Network Working Group V. Torvinen
Internet-Draft J. Arkko
Expires: April 28, 2005 M. Naslund
Ericsson
October 28, 2004
Hypertext Transfer Protocol (HTTP) Digest Authentication Using
Authentication and Key Agreement (AKA) Version-2
draft-torvinen-http-digest-aka-v2-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
HTTP Digest as specified in [4] 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
exist not just with HTTP Digest but also with other IETF protocols
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that use tunneled authentication. This document specifies version 2
of the HTTP Digest AKA algorithm [6]. This algorithm can be
implemented in a way that it is resistant to the man-in-the-middle
attack.
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
3. HTTP Digest AKAv2 . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Password generation . . . . . . . . . . . . . . . . . . . 6
3.2 Session keys . . . . . . . . . . . . . . . . . . . . . . . 7
4. Example Digest AKAv2 Operation . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5.1 Multiple Authentication Schemes and Algorithms . . . . . . 8
5.2 Session Protection . . . . . . . . . . . . . . . . . . . . 8
5.3 Man-in-the-middle attacks . . . . . . . . . . . . . . . . 8
5.4 Entropy . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6.1 Registration Information . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
7.2 Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Requirements notation
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].
2. Introduction
The Hypertext Transfer Protocol (HTTP) Digest Authentication,
described in [4], has been extended in [6] to support Authentication
and Key Agreement (AKA) mechanism [8]. AKA mechanism performs
authentication and session key distribution 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 general attack that
affects a number of IETF protocols such as PIC. The general problem
is discussed in [9] and [10].
Because of the previous vulnerability, the use of HTTP Digest "AKAv1"
should be limited to the situations where the client is able to
demonstrate that in addition to the AKA response, it possess 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 "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 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.
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2. The keys used in the underlying security protocols are somehow
bind 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
application layer. Furthermore, this solution would only solve
the problem when HTTP Digest is used over one hop, and leave the
problem of using HTTP Digest via multiple hops, e.g. via proxy
servers, unsolved.
3. Authentication credentials are used in 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 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 either on HTTP
Digest authentication or UMTS authentication (cf. approach 4 above).
The only difference to "AKAv1" is that in addition to AKA response
RES the AKA related session keys, IK and CK, are also used as the
password for HTTP Digest. AKAv2 is immune to 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 in 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 that the implementators should be aware of. The
extension version and security implications are presented in this
document.
2.1 Terminology
This chapter explains the terminology used in this document.
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
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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.
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 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
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UMTS Subscriber Identity Module. UMTS counter part for ISIM and
SIM.
XRES
Expected Authentication Response. In a successful authentication
this is equal to RES.
3. 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
3.1 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.
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The string "http-digest-akav2-password" included in the key
derivation is case sensitive.
3.2 Session keys
Even though 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 HTTP authentication framework is allowed to use
these masked session keys. The unmaksed session keys MAY also be
re-used in some other context if some other application specific
strings 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 HMAC format as defined in [2].
The strings "http-digest-akav2-integritykey" and
"http-digest-akav2-cipherkey" included in the key derivation are case
sensitive.
4. 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] applies
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,
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"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. "AKAv2"
password is base64 encoded [1].
5. Security Considerations
5.1 Multiple Authentication Schemes and Algorithms
The rules for an 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 attack
in environments where multiple authentication schemes and/or
algorithms are used, the system implementators should pay special
attention for scenarios where both "AKAv1" and "AKAv2" are used.
Especially if the AKA generated sessions keys or some other
additional security measures to authenticate the clients, such as
client certificates, are not used, the use of both AKA algorithm
versions should be avoided.
5.2 Session Protection
Even though "AKAv2" uses the additional integrity (IK) and
confidentiality (CK) keys as a part of 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
computational information about IK and CK. Furthermore, the session
keys are masked into IK' and CK' before they can be used for session
protection.
5.3 Man-in-the-middle attacks
[9] describe a "man-in-the-middle" attack related to tunnelled
authentication protocols. The attack can occur e.g. in 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
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challenges and responses can be fetched from and to different TLS
tunnels without noticing where they originally came from.
Especially, the attack is easy to perform if the client fails to
authenticate the server. If the same HTTP credentials are used with
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 Session Initiation Protocol (SIP).
2. The attacker contacts some real proxy/server also using TLS and
some HTTP authentication compatible protocol. The proxy/server
responds to the attacker with 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
check that the identity in the server certificate in TLS matches
the realm in the HTTP authentication challenge, it may send a new
request which 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.
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 HTTP Digest
challenge.
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3. The attacker challenges the victim in some other context using
the challenge carried in the HTTP Digest challenge. The HTTP
Digest challenge need 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 for this interleaving attack, and
it can be used with tunneled security protocols without using the
related AKA session keys.
5.4 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 reuse of the same RES value in authenticating
subsequent requests and responses is not recommended. Furthermore,
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 can not be
re-used with HTTP Digest AKAv2 algorithm because such 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
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keep CK and IK confidential but will typically send RES in the clear.
We note that even if (by some unfortunate missuse of AKA) RES values
were revealed, the inclusion of RES in PRF(RES||IK||CK) is still
benefitial as it makes pre-calculated dictionaries of IK||CK values
rather useless (though such dictionaries are anyway infeasible for
typical sizes of IK and CK).
6. 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].
6.1 Registration Information
To: ietf-digest-aka@iana.org
Subject: Registration of a new AKA version
Version identifier: "AKAv2"
Contacts for further information: vesa.torvinen@ericsson.com,
jari.arkko@ericsson.com or mats.naslund@ericsson.com
7. References
7.1 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
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Key Agreement (AKA)", RFC 3310, September 2002.
7.2 Informative References
[7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[8] 3rd Generation Partnership Project, "Security Architecture
(Release 4)", TS 33.102, December 2001.
[9] 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.
[10] Puthenkulam, J., Lortz, V., Palekar, A. and D. Simon, "The
Compound Authentication Binding Problem", IETF, Work in
progress draft-puthenkulam-eap-binding-02.txt, March 2003.
Authors' Addresses
Vesa Torvinen
Ericsson
Joukahaisenkatu 1
Turku FIN 20520
Finland
Phone: +358 40 7230822
EMail: vesa.torvinen@ericsson.com
Jari Arkko
Ericsson
Hirsalantie 1
Jorvas FIN 02420
Finland
Phone: +358 40 5079256
EMail: jari.arkko@ericsson.com
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Mats N„slund
Ericsson
Torshamnsgatan 23
Stockholm SE 16480
Sweden
Phone: +46 8 58533739
EMail: mats.naslund@ericsson.com
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