One document matched: draft-zeilenga-sasl-yap-06.txt
Differences from draft-zeilenga-sasl-yap-05.txt
INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Experimental Isode Limited
Expires in six months 30 May 2009
SASL Yet Another Password Mechanism
<draft-zeilenga-sasl-yap-06.txt>
Status of this Memo
This document is intended to be, after appropriate review and
revision, submitted to the RFC Editor as a Experimental document.
Distribution of this memo is unlimited. Technical discussion of this
document may take place on the IETF SASL WG mailing list <ietf-
sasl@imc.org>. Please send editorial comments directly to the author
<Kurt.Zeilenga@Isode.COM>.
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Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
Abstract
This document describes a password authentication mechanism, called
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YAP-SHA-256-TLS-UNIQ, for use in protocols which support Simple
Authentication and Security Layer (SASL) framework. The mechanism
relies on security services provided by a lower layer, such as
Transport Layer Security (TLS), to protect the authentication
exchange, and subsequent application data exchange, from common
attacks. The YAP-SHA-256-TLS-UNIQ mechanism can be viewed as an
alternative to other password-based SASL mechanism, such as PLAIN,
CRAM-MD5, and DIGEST-MD5.
1. Introduction
There exist multiple password-based mechanisms for use in the Simple
Authentication and Security Layer (SASL) [RFC4422] framework. These
include the PLAIN [RFC4616], CRAM-MD5 [RFC2195], and DIGEST-MD5
[RFC2831]. None of these mechanisms, themselves, provide integrity
and confidential protection over the entirety of the authentication
exchange. Only DIGEST-MD5 offers a security layer and, even so, the
specification and its implementations suffer from multiple problems.
And while these mechanisms may be used in conjunction with lower-level
security services, these mechanism do not offer any facility to bind
the channels [RFC5056].
This situation has lead to multiple efforts to design "better" SASL
password-based mechanism. This document not only specifies yet
another password mechanism, YAP-SHA-256-TLS-UNIQ, but defines a family
of related password mechanisms, YAP-*.
YAP-* is a family of simple password SASL mechanisms based upon the
Keyed-Hash Message Authentication Code (HMAC) [RFC2104] algorithm and
unique channel bindings [RFC5056].
The YAP-SHA256-TLS-UNIQ is a YAP mechanism which uses the SHA-256
[FIPS180-2] cryptographic hash function in conjunction with the HMAC
algorithm and the tls-unique [CBT-TLS-U] unique channel bindings.
YAP is specified as a family of SASL mechanisms to provide hash
agility and channel binding type agility.
YAP mechanisms rely on services provided at a lower level, such as
Transport Layer Security (TLS) [RFC5246], to secure the authentication
exchange and subsequent application data exchange and, hence, YAP
mechanisms do not offer a SASL security layer. YAP mechanisms require
the lower-level security layer to be bound in the authentication using
unique channel bindings [RFC5056]. YAP relies on client to
authenticate the server within this lower-level security layer to
avoid information disclosure to rogue servers.
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1.1 Experimental
This specification is part of a research and development effort
exploring alternatives to current password-based authentication
mechanisms.
Implementors of this specification ought to considered implementing
the SCRAM [SCRAM] mechanism being developed by the IETF for
publication on the Standards Track.
1.2 Terminology
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 RFC 2119 [RFC2119].
2. The YAP-* Family of Mechanisms
Each mechanism in this family differs by the choice of hash algorithm
and the choice of unique channel binding type. Each mechanism has a
name of the form YAP-HA-CBT where HA is a string chosen to reflect the
hash algorithm used and CBT is a string choosen to reflect the channel
binding type. HA and CBT are to be choose so the mechanism name does
not exceed 20 characters imposed by the SASL Technical Specification
[RFC4422]. While it not required that each mechanism use the same HA
string for a particular hash algorithm or the same CBT for a
particular channel binding type as those used in previously registered
mechanisms, reuse of the encouraged.
To define a new mechanism within the YAP family of mechanisms, the
mechanism specification must indicate that it is a YAP mechanism,
identify the hash algorithm used, identify the channel binding type
used and specify the name the mechanism and cause this name to be
registered with IANA in accordance with the SASL Technical
Specification. The mechanism specification should detail security
considerations specific to hash algorithm and channel binding types
selected.
