Network Working Group Abhijit Menon-Sen
Internet-Draft Oryx Mail Systems GmbH
Intended Status: Proposed Standard Chris Newman
Expires: August 2009 Sun Microsystems
Alexey Melnikov
Isode Ltd
February 21, 2009
Salted Challenge Response (SCRAM) SASL Mechanism
draft-newman-auth-scram-10.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
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 a family of authentication mechanisms
called 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, a mechanism from this family 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 [RFC5234] including the core rules
defined in Appendix B of [RFC5234].
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.
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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, iteration count, the "StoredKey"
and "ServerKey" (as defined in the algorithm overview) for each
supported cryptographic hash function.
- 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]).
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- 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.
- 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.
- Hi(str, salt):
U0 := HMAC(str, salt + INT(1))
U1 := HMAC(str, U0)
U2 := HMAC(str, U1)
...
Ui-1 := HMAC(str, Ui-2)
Ui := HMAC(str, Ui-1)
Hi := U0 XOR U1 XOR U2 XOR ... XOR Ui
where "i" is the iteration count, "+" is the string concatenation
operator and INT(g) is a four-octet encoding of the integer g,
most significant octet first.
This is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and
with dkLen == output length of HMAC() == output length of H().
2. Introduction
This specification describes a family of authentication mechanisms
called 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 [TLS])
or an equivalent security layer, a mechanism from this family 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 family of 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
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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]).
- Both the client and server can be authenticated by the protocol.
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
Note that this section omits some details, such as client and server
nonces. See Section 5 for more details.
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,
ServerKey and the iteration count i. (Note that a server
implementation may chose to use the same iteration count for all
account.) The server sends the salt and the iteration count to the
client, which then computes the following values and sends a
ClientProof to the server:
SaltedPassword := Hi(password, salt)
ClientKey := H(SaltedPassword)
StoredKey := H(ClientKey)
AuthMessage := client-first-message + "," +
server-first-message + "," +
client-final-message-without-proof
ClientSignature := HMAC(StoredKey, AuthMessage)
ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, salt)
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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 ServerKey.
The AuthMessage is computed by concatenating messages from the
authentication exchange. The format of these messages is defined in
the Formal Syntax section.
4. SCRAM mechanism names
A SCRAM mechanism name is a string "SCRAM-HMAC-" followed by the
uppercased name of the underlying hashed function taken from the
IANA "Hash Function Textual Names" registry (see
http://www.iana.org).
For interoperability, all SCRAM clients and servers MUST implement
the SCRAM-HMAC-SHA-1 authentication mechanism, i.e. an
authentication mechanism from the SCRAM family that uses the SHA-1
hash function as defined in [RFC3174].
5. 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 5.1, and defined in the Formal
Syntax section.
This is a simple example of a SCRAM-HMAC-SHA-1 authentication
exchange:
C: n=Chris Newman,r=ClientNonce
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
C: r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
S: v=WxPv/siO5l+qxN4
With channel-binding data sent by the client this might look like this:
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C: n=Chris Newman,r=ClientNonce
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
C: c=0123456789ABCDEF,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
S: v=WxPv/siO5l+qxN4
<<Note that the channel-bind data above, as well as all hashes are fake>>
First, the client sends a message containing the username, and a
random, unique nonce. In response, the server sends the user's
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 using the selected hash function as
explained earlier. In this step the client can also include an
optional authorization identity. The server verifies the nonce and
the proof, verifies that the authorization identity (if supplied by
the client in the second message) is authorized to act as the
authentication identity, and, finally, it responds with a
ServerSignature, concluding the authentication exchange. The client
then authenticates the server by computing the ServerSignature and
comparing it to the value sent by the server. If the two are
different, the client MUST consider the authentication exchange to
be unsuccessful and it might have to drop the connection.
5.1 SCRAM attributes
This section describes the permissible attributes, their use, and
the format of their values. All attribute names are single US-ASCII
letters and are case-sensitive.
- a: This optional attribute specifies an authorization identity. A
client may include it in its second 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.
Upon the receipt of this value the server verifies its correctness
according to the used SASL protocol profile. Failed verification
results in failed authentication exchange.
If this attribute is omitted (as it normally would be), or
specified with an empty value, the authorization identity is
assumed to be derived from 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
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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
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.
Before sending the username to the server, the client MUST prepare
the username using the "SASLPrep" profile [SASLPrep] of the
"stringprep" algorithm [RFC3454]. If the preparation of the
username fails or results in an empty string, the client SHOULD
abort the authentication exchange (*).
