Network Working Group Abhijit Menon-Sen
Internet-Draft Oryx Mail Systems GmbH
Intended Status: Proposed Standard Chris Newman
Expires: September 2009 Sun Microsystems
Alexey Melnikov
Isode Ltd
Simon Josefsson
SJD AB
March 9, 2009
Salted Challenge Response Authentication Mechanism (SCRAM)
as a GSS-API Mechanism
draft-newman-auth-scram-gs2-01.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 an authentication mechanism 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, SCRAM 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.
The purpose of this document is to describe the general SCRAM
protocol, and how it is used in the GSS-API environment. Through
GS2, this makes the protocol available in the SASL environment as
well.
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1.0. 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.
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.
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- 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.
- 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
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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 an authentication mechanism 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 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]).
- 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.
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Comments regarding this draft may be sent either to the ietf-
sasl@imc.org mailing list or to the authors.
3. SCRAM Algorithm and Protocol 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)
ServerSignature := HMAC(ServerKey, AuthMessage)
ScramKey := HMAC(ClientKey, AuthMessage)
MicKey := HMAC(ScramKey, "SCRAM MIC constant")
ClientMic := HMAC(MicKey, client-gs2-to-be-protected)
ServerMic := HMAC(MicKey, server-gs2-to-be-protected)
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.
Once authentication is successful both the client and the server are
in possesion of the ClientKey. The ClientKey is used to construct
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the shared SCRAM key (ScramKey), which is then used to produce the
MicKey. The MicKey is used to verify channel binding and
authorization identity by the server, and to confirm that the
channel binding information was verified by the client.
The AuthMessage is computed by concatenating messages from the
authentication exchange. client-gs2-to-be-protected and server-
gs2-to-be-protected are also parts of the authentication exchange.
The format of these messages is defined in the Formal Syntax
section.
4. Use of SCRAM in GSS-API and SASL
The SCRAM protocol defined in this document is not specific to a
particular authentication framework, such as GSS-API, SASL or EAP.
The purpose of this section is to describe how the SCRAM protocol is
implemented within a particular framework. The focus here is on
GSS-API and SASL. If desirable, it may be possible to write similar
mappings for other authentication frameworks in the future (e.g.,
EAP).
4.1 Use of SCRAM in GSS-API
Context establishment consists of sending and receiving the SCRAM
Authentication Exchange protocol. The GSS-API OID allocated for
SCRAM is 1.3.6.1.4.1.11591.4.2. The PROT_READY should be set after
the authentication exchange completed. When the context has been
established, message integrity services through GSS_Wrap/GSS_Unwrap
are implemented by using the ClientMic and ServerMic keys derived
from the authentication protocol.
<<describe syntax of gss_wrap/gss_unwrap output better>>
4.2 Use of SCRAM in SASL via GS2.
Through GS2, each GSS-API mechanism is supported in SASL. To use
SCRAM in SASL, we must derive the SASL mechanism name using the
algorithm described in GS2. The DER encoding of the OID is (in hex)
06 09 2B 06 01 04 01 DA 47 04 02. The SHA-1 hash is 29 06 29 12 AB
25 83 CD 02 92 1B 4E 2D D8 6A 40 CD D0 5D C2. Convert the first ten
octets to binary, and re-group them in groups of 5, and convert them
back to decimal, which results in these computations:
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hex:
29 06 29 12 AB 25 83 CD 02 92
binary:
00101001 00000110 00101001 00010010 10101011
00100101 10000011 11001101 00000010 10010010
binary in groups of 5:
00101 00100 00011 00010 10010 00100 10101 01011
00100 10110 00001 11100 11010 00000 10100 10010
decimal of each group:
5 4 3 2 18 4 21 11 4 22 1 28 26 0 20 18
base32 encoding:
F E D C S E V L E W B 4 2 A U S
The last step translate each decimal value using table 3 in Base32
[RFC4648]. Thus the SASL mechanism name for SCRAM is
"GS2-FEDCSEVLEWB42AUS".