3. The YAP Mechanism
The mechanism involves a single message from the client to the server.
message = authzid separator [ authcid ] separator data
separator = %x00
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where:
- <authzid>, when present, is the authorization identity in the
form specified by the application protocol specification,
represented in UTF-8 [RFC3629], and
- <authcid> is authentication identity, a simple user name
[RFC4013], prepared using the SASLprep [RFC4013] and represented
in UTF-8 [RFC3629],
- <data> is a Keyed-Hash Message Authentication Code (HMAC)
[RFC2104] produced as described below.
Implementors should note that the data portion of the message may
contain a zero-valued octet and hence should parse the message
front-to-back.
The HMAC is produced using the mechanism-specific hash algorithm, such
as SHA-256, as the cryptographic hash function, H. The secret key, K,
is the unique channel binding [RFC5056] for the lower-level security
protocol, padded with zero octets to the block size of the hash
function. Where the unique channel binding is longer than the block
size of the hash function, K is hash of the unique channel binding.
The text is the concatenation of the authcid, the authzid, and the
hash of the user's password, a simple password [RFC4013], prepared
using SASLprep [RFC4013] and represented in UTF-8 [RFC3629]. That is,
the <data> is computed as illustrated by the following pseudo code.
HMAC(
Pad( Length(ChannelBinding)>HashBlockSize
? H(ChannelBinding) : ChannelBinding, 0, HashBlockSize),
Concat(authzid, authcid, H(UTF8(SASLprep(password)))))
Note, in this pseudo code, the first argument of the HMAC function is
the secret key and the second is the text. The cryptographic hash
function used in the HMAC is implicitly H. The Pad function pads the
first argument to the length specified in the third argument with the
octet value provided in the second argument. The variable
HashBlockSize is the block size of hash function, H. The Length
function returns the length of its argument. The Concat function
returns an octet which is the concatenation of its arguments. The
UTF8 function returns the UTF-8 encoding of its argument. The
SASLprep function prepares it argument according to the SASLprep
algorithm. The H function returns the hash of its argument.
The hash of the user's password is a password equivalent. Servers may
choose to store this hash instead of the user's password. In either
case, the stored value must be adequately protected.
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Implementations SHOULD NOT advertise availability of any mechanism in
this family unless a lower-level security service providing both data
integrity and data confidentiality protection is in place. Client
implementations SHOULD NOT utilize any mechanism in this family
without first verifying the identity of the server within the
lower-level security service. Client implementors should consult the
application protocol specification, in conjunction with the
specification of the lower-level security service, for details on how
to implement this verification.
4. The YAP-SHA-256-TLS-UNIQ Mechanism
The YAP-SHA-256-TLS-UNIQ mechanism is a YAP mechanism which utilizes
the SHA-256 [FIPS180-2][RFC4634] hash algorithm and the tls-unique
unique channel binding type [CBT-TLS-U]. This type is for use with
Transport Layer Security (TLS) [RFC5246].
The mechanism is named "YAP-SHA-256-TLS-UNIQ".
5. YAP-SHA-256-TLS-UNIQ Example
Consider a client authenticating as "kurt" with the password "secret"
who is not wishing to act as another user which has established TLS
channel which has the tls-unique binding of
zHsxigXXUssRg9iVRbw5AX/dgRVlUgBz/RfjI7c4woM= (base64).
The client compute the HMAC over the authzid, authcid, and password as
described in section 3 with the binding as the secret key. The
client would construct text input the HMAC by concatenating the empty
authzid string with the "kurt" authcid string with the hash of the
properly prepared password. SASLprep("secret") returns "secret". The
hash of this string (when encoded as UTF-8), is
K7gNU3sdo+OL0wNhqoVWhr3g6s1xYv72ol/pe/Unols= (base64). The HMAC for
this text and key is Ksarn7PFnqCgi4ewSYOfXIyP8ImNcmpoWmtCgA0QqT4=
(base64).
The client would construct and send the message which contained first
zero octets for the authzid, then a 00 (hex) octet for a separator,
followed by 4 octets 6b 75 72 74 (hex) representing the authcid
"kurt", followed by 00 (hex) octet for a separator, followed by 32
octets HMAC value. This would produce a message of
AGt1cnQAKsarn7PFnqCgi4ewSYOfXIyP8ImNcmpoWmtCgA0QqT4= (base64).
