Network Working Group Abhijit Menon-Sen
Internet-Draft Oryx Mail Systems GmbH
Intended Status: Proposed Standard Chris Newman
Expires: September 6, 2007 Sun Microsystems
March 2007
Salted Challenge Response Authentication Mechanism (SCRAM)
draft-newman-auth-scram-04.txt
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Copyright (C) The IETF Trust (2007).
Abstract
The secure authentication mechanism most widely deployed and used by
Internet application protocols, is the transmission of clear-text
passwords over a channel protected by Transport Layer Security
(TLS). There are some significant security concerns with that
mechanism which could be addressed by the use of a challenge
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response authentication mechanism protected by TLS. Unfortunately,
the challenge response mechanisms presently on the standards track
all fail to meet requirements necessary for widespread deployment
and have had success only in limited use.
This specification describes the Salted Challenge Response
Authentication Mechanism (SCRAM) which addresses the deployability
requirements. When used in combination with Transport Layer
Security or an equivalent security layer, this mechanism could
improve the status-quo for application protocol authentication and
provide a suitable choice for a mandatory-to-implement mechanism for
future application protocol standards.
1. Conventions 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 [RFC2119].
Formal syntax is defined by [RFC4234] including the core rules
defined in Appendix B of [RFC4234].
Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server. If a single "C:" or "S:" label
applies to multiple lines, then the line breaks between those lines
are for editorial clarity only and are not part of the actual
protocol exchange.
1.1. Terminology
This document uses several terms defined in [RFC2828] ("Internet
Security Glossary") including the following: authentication,
authentication exchange, authentication information, brute force,
challenge-response, cryptographic hash function, dictionary attack,
eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
one-way encryption function, password, replay attack and salt.
Readers not familiar with these terms should use that glossary as a
reference.
Some clarifications and additional definitions follow:
- Authentication information: Information used to verify an identity
claimed by a SCRAM client. The authentication information for a
SCRAM identity consists of salt, and for each cryptographic hash
function supported, it includes the "StoredKey" and the
"ServerKey" (the latter two are defined in the algorithm
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overview).
- Authentication database: The database used to look up the
authentication information associated with a particular identity.
For application protocols, LDAPv3 (see [RFC4510]) is frequently
used as the authentication database. For network-level protocols
such as PPP or 802.11x, the use of RADIUS is more common.
- Base64: An encoding mechanism defined in [RFC2045] which converts
an octet string input to a textual output string which can be
easily displayed to a human. Although MIME permits arbitrary
insertion of whitespace and other characters, use of base64 in
SCRAM is restricted to the canonical form with no whitespace.
- Octet: An 8-bit byte.
- Octet string: A sequence of 8-bit bytes.
- Salt: A random octet string that is combined with a password
before applying a one-way encryption function. This value is used
to protect passwords that are stored in an authentication
database.
1.2. Notation
The pseudocode description of the algorithm uses the following
notations:
- ":=": The variable on the left hand side represents the octet
string resulting from the expression on the right hand side.
- "+": Octet string concatenation.
- "[ ]": A portion of an expression enclosed in "[" and "]" may not
be included in the result under some circumstances. See the
associated text for a description of those circumstances.
- HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in
[RFC2104]) using the octet string represented by "key" as the key
and the octet string "str" as the input string. The size of the
result is the hash result size for the hash function in use. For
example, it is 20 octets for SHA-1 (see [RFC3174]).
- H(str): Apply the cryptographic hash function to the octet string
"str", producing an octet string as a result. The size of the
result depends on the hash result size for the hash function in
use.
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- Hi(str): Apply the cryptographic hash function to the octet string
"str", then repeat the application on the output string for a
number of iterations equal to the integer i minus 1.
- XOR: Apply the exclusive-or operation to combine the octet string
on the left of this operator with the octet string on the right of
this operator. The length of the output and each of the two
inputs will be the same for this use.
2. Introduction
This specification describes the Salted Challenge Response
Authentication Mechanism (SCRAM) which addresses the requirements
necessary to deploy a challenge-response mechanism more widely than
past attempts. When used in combination with Transport Layer
Security (TLS, see [RFC4346]) or an equivalent security layer, this
mechanism could improve the status-quo for application protocol
authentication and provide a suitable choice for a mandatory-to-
implement mechanism for future application protocol standards.
For simplicity, this mechanism does not presently include
negotiation of a security layer. It is intended to be used with an
external security layer such as that provided by TLS or SSH.
SCRAM provides the following protocol features:
- The authentication information stored in the authentication
database is not sufficient by itself to impersonate the client.
The information is salted to prevent a pre-stored dictionary
attack if the database is stolen.
- The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies).
- The mechanism permits the use of a server-authorized proxy without
requiring that proxy to have super-user rights with the back-end
server.
