Network Working Group Abhijit Menon-Sen
Internet-Draft Oryx Mail Systems GmbH
Intended Status: Proposed Standard Chris Newman
Expires: May 1, 2008 Sun Microsystems
December 2007
Salted Challenge Response Authentication Mechanism (SCRAM)
draft-newman-auth-scram-05.txt
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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 security
concerns and meets the deployability requirements. When used in
combination with TLS 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 [RFC4949] ("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 the "StoredKey" and
"ServerKey" (as defined in the algorithm overview) for each
supported cryptographic hash function.
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- 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 [RFC4648] which converts
an octet string input to a textual output string which can be
easily displayed to a human. The 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.
- Hi(str): Apply the cryptographic hash function to the octet string
"str", then repeat the application on the output string for a
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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
upgraded gracefully without breaking backwards compatibility or
risking downgrade attacks.
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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
To begin with, the client is in possession of a username and
password. It sends the username to the server, which retrieves the
corresponding authentication information, i.e. a salt, StoredKey,
and ServerKey. The server sends the salt and an iteration count to
the client, which then computes the following values and sends a
ClientProof to the server:
SaltedPassword := Hi(HMAC(password, salt))
ClientKey := H(SaltedPassword)
StoredKey := H(ClientKey)
AuthMessage := client-first-message + "," +
server-first-message + "," +
final-client-message-without-proof
ClientSignature := HMAC(StoredKey, AuthMessage)
ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, salt)
ServerSignature := HMAC(ServerKey, AuthMessage)
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. If the ClientKey is correct, this proves that the client
has access to the user's password.
Similarly, the client authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server.
If the two are equal, it proves that the server had access to the
user's SaltedPassword.
The AuthMessage is computed by concatenating messages from the
authentication exchange. The format of these messages is defined in
the Formal Syntax section.
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4. SCRAM Authentication Exchange
SCRAM is a text protocol where the client and server exchange
messages containing one or more attribute-value pairs separated by
commas. Each attribute has a one-letter name. The messages and their
attributes are described in section 4.1, and defined in the Formal
Syntax section.
This is a simple example of a SCRAM authentication exchange:
C: n=Chris Newman,h=md5,r=ClientNonce
S: r=ClientNonceServerNonce,h=md5,s=PxR/wv+epq,i=128
C: r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
S: v=WxPv/siO5l+qxN4
First, the client sends a message containing the username, a list of
the hash functions it supports, and a random, unique nonce. In
response, the server sends its list of supported hash functions, an
iteration count i, the user's salt, and appends its own nonce to the
client-specified one. The client then responds with the same nonce
and a ClientProof computed as explained earlier. The server verifies
the proof and responds with a ServerSignature, concluding the
authentication exchange.
4.1 SCRAM attributes
This section describes the permissible attributes, their use, and
the format of their values.
- a: This optional attribute specifies an authorization identity. A
client may include it in its first message to the server if it
wants to authenticate as one user, but subsequently act as a
different user. This is typically used by an administrator to
perform some management task on behalf of another user, or by a
proxy in some situations (see appendix A for more details).
If this attribute is omitted (as it normally would be), or
specified with an empty value, the authorization identity is
assumed to be the same as the username specified with the
(required) "n" attribute.
The server always authenticates the user specified by the "n"
attribute. If the "a" attribute specifies a different user, the
server associates that identity with the connection after
successful authentication and authorization checks.
The syntax of this field is the same as that of the "n" field with
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respect to quoting of '=' and ','.
- n: This attribute specifies the name of the user whose password is
used for authentication. A client must include it in its first
message to the server. If the "a" attribute is not specified
(which would normally be the case), this username is also the
identity which will be associated with the connection subsequent
to authentication and authorization.
The characters ',' or '=' in usernames are sent as '=2C' and '=3D'
respectively. If the server receives a username which contains '='
not followed by either '2C' or '3D', then the server MUST fail the
authentication.
- h: This attribute is a colon-separated list of supported hash
function names, as defined in the IANA "Hash Function Textual
Names" registry.
- r: This attribute specifies 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).
As described earlier, the client supplies an initial value in its
first message, and the server augments that value with its own
nonce in its first response. It is important that this be value
different for each authentication. The client must verify that the
initial part of the nonce used in subsequent messages is the same
as the nonce it initially specified.
- c: This optional attribute specifies base64-encoded channel-
binding data. It may be sent by either the client or the server.
If specified, the authentication MUST fail unless the value is
successfully verified. Whether this attribute is included, and
the meaning and contents of the channel-binding data depends on
the external security layer in use. This is necessary to detect a
man-in-the-middle attack on the security layer.
- s: This attribute specifies the base64-encoded salt used by the
server for this user. It is sent by the server in its first
message to the client.