6. Security Considerations
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Security is discussed throughout this document.
This family of mechanisms was specifically designed to rely on
security services offered at lower-levels to secure mechanism
negotiation, the authentication exchange and subsequent data
exchanges. To ensure lower-level security services are provided
end-to-end, the mechanisms utilize unique channel bindings [RFC5056].
To avoid disclosing the identity information to a rogue server, the
client verifies the server's identity using the lower-layer security
service before utilizing any mechanism in this family.
Hash agility and channel binding type agility is provided in the
family of mechanisms through the specification of additional
mechanisms.
To avoid requiring server implementations maintain access to the
user's password, a password equivalent is used. The password
equivalent is a simple hash of the password.
While it is likely that those choosing to store the password
equivalent instead of the password would prefer the equivalent be
designed to hinder dictionary attack with precomputed dictionary
entries, a simple hash was chosen to avoid adding a server challenge.
Use of the authcid as a salt was considered but rejected as it would
tie the password equivalent to a particular authcid. It is desirable
for the password equivalent to be usable with multiple authcid values
(kurt and KURT) representing the same entity. It was also realized
that it likely that implementors would (continue to) choose to store
the password instead of a mechanism-specific password equivalent.
Storing the password avoids significant implementation complexity and
facilitates mechanism agility.
YAP-SHA-256-TLS-UNIQ uses the SHA-256 hash algorithm and tls-unique
channel binding type. At the time of this writing, there are no known
attacks on SHA-256 hash algorithm or tls-unique channel binding type
which are applicable to this mechanism.
7. IANA Considerations
It is requested that IANA process the following request(s) upon
approval of this document for publication as an RFC.
Subject: Registration of SASL YAP family of mechanisms
SASL family name (or prefix for the family): YAP-*
Security considerations: see RFC XXXX
Published specification (recommended): RFC XXXX
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Person & email address to contact for further information:
Kurt Zeilenga <kurt.zeilenga@isode.com>
Intended usage: COMMON
Subject: Registration of SASL YAP SHA-256 tls_endpoint mechanism
SASL name (or prefix for the family): YAP-SHA-256-TLS-UNIQ
Security considerations: see RFC XXXX
Published specification (recommended): RFC XXXX
Person & email address to contact for further information:
Kurt Zeilenga <kurt.zeilenga@isode.com>
Intended usage: COMMON
7. Acknowledgments
TBD.
8. Author's Address
Kurt D. Zeilenga
Isode Limited
Email: Kurt.Zeilenga@Isode.COM
9. References
[[Note to the RFC Editor: please replace the citation tags used in
referencing Internet-Drafts with tags of the form RFCnnnn where
possible.]]
9.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14 (also RFC 2119), March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629 (also STD 63), November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[RFC4422] Melnikov, A. (Editor), K. Zeilenga (Editor), "Simple
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Authentication and Security Layer (SASL)", RFC 4422,
June 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5056] Williams, N., "on the Use of Channel Bidnings to Secure
Channels", RFC 5056, November 2007.
[FIPS180-2] National Institute of Standards and Technology, "Secure
Hash Algorithm. NIST FIPS 180-2", August 2002.
[CBT-TLS-U] Zhu, Larry, "Registration of TLS unique channel bindings
(generic)", http://www.iana.org/assignments/channel-
binding-types/tls-unique, June 26, 2008.
9.2. Informative References
[RFC2195] Klensin, J., R. Catoe, and P. Krumviede, "IMAP/POP
AUTHorize Extension for Simple Challenge/Response", RFC
2195, September 1997.
[RFC2831] Leach, P. and C. Newman, "Using Digest Authentication as
a SASL Mechanism", RFC 2831, May 2000.
[RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616, August 2006.
[RFC4634] Eastlake 3rd, D. and T. Hansen, "US Secure Hash
Algorithms (SHA and HMAC-SHA)", RFC 4634, August 2006.
[RFC5246] Dierks, T. and, E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246, August
2008.
[SCRAM] Menon-Sen, A., el. al, "Salted Challenge Response
(SCRAM) SASL Mechanism", draft-ietf-sasl-scram-xx.txt, a
work in progress.
Full Copyright
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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