- A standard attribute is defined to enable storage of the
authentication information in LDAPv3 (see [RFC4510]).
- Bindings to several authentication frameworks are provided so the
mechanism is not limited to a small subset of protocols.
- Both the client and server can be authenticated by the protocol.
- The cryptographic hash function used to authenticate can be
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upgraded gracefully without breaking backwards compatibility or
risking downgrade attacks.
For an in-depth discussion of why other challenge response
mechanisms are not considered sufficient, see appendix A. For more
information about the motivations behind the design of this
mechanism, see appendix B.
Comments regarding this draft may be sent either to the ietf-
sasl@imc.org mailing list or to the authors.
3. SCRAM Algorithm Overview
Here is a psuedocode overview of the complete SCRAM algorithm:
SaltedPassword := Hi(HMAC(password, salt))
ClientKey := H(SaltedPassword)
StoredKey := H(ClientKey)
Message := [FirstClientMessage + "," +
FirstServerMessage + "," +]
FinalClientMessageExcludingP
ClientSignature := HMAC(StoredKey, Message)
ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, salt)
ServerSignature := HMAC(ServerKey, Message)
The client has access to the user's password and once it gets the
value of the salt and the value of i from the server, it can
determine all of the above values. The authentication information
which the server stores consists of the "salt", the "StoredKey" and
the "ServerKey". The server authenticates the client by computing
the ClientSignature, exclusive-oring that with the ClientProof to
recover the ClientKey and verifying the correctness of the ClientKey
by applying the hash function and comparing the result to the
StoredKey.
4. SCRAM Authentication Exchange
The SCRAM protocol is a textual protocol with a comma-separated list
of attribute value pairs. Each attribute has a name with one
letter. An example of a simple protocol exchange follows:
C: r=ClientNonce,n=Chris Newman
S: r=ClientNonceServerNonce,i=128,s=PxR/wv+epq
C: r=ClientNonceServerNonce,
c=QWBv+XbwcaI/wp,p=WxPv/siO5l+qxN4
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S: v=WxPv/siO5l+qxN4
The client sends the first message to provide the user name whose
password will be used, a random and unique nonce and a list of the
cryptographic hash functions the client supports. The server
responds by appending additional randomness on the end of the nonce,
providing the list of hash functions it supports, and providing the
base64-encoded salt from that user's authentication information.
The client then authenticates by repeating the nonce, stating the
resource to which it is authenticating, an optional base64-encoded
channel binding (discussed later) and a proof that it has the user's
password. Unless the client has choosen to skip the server
authentication step (discussed later), the server responds with a
verification proof that the server has the appropriate password
verifier.
A more detailed description of the protocol elements follows:
- a: This attribute can be provided in the client's first message.
It specifies the authorization identity. When this is omitted or
empty, it is presumed to be the same as the "n" attribute. When
this is different from the "n" attribute, then the "n" attribute
specifies the user name whose password will be used for
authentication, while the "a" attribute specifies the user whose
identity will be associated with the connection subsequent to
successful authentication and authorization. This is typically
used when an administrative user wishes to impersonate another
user to perform some management role for that user. It can also
be used by proxies in some situations (see appendix A for more
information). The syntax of this field is the same as that of the
"n" field with respect to quoting of '=' and ','.
- c: This base64-encoded string is the channel binding. Whether
this is included and the meaning of the contents of this string
depends on the external security layer used with this mechanism.
See section XXX for more details. If this is present, the
authentication MUST fail unless this value is successfully
verified. In other words, if it appears when not expected then an
automatic authentication failure results. This is necessary to
detect a man-in-the-middle attack on the security layer (see
security considerations for details).
- n: This is the name of the user whose password is used for
authentication. In most cases, the "a" attribute is omitted and
this also specifies the identity which will be associated with the
connection subsequent to authentication and authorization. If the
actual name contains the characters ',' or '=', then a quoting
mechanism is applied which converts these characters to '=2C' and
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'=3D' respectively. If the server receives a name string which
contains '=' not followed by either '2C' or '3D', then the server
MUST fail the authentication.
- p: This base64-encoded string provides the client's proof of
authentication. This is the last attribute in the message. See
the algorithm overview for the computation.
- r: This is a sequence of random printable characters excluding ','
which forms the nonce used as input to the hash function. No
quoting is applied to this string (unless the binding of SCRAM to
a particular protocol states otherwise). The client provides the
first part of the string and the server provides the second part.
It is important that this be different for each authentication and
it is important that the client verifies the first part of this
string used in all messages was provided by the client.
- s: This is a base64-encoded string which represents the salt used
by the server for this user.
- v: This is used to verify the server has access to the user's
authentication information. If the client supplies the "v"
attribute with an empty value in the message with a "p", that
indicates the client does not want the server to provide
verification. When the server includes this in the final message,
it is always the last attribute in the message. See the next
section for the computation.