- i: This attribute specifies an iteration count for the selected
hash function, and must be sent by the server along with the
user's salt.
- p: This attribute specifies a base64-encoded ClientProof. The
client computes this value as described in the overview and sends
it to the server.
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- v: This attribute specifies a base64-encoded ServerSignature. It
is sent by the server in its final message, and may be used by the
client to verify that the server has access to the user's
authentication information. This value is computed as explained in
the overview.
5. Hash functions
SCRAM can use hash functions defined by the IANA "Hash Function
Textual Names" registry.
For interoperability, all SCRAM clients and servers MUST implement
the MD5 hash function as defined in [RFC1321].
Servers SHOULD announce a hash iteration-count of at least 128.
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC4234].
generic-message = attr-val *("," attr-val)
attr-val = ALPHA "=" value
value = *(value-char)
value-safe-char = %20-2B / %2D-3C / %3E-7E /
UTF8-2 / UTF8-2 / UTF-3 / UTF8-4
;; UTF8-char except CTL, "=", and ",".
value-char = value-safe-char / "="
base64-char = ALPHA / DIGIT / "/" / "+"
base64-4 = 4*4(base64-char)
base64-3 = 3*3(base64-char) "="
base64-2 = 2*2(base64-char) "=="
base64 = *(base64-4) [base64-3 / base64-2]
saslname = 1*(value-safe-char / "=2C" / "=3D")
;; Conforms to <value>
;; Usernames are prepared using SASLPrep.
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authzid = "a=" saslname
username = "n=" saslname
channel-binding = "c=" base64
proof = "p=" base64
nonce = "r=" value [value]
;; Second part provided by server.
salt = "s=" base64
verifier = "v=" base64
hash-list = "h=" hash-name *(":" hash-name)
hash-name = value
;; Hash Function Textual Name, from
;; http://www.iana.org/assignments/hash-
function-text-names
iteration-count = "i=" 1*DIGIT
client-first-message =
[authzid ","] username "," hash-list "," nonce
server-first-message =
nonce "," hash-list "," salt "," iteration-count
client-final-message-without-proof =
nonce "," channel-binding
client-final-message =
client-final-message-without-proof "," proof
server-final-message =
verifier
7. 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
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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.
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 either eavesdrops on one
authentication exchange or impersonates a server, the attacker gains
the ability to impersonate that user to all servers providing SCRAM
access using the same password and salt. For this reason, it is
important to use randomly-generated salt values.
If the server detects (from the value of the client-specified "h"
attribute) that both endpoints support a stronger hash function that
the one the client actually chooses to use, then it SHOULD treat
this as a downgrade attack and reject the authentication attempt.
8. IANA considerations
(Hash function names registry, SASL mechanism registration.)
9. Acknowedgements
The authors would like to thank Alexey Melnikov and Dave Cridland
for their contributions to this document.
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10. Normative References
[RFC4648] Josefsson, "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, SJD, October 2006.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2104] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for
Message Authentication", IBM, February 1997.
[RFC2119] Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March
1997.
[RFC3174] Eastlake, Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC
3174, Motorola, September 2001
[RFC4234] Crocker, Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, Brandenburg
Internetworking, Demon Internet Ltd, October 2005.
[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.
11. 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.
[RFC4949] Shirey, "Internet Security Glossary, Version 2", RFC
4949, FYI 0036, August 2007.
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[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-12.txt, Isode
Ltd., March 2007
12. Authors' Addresses
Abhijit Menon-Sen
Oryx Mail Systems GmbH
Email: ams@oryx.com
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
USA
Email: chris.newman@sun.com
Appendix A: Other Authentication Mechanisms
The DIGEST-MD5 mechanism has proved to be too complex to implement
and test, and thus has poor interoperability. The security layer is
often not implemented, and almost never used; everyone uses TLS
instead.
The PLAIN SASL mechanism allows a malicious server or eavesdropper
to impersonate the authenticating user to any other server for which
the user has the same password. It also sends the password in the
clear over the network, unless TLS is used. Server authentication is
not supported.
(To be completed.)
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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
- The appendices need to be written.
- Should the server send a base64-encoded ServerSignature for the
value of the "v" attribute, or should it compute a ServerProof the
way the client computes a ClientProof?
- What about this LDAP attribute to store authentication
information?
- It is entirely unclear to me how best to handle channel bindings.
Should the channel binding data be sent directly at all?
- It's not clear from the document that the hash function to use for
a particular authentication exchange is selected by the client
before the exchange begins.
- Should the title include the acronym SASL to help the greppers?
Changes since -04
- Update Base64 and Security Glossary references.
- Add Formal Syntax section.
- Don't bother with "v=".
- Make MD5 mandatory to implement. Suggest i=128.
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 back from the dead.
- Be hash agnostic, so MD5 can be replaced more easily.
- General simplification.
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