mechanism, ripe to
result in problems whenever client configuration, routing of
authentication requests, or server configuration does not match
expectations. It also does not help to assume that the EAP client and
server are running a particular release of 3GPP network
specifications. Network vendors often provide features from future
releases early or do not provide all features of the current
release. And obviously, there are many EAP and even some EAP-AKA
implementations that are not bundled with the 3GPP network
offerings. In general, these approaches are expected to lead to
hard-to-diagnose problems and increased support calls.Test VectorsTest 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 .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.ContributorsThe test vectors in were provided by and
, based on two independent implementations of this
specification. provided suggested text for . provided much of the
text for . Karl Norrman was the source of
much of the information in .AcknowledgmentsThe authors would like to thank , , , ,
, , , , , ,
, , , , , , , , ,
, , , , , , , , , , ,
, , and for their
in-depth reviews and interesting discussions in this problem
space.