5. Security Considerations
If the authentication exchange is performed without a strong
security layer, then a passive eavesdropper can gain sufficient
information to mount an offline dictionary or brute-force attack
which can be used to recover the user's password. The amount of
time necessary for this attack depends on the cryptographic hash
function selected, the strength of the password and the iteration
count supplied by the server. An external security layer with
strong encryption will prevent this attack.
If the external security layer used to protect the SCRAM exchange
uses an anonymous key exchange, then the SCRAM channel binding
mechanism can be used to detect a man-in-the-middle attack on the
security layer and cause the authentication to fail as a result.
However, the man-in-the-middle attacker will have gained sufficient
information to mount an offline dictionary or brute-force attack.
For this reason, SCRAM includes the ability to increase the
iteration count over time.
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If the authentication information is stolen from the authentication
database, then an offline dictionary or brute-force attack can be
used to recover the user's password. The use of salt mitigates this
attack somewhat by requiring a separate attack on each password.
Authentication mechanisms which protect against this attack are
available (e.g., the EKE class of mechanisms), but the patent
situation is presently unclear.
If an attacker obtains the authentication information from the
authentication repository and eavesdrops on one authentication
exchange, the attacker gains the ability to impersonate that user to
all servers providing SCRAM access using the same password and salt.
6. IANA considerations
(Hash function names registry, SASL mechanism registration.)
7. Acknowedgements
(Frank Ellermann already?)
8. Normative References
[RFC2045] Freed, Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies", RFC
2045, Innosoft, November 1996.
[RFC2104] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for
Message Authentication", IBM, February 1997.
[RFC2119] Bradner, "Key words for use in RFCs to Indicate
pRequirement Levels", RFC 2119, Harvard University, March
1997.
[RFC3174] Eastlake, Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC
3174, Motorola, September 2001
[RFC3986] Berners-Lee, Fielding, Masinter, "Uniform Resource
Identifier (URI): Generic Syntax", RFC 3986, W3C/MIT,
January 2005.
[RFC4234] Crocker, Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, Brandenburg
Internetworking, Demon Internet Ltd, October 2005.
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[RFC4346] Dierks, Rescorla, "The Transport Layer Security (TLS)
Protocol, Version 1.1", RFC 4346, Brandenburg
Internetworking, April 2006.
[RFC4422] Melnikov, Zeilenga, "Simple Authentication and Security
Layer (SASL)", RFC 4422, Isode Limited, June 2006.
9. Informative References
[RFC1939] Myers, Rose, "Post Office Protocol - Version 3", RFC
1939, Carnegie Mellon, May 1996.
[RFC2195] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension
for Simple Challenge/Response", RFC 2195, MCI, September
1997.
[RFC2202] Cheng, Glenn, "Test Cases for HMAC-MD5 and HMAC-SHA-1",
RFC 2202, IBM, September 1997
[RFC2289] Haller, Metz, Nesser, Straw, "A One-Time Password
System", RFC 2289, STD0061, February 1998.
[RFC2828] Shirey, "Internet Security Glossary", RFC 2828, FYI 0036,
May 2000.
[RFC4086] Eastlake, Schiller, Crocker, "Randomness Requirements for
Security", RFC 4086, BCP 0106, Motorola Laboratories,
June 2005.
[RFC4120] Neuman, Yo, Hartman, Raebun, "The Kerberos Network
Authentication Service (V5)", RFC 4120, USC-ISI, July
2005.
[RFC4510] Zeilenga, "Lightweight Directory Access Protocol (LDAP):
Technical Specification Road Map", RFC 4510, June 2006.
[DIGEST-MD5] Melnikov, "Using Digest Authentication as a SASL
Mechanism", draft-ietf-sasl-rfc2831bis-11.txt, Isode
Ltd., November 2006
10. Authors' Addresses
Abhijit Menon-Sen
Oryx Mail Systems GmbH
Email: ams@oryx.com
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Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
USA
Email: chris.newman@sun.com
Appendix A: Other Authentication Mechanisms
(To be written.)
Appendix B: Design Motivations
(To be written.)
Appendix C: SCRAM Examples
(To be written.)
Appendix D: SCRAM Interoperability Testing
(To be written.)
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(RFC Editor: Please delete everything after this point)
Open Issues
Appendixes A, B and C need to be written.
D too, and the framework implemented.
Should the title include the acronym SASL to help the greppers?
Changes since -03
- Seven years have passed, in which it became clear that DIGEST-MD5
suffered from unacceptably bad interoperability, so SCRAM-MD5 is
now bad from the dead.
- Be hash agnostic, so MD5 can be replaced more easily
- General simplification.
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