Network Working Group V. Torvinen

Network Working Group V. Torvinen
Request for Comments: 4169 Turku Polytechnic
Category: Informational J. Arkko
M. Naslund
Ericsson
November 2005

Hypertext Transfer Protocol (HTTP) Digest Authentication Using
Authentication and Key Agreement (AKA) Version-2

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

HTTP Digest, as specified in RFC 2617, is known to be vulnerable to
man-in-the-middle attacks if the client fails to authenticate the
server in TLS, or if the same passwords are used for authentication
in some other context without TLS. This is a general problem that
exists not just with HTTP Digest, but also with other IETF protocols
that use tunneled authentication. This document specifies version 2
of the HTTP Digest AKA algorithm (RFC 3310). This algorithm can be
implemented in a way that it is resistant to the man-in-the-middle
attack.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . 4
2. HTTP Digest AKAv2 . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Password generation . . . . . . . . . . . . . . . . . . 6
2.2. Session keys . . . . . . . . . . . . . . . . . . . . . . 6
3. Example Digest AKAv2 Operation . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Multiple Authentication Schemes and Algorithms . . . . . 7
4.2. Session Protection . . . . . . . . . . . . . . . . . . . 7
4.3. Man-in-the-middle attacks . . . . . . . . . . . . . . . 8
4.4. Entropy . . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. Registration Information . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . 11

1. Introduction

The Hypertext Transfer Protocol (HTTP) Digest Authentication,
described in [4], has been extended in [6] to support the
Authentication and Key Agreement (AKA) mechanism [7]. The AKA
mechanism performs authentication and session key agreement in
Universal Mobile Telecommunications System (UMTS) networks. HTTP
Digest AKA enables the usage of AKA as a one-time password generation
mechanism for Digest authentication.

HTTP Digest is known to be vulnerable to man-in-the-middle attacks,
even when run inside TLS, if the same HTTP Digest authentication
credentials are used in some other context without TLS. The attacker
may initiate a TLS session with a server, and when the server
challenges the attacker with HTTP Digest, the attacker masquerades
the server to the victim. If the victim responds to the challenge,
the attacker is able to use this response towards the server in HTTP
Digest. Note that this attack is an instance of a general attack
that affects a number of IETF protocols, such as PIC. The general
problem is discussed in [8] and [9].

Because of the vulnerability described above, the use of HTTP Digest
"AKAv1" should be limited to the situations in which the client is
able to demonstrate that, in addition to the AKA response, it
possesses the AKA session keys. This is possible, for example, if
the underlying security protocol uses the AKA-generated session keys
to protect the authentication response. This is the case, for
example, in the 3GPP IP Multimedia Core Network Subsystem (IMS),
where HTTP Digest "AKAv1" is currently applied. However, HTTP Digest

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"AKAv1" should not be used with tunnelled security protocols that do
not utilize the AKA session keys. For example, the use of HTTP
Digest "AKAv1" is not necessarily secure with TLS if the server side
is authenticated using certificates and the client side is
authenticated using HTTP Digest AKA.

There are at least four potential solutions to the problem:

1. The use of the authentication credentials is limited to one
application only. In general, this approach is good and can be
recommended from the security point of view. However, this will
increase the total number of authentication credentials for an
end-user, and may cause scalability problems in the server side.

2. The keys used in the underlying security protocols are somehow
bound to the keys used in the tunneled authentication protocol.
However, this would cause problems with the current
implementations of underlying security protocols. For example,
it is not possible to use the session keys from TLS at the
application layer. Furthermore, this solution would only solve
the problem when HTTP Digest is used over one hop, and would
leave the problem of using HTTP Digest via multiple hops (e.g.,
via proxy servers) unsolved.

3. Authentication credentials are used in a cryptographically
different way for each media and/or access network. However, it
may be difficult to know which underlying media is used below the
application.

4. Authentication credentials are used in a cryptographically
different way for each application.

This document specifies a new algorithm version for HTTP Digest AKA
(i.e., "AKAv2"). "AKAv2" specifies a cryptographically different way
to use AKA credentials in use cases that are based on either HTTP
Digest authentication or UMTS authentication (cf. approach 4 above).
The only difference to "AKAv1" is that, in addition to an AKA
response RES, the AKA related session keys, IK and CK, are also used
as the password for HTTP Digest. AKAv2 is immune to the
man-in-the-middle attack described above. However, if AKAv2 is used
in some environment, both with and without some underlying security,
such as TLS, the problem still exists.