(*) An interactive client can request a repeated entry of the
username value.
Upon receipt of the username by the server, the server SHOULD
prepare it using the "SASLPrep" profile [SASLPrep] of the
"stringprep" algorithm [RFC3454]. If the preparation of the
username fails or results in an empty string, the server SHOULD
abort the authentication exchange.
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.
- m: This attribute is reserved for future extensibility. In this
version of SCRAM, its presence in a client or a server message
MUST cause authentication failure when the attribute is parsed by
the other end.
- 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. The server MUST verify that
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the nonce sent by the client in the second message is the same as
the one sent by the server in its first message.
- c: This optional attribute specifies base64-encoded channel-
binding data. It is sent by the client in the second step. If
specified by the client, if the server supports the specified
channel binding type and if the server can't verify it, then the
server MUST fail the authentication exchange. 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 user, and must be sent by the server along with
the user's salt.
For SCRAM-HMAC-SHA-1 SASL mechanism servers SHOULD announce a hash
iteration-count of at least 128.
- p: This attribute specifies a base64-encoded ClientProof. The
client computes this value as described in the overview and sends
it to the server.
- 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.
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3"
and "UTF8-4" non-terminal are defined in [UTF-8].
generic-message = attr-val *("," attr-val)
;; Generic syntax of any server challenge
;; or client response
attr-val = ALPHA "=" value
value = 1*(value-char)
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value-safe-char = %01-2B / %2D-3C / %3E-7F /
UTF8-2 / UTF-3 / UTF8-4
;; UTF8-char except NUL, "=", 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]
posit-number = (%x31-39) *DIGIT
;; A positive number
saslname = 1*(value-safe-char / "=2C" / "=3D")
;; Conforms to <value>
authzid = "a=" saslname
;; Protocol specific.
username = "n=" saslname
;; Usernames are prepared using SASLPrep.
reserved-mext = "m=" 1*(value-char)
;; Reserved for signalling mandatory extensions.
;; The exact syntax will be defined in
;; the future.
channel-binding = "c=" base64
proof = "p=" base64
nonce = "r=" c-nonce [s-nonce]
;; Second part provided by server.
c-nonce = value
s-nonce = value
salt = "s=" base64
verifier = "v=" base64
;; base-64 encoded ServerSignature.
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iteration-count = "i=" posit-number
;; A positive number
client-first-message =
[reserved-mext ","] username "," nonce [","
extensions]
server-first-message =
[reserved-mext ","] nonce "," salt ","
iteration-count ["," extensions]
client-final-message-without-proof =
[authzid ","] [channel-binding ","] nonce [","
extensions]
client-final-message =
client-final-message-without-proof "," proof
server-final-message =
verifier ["," extensions]
extensions = attr-val *("," attr-val)
;; All extensions are optional,
;; i.e. unrecognized attributes
;; not defined in this document
;; MUST be ignored.
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
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 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 hash function, password, iteration count 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.
A hostile server can perform a computational denial-of-service
attack on clients by sending a big iteration count value.
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8. IANA considerations
IANA is requested to add the following entry to the SASL Mechanism
registry established by [RFC4422]:
To: iana@iana.org
Subject: Registration of a new SASL mechanism SCRAM-HMAC-SHA-1
SASL mechanism name (or prefix for the family): SCRAM-HMAC-SHA-1
Security considerations: Section 7 of [RFCXXXX]
Published specification (optional, recommended): [RFCXXXX]
Person & email address to contact for further information:
IETF SASL WG <ietf-sasl@imc.org>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note:
Note that even though this document defines a family of SCRAM-HMAC
mechanisms, it doesn't register a family of SCRAM-HMAC mechanisms in
the SASL Mechanisms registry. IANA is requested to prevent future
registrations of SASL mechanisms starting with SCRAM-HMAC- without
consulting the SASL mailing list <ietf-sasl@imc.org> first.
Note to future SCRAM-HMAC mechanism designers: each new SCRAM-HMAC
SASL mechanism MUST be explicitly registered with IANA and MUST
comply with SCRAM-HMAC mechanism naming convention defined in
Section 4 of this document.
9. Acknowedgements
The authors would like to thank Dave Cridland for his contributions
to this document.
10. Normative References
[RFC4648] Josefsson, "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, SJD, October 2006.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC2104] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for
Message Authentication", IBM, February 1997.
[RFC2119] Bradner, "Key words for use in RFCs to Indicate
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Requirement Levels", RFC 2119, Harvard University, March
1997.