New HTTP Digest AKA algorithm versions can be registered with IANA,
based on Expert Review. Documentation of new algorithm versions is
not mandated as RFCs. However, "AKAv2" is documented as an RFC
because the use of different AKA algorithm versions includes security
implications of which the implementors should be aware. The

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extension version and security implications are presented in this
document.

1.1. Terminology

This chapter explains the terminology used in this document.

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 [3].

AKA

Authentication and Key Agreement.

AKA is a challenge-response based mechanism that uses symmetric
cryptography. AKA can be run in a UMTS IM Services Identity
Module (ISIM) or in UMTS Subscriber Identity Module (USIM), which
reside on a smart-card-like device that also provides tamper
resistant storage of shared secrets.

CK

Cipher Key. An AKA session key for encryption.

CK'

Cipher Key. HTTP Digest AKAv2 session key for encryption. CK' is
derived from CK using a pseudo-random function.

IK

Integrity Key. An AKA session key for integrity check.

IK'

Integrity Key. HTTP Digest AKAv2 session key for integrity check.
IK' is derived from IK using a pseudo-random function.

ISIM

IP Multimedia Services Identity Module. Sometimes ISIM is
implemented using USIM.

RES

Authentication Response. Generated by the ISIM.

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PRF

Pseudo-random function that is used to construct the AKAv2
password and related session keys IK' and CK'. In this document,
PRF is presented in the format KD(secret, data), denoting a keyed
digest algorithm (KD) performed to the data ("data") with the
secret ("secret").

SIM

Subscriber Identity Module. GSM counter part for ISIM and USIM.

UMTS

Universal Mobile Telecommunications System.

USIM

UMTS Subscriber Identity Module. UMTS counter part for ISIM and
SIM.

XRES

Expected Authentication Response. In a successful authentication,
this is equal to RES.

2. HTTP Digest AKAv2

In general, the Digest AKAv2 operation is identical to the Digest
AKAv1 operation described in [6]. This chapter specifies the parts
in which Digest AKAv2 is different from Digest AKAv1 operation. The
notation used in the Augmented BNF definitions for the new and
modified syntax elements in this section is as used in SIP [5], and
any elements not defined in this section are as defined in [6].

In order to direct the client into using AKAv2 for authentication
instead of other AKA versions or other HTTP Digest algorithms, the
AKA version directive of [6] shall have the following new value:

aka-version = "AKAv2"

The AKA version directive is used as a part of the algorithm field as
defined in [6].

Example: algorithm=AKAv2-MD5

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2.1. Password Generation

The client shall use base64 encoded [1] parameters PRF(RES||IK||CK,
"http-digest-akav2-password") as a "password" when calculating the
HTTP Digest response directive for AKAv2.

The server shall use base64 encoded [1] parameters PRF(XRES||IK||CK,
"http-digest-akav2-password") as a "password" when checking the HTTP
Digest response or when calculating the "response-auth" of the
"Authentication-Info" header.

The pseudo-random function (PRF) used to construct the HTTP Digest
password is equal to HMAC [2] using the hash algorithm that is used
in producing the digest and the checksum. For example, if the
algorithm is AKAv2-MD5, then the PRF is HMAC_MD5.

The string "http-digest-akav2-password" included in the key
derivation is case sensitive.

2.2. Session keys

Even though the HTTP Digest AKA framework does not specify the use of
the session keys IK and CK for confidentiality and integrity
protection, the keys may be used for creating additional security
within HTTP authentication or some other security mechanism.
However, the original session keys IK and CK MUST NOT be directly
re-used for such additional security in "AKAv2". Instead, session
keys IK' and CK' are derived from the original keys IK and CK in the
following way:

IK' = PRF(IK, "http-digest-akav2-integritykey")

CK' = PRF(CK, "http-digest-akav2-cipherkey")

Any application using the HTTP authentication framework is allowed to
use these masked session keys. The unmasked session keys MAY also be
re-used in some other context if application-specific strings other
than "http-digest-akav2-integritykey" or
"http-digest-akav2-cipherkey" are used to mask the original session
keys.

The pseudo-random function (PRF) used to construct the HTTP Digest
session keys is equal to HMAC [2] using the hash algorithm that is
used in producing the digest and the checksum. For example, if the
algorithm is AKAv2-MD5, then the PRF is HMAC_MD5. The algorithm MUST
be used in the HMAC format, as defined in [2].

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The strings "http-digest-akav2-integritykey" and "http-digest-akav2-
cipherkey" included in the key derivation are case sensitive.

3. Example Digest AKAv2 Operation

This document does not introduce any changes to the operations of
HTTP Digest or HTTP Digest AKA. Examples defined in [6] apply
directly to AKAv2 with the following two exceptions:

1. The algorithm directive has a prefix "AKAv2" instead of "AKAv1".

2. The HTTP Digest password is derived from base64 encoded PRF(RES||
IK||CK, "http-digest-akav2-password") or PRF(XRES||IK||CK, "http-
digest-akav2-password") instead of (RES) or (XRES) respectively.

3. The optional session keys are derived from PRF(IK, "http-digest-
akav2-integritykey") and PRF(CK, "http-digest-akav2-cipherkey")
instead of IK and CK respectively.

Note that the password in "AKAv1" is in binary format. The "AKAv2"
password is base64 encoded [1].

4. Security Considerations

4.1. Multiple Authentication Schemes and Algorithms

The rules for a user agent for choosing among multiple authentication
schemes and algorithms are as defined in [6], except that the user
agent MUST choose "AKAv2" if both "AKAv1" and "AKAv2" are present.

Since HTTP Digest is known to be vulnerable for bidding-down attacks
in environments where multiple authentication schemes and/or
algorithms are used, the system implementors should pay special
attention to scenarios in which both "AKAv1" and "AKAv2" are used.
The use of both AKA algorithm versions should be avoided, especially
if the AKA generated sessions keys or some other additional security
measures to authenticate the clients (e.g., client certificates) are
not used.

4.2. Session Protection

Even though "AKAv2" uses the additional integrity (IK) and
confidentiality (CK) keys as a part of the HTTP Digest AKA password,
these session keys may still be used for creating additional security
within HTTP authentication or some other security mechanism. This
recommendation is based on the assumption that algorithms used in
HTTP Digest, such as MD5, are sufficiently strong one-way functions,
and, consequently, HTTP Digest responses leak no or very little

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computational information about IK and CK. Furthermore, the session
keys are masked into IK' and CK' before they can be used for session
protection.

4.3. Man-in-the-Middle Attacks

Reference [8] describes a "man-in-the-middle" attack related to
tunnelled authentication protocols. The attack can occur in an EAP
context or any similar contexts where tunnelled authentication is
used and where the same authentication credentials are used without
protection in some other context or the client fails to authenticate
the server.

For example, the use of TLS with HTTP Digest authentication (i.e.,
TLS for server authentication, and subsequent use of HTTP Digest for
client authentication) is an instance of such scenario. HTTP
challenges and responses can be fetched from and to different TLS
tunnels without noticing their origin. The attack is especially easy
to perform if the client fails to authenticate the server. If the
same HTTP credentials are used with an unsecured connection, the
attack is also easy to perform.

This is how the "man-in-the-middle" attack works with HTTP Digest and
TLS if the victim (i.e., the client) fails to authenticate the
server:

1. The victim contacts the attacker using TLS. If the attacker has
a valid server certificate, the client may continue talking to
the attacker and use some HTTP authentication compatible
protocol, such as the Session Initiation Protocol (SIP).

2. The attacker contacts a real proxy/server also using TLS and an
HTTP-authentication-compatible protocol. The proxy/server
responds to the attacker with the HTTP Authentication challenge.

3. The attacker forwards the HTTP Authentication challenge from the
proxy/server to the victim. If the victim is not careful, and
does not check whether the identity in the server certificate in
TLS matches the realm in the HTTP authentication challenge, it
may send a new request that carries a valid response to the HTTP
Authentication challenge.

4. The attacker may use the response with the victims HTTP Digest
username and password to authenticate itself to the proxy/server.

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The man-in-the-middle attack is not possible if the client compares
the identities in the TLS server certificate and the HTTP Digest
authentication challenge. Note that with HTTP Basic, the client
would send the password to the attacker.