[RFC3174] Eastlake, Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC
3174, Motorola, September 2001
[RFC5234] Crocker, Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008.
[RFC4422] Melnikov, Zeilenga, "Simple Authentication and Security
Layer (SASL)", RFC 4422, Isode Limited, June 2006.
[SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user
names and passwords", RFC 4013, February 2005.
[RFC3454] Hoffman, P., Blanchet, M., "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
11. Informative References
[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
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[TLS] Dierks, Rescorla, "The Transport Layer Security (TLS)
Protocol, Version 1.2", RFC 5246, August 2008.
[RFC4949] Shirey, "Internet Security Glossary, Version 2", RFC
4949, FYI 0036, August 2007.
[RFC4086] Eastlake, Schiller, Crocker, "Randomness Requirements for
Security", RFC 4086, BCP 0106, Motorola Laboratories,
June 2005.
[RFC4510] Zeilenga, "Lightweight Directory Access Protocol (LDAP):
Technical Specification Road Map", RFC 4510, June 2006.
[DIGEST-MD5] Leach, P. and C. Newman , "Using Digest Authentication
as a SASL Mechanism", RFC 2831, May 2000. <<Also draft-
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ietf-sasl-rfc2831bis-12.txt>>
[DIGEST-HISTORIC] Melnikov, "Moving DIGEST-MD5 to Historic", work in
progress, draft-ietf-sasl-digest-to-historic-00.txt, July
2008
[CRAM-HISTORIC] Zeilenga, "CRAM-MD5 to Historic", work in progress,
draft-ietf-sasl-crammd5-to-historic-00.txt, November
2008.
[PLAIN] Zeilenga, "The PLAIN Simple Authentication and Security
Layer (SASL) Mechanism" RFC 4616, August 2006.
12. Authors' Addresses
Abhijit Menon-Sen
Oryx Mail Systems GmbH
Email: ams@oryx.com
Alexey Melnikov
Isode Ltd
EMail: Alexey.Melnikov@isode.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 [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. For a more complete list of problems
with DIGEST-MD5 which lead to the creation of SCRAM see [DIGEST-
HISTORIC].
The CRAM-MD5 SASL mechanism, while widely deployed has also some
problems, in particular it is missing some modern SASL features such
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as support for internationalized usernames and passwords, support
for passing of authorization identity, support for channel bindings.
It also doesn't support server authentication. For a more complete
list of problems with CRAM-MD5 see [CRAM-HISTORIC].
The PLAIN [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.
Appendix B: Design Motivations
The following design goals shaped this document. Note that some of
the goals have changed since the initial version of the document.
The SASL mechanism has all modern SASL features: support for
internationalized usernames and passwords, support for passing of
authorization identity, support for channel bindings.
Both the client and server can be authenticated by the protocol.
The authentication information stored in the authentication
database is not sufficient by itself to impersonate the client.
<<The server does not gain the ability to impersonate the client
to other servers (with an exception for server-authorized
proxies).>>
The mechanism is extensible, but [hopefully] not overengineered in
this respect.
Easier to implement than DIGEST-MD5 in both clients and servers.
Appendix C: SCRAM Examples
<<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?
Changes since -07
Updated References.
Clarified purpose of the m= attribute.
Fixed a problem with authentication/authorization identity's ABNF
not allowing for some characters.
Updated ABNF for nonce to show client-generated and server-generated
parts.
Only register SCRAM-HMAC-SHA-1 with IANA and require explicit
registrations of all other SCRAM-HMAC- mechanisms.
Changes since -06
Removed hash negotiation from SCRAM and turned it into a family of
SASL mechanisms.
Start using "Hash Function Textual Names" IANA registry for SCRAM
mechanism naming.
Fixed definition of Hi(str, salt) to be consistent with [RFC2898].
Clarified extensibility of SCRAM: added m= attribute (for future
mandatory extensions) and specified that all unrecognized
attributes must be ignored.
Changes since -05
Changed the mandatory to implement hash algorithm to SHA-1 (as per
Menon-Sen & Co Expires August 2009 FF[Page 17]
Internet-draft February 2009
WG consensus).
Added text about use of SASLPrep for username
canonicalization/validation.
Clarified that authorization identity is canonicalized/verified
according to SASL protocol profile.
Clarified that iteration count is per-user.
Clarified how clients select the authentication function.
Added IANA registration for the new mechanism.
Added missing normative references (UTF-8, SASLPrep).
Various editorial changes based on comments from Hallvard B
Furuseth, Nico William and Simon Josefsson.
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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