Another variant of the "man-in-the-middle" attack is the so-called
"interleaving attack". This attack is possible if the HTTP Digest
authentication credentials are used in several contexts, and in one
of them without protection.

This is how the attack could proceed:

1. The attacker establishes a TLS tunnel to the proxy/server using
one-way server authentication. The attacker sends a request to
the proxy/server.

2. The proxy/server challenges the attacker with the HTTP Digest
challenge.

3. The attacker challenges the victim in some other context using
the challenge carried in the HTTP Digest challenge. The HTTP
Digest challenge needs to be modified to the format used in the
protocol of this other context.

4. The victim responds with a response.

5. The attacker uses the response from the other context for
authentication in HTTP Digest.

6. The proxy/server accepts the response, and delivers the service
to the attacker.

In some circumstances, HTTP Digest AKAv1 may be vulnerable for the
interleaving attack. In particular, if ISIM is implemented using
USIM, the HTTP Digest AKAv1 should not be used with tunneled security
protocols unless the AKA-related session keys, IK and CK, are somehow
used with the solution.

HTTP Digest AKAv2 is not vulnerable to this interleaving attack, and
it can be used with tunneled security protocols without using the
related AKA session keys.

4.4. Entropy

AKAv1 passwords should only be used as one-time passwords if the
entropy of the used RES value is limited (e.g., only 32 bits). For
this reason, the re-use of the same RES value in authenticating
subsequent requests and responses is not recommended. Furthermore,

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algorithms such as "MD5-sess", which limit the amount of material
hashed with a single key by producing a session key for
authentication, should not be used with AKAv1.

Passwords generated using AKAv2 can more securely be used for
authenticating subsequent requests and responses because the
concatenation of AKA credentials (i.e., RES||IK||CK) makes the
passwords significantly longer, and the pseudo-random function
heuristically provides an entropy equal to the length of this string,
or the length of the PRF output, whichever is the shortest. The user
agent does not need to assume that AKAv2 passwords are limited to
one-time use only, and it may try to re-use the AKAv2 passwords with
the server. However, note that AKAv2 passwords cannot be re-used
with the HTTP Digest AKAv2 algorithm because such an authentication
challenge will automatically generate a fresh password. AKAv2
passwords can be used with other HTTP Digest algorithms, such as
"MD5".

The underlying AKA protocol (e.g., UMTS AKA) has been designed to
keep CK and IK confidential, but will typically send RES in the
clear. We note that, even if (by some unfortunate misuse of AKA) RES
values were revealed, the inclusion of RES in PRF(RES||IK||CK) is
still beneficial, as it makes pre-calculated dictionaries of IK||CK
values rather useless (though such dictionaries are infeasible for
typical sizes of IK and CK).

5. IANA Considerations

This document specifies a new aka-version, "AKAv2", to the
aka-version namespace maintained by IANA. The procedure for
allocation of new aka-versions is defined in [6].

5.1. Registration Information

To: [email protected]

Subject: Registration of a new AKA version

Version identifier: "AKAv2"

Contacts for further information: [email protected],
[email protected], or [email protected]

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6. References

6.1. Normative References

[1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.

[2] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.

[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[4] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.

[5] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.

[6] Niemi, A., Arkko, J., and V. Torvinen, "Hypertext Transfer
Protocol (HTTP) Digest Authentication Using Authentication and
Key Agreement (AKA)", RFC 3310, September 2002.

6.2. Informative References

[7] 3rd Generation Partnership Project, "Security Architecture
(Release 4)", TS 33.102, December 2001.

[8] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle in
Tunnelled Authentication Protocols", Cryptology ePrint Archive,
http://eprint.iacr.org Report 2002/163, October 2002.

[9] Puthenkulam, J., Lortz, V., Palekar, A., and D. Simon, "The
Compound Authentication Binding Problem", Work in Progress,
March 2003.

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Authors' Addresses

Vesa Torvinen
Turku Polytechnic
Ylhaistentie 2
Salo FIN 24130
Finland

Phone: +358 10 5536210
EMail: [email protected]

Jari Arkko
Ericsson
Hirsalantie 1
Jorvas FIN 02420
Finland

Phone: +358 40 5079256
EMail: [email protected]

Mats Naeslund
Ericsson
Torshamnsgatan 23
Stockholm SE 16480
Sweden

Phone: +46 8 58533739
EMail: [email protected]

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