The wire syntax of SCRAM in SASL is described normatively in [GS2],
based on the wire format describe above for GSS-API.
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 authentication exchange:
C: n=Chris Newman,r=ClientNonce[^A]
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128[^A]
C: r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4[^A]mic=<<base64>>,
d=qop=none
S: v=WxPv/siO5l+qxN4[^A]mic=<<base64>>,d=qop=none
<< oidgunk required at the beginning of the first client message?
However we can assume GS2 compression as discuss on the mailing list
>>
<<+cbgood in the last server step implies that the channel binding
was verified. But is it optional?>>
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With channel bindings this might look like:
C: n=Chris Newman,r=ClientNonce[^A]
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128[^A]
C: r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4[^A]mic=<<base64>>,
d=qop=none,cbqop=none,c=<<base64>>
S: v=WxPv/siO5l+qxN4[^A]mic=<<base64>>,d=qop=none+cbgood
Note that [^A] here represents 1 octet with value %x01.
<<This text needs to be updated to match ABNF:>>
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 (<<see appendix A for more details>>).
Upon the receipt of this value the server verifies its correctness
and makes the authorization decision. Failed verification results
in failed authentication exchange.
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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
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 %x01, '=' 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 %x01, ',' or '=' in usernames are sent as '=01',
'=2C' and '=3D' respectively. If the server receives a username
which contains '=' not followed by either '01', '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
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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
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.
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].
attr-val = ALPHA "=" value
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value = *(value-char)
value-safe-char = %02-2B / %2D-3C / %3E-7F /
UTF8-2 / UTF-3 / UTF8-4
;; UTF8-char except NUL, %x01 (CTRL+A), "=",
;; 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 / "=01" / "=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
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verifier = "v=" base64
;; base-64 encoded ServerSignature.
iteration-count = "i=" posit-number
delim = %x01
client-first-message =
scram-client-first-message delim
server-first-message =
scram-server-first-message delim
client-final-message =
scram-client-final-message-without-proof ","
proof delim
gss-mic client-gss-wrap
;; <<GS2 extensions omitted after "gss-mic">>
server-final-message =
scram-server-final-message delim
gss-mic server-gss-wrap
;; <<GS2 extensions omitted after "gss-mic">>
gss-mic = "mic=" base64 ","
;; base-64 encoding of ClientMic
;; for the client and ServerMic
;; for the server
client-gss-wrap = "d=" client-gs2-to-be-protected
;; A particular case of <gss-wrap>
client-gs2-to-be-protected = "qop=none" [",cbqop=none," channel-
binding]
["," authzid]
;; A particular case of <gs2-to-be-protected>
server-gss-wrap = "d=" server-gs2-to-be-protected
;; A particular case of <gss-wrap>
server-gs2-to-be-protected = "qop=none" [ "+cbgood" ]
;; A particular case of <gs2-to-be-protected>
;; Note that "+cbgood" is included if
;; channel binding verification succeeded.
gss-wrap = "d=" gs2-to-be-protected
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gs2-to-be-protected = qop ["," maxbuf]
["," cbqop "," channel-binding] ["," authzid]
;; <<GS2- specific extensions -
;; "["," extensions]"
;; omitted at the end>>
qop = "qop=" qopvalue *( "+" qopvalue)
qopvalue = "none" ; no security layer
/ "integ" ; integrity protection
/ "conf" ; confidentiality protection
/ "cbgood" ; channel binding validated
; (server to client)
maxbuf = "maxbuf=" posit-number
cbqop = "cbqop=" qopvalue *( "+" qopvalue)
;; QOPs that can be used if channel binding
;; succeeds
scram-client-first-message =
[reserved-mext ","] username "," nonce
["," extensions]
scram-server-first-message =
[reserved-mext ","] nonce "," salt ","
iteration-count ["," extensions]
scram-client-final-message-without-proof =
nonce ["," extensions]
;; <<Note, we used to have GSS-API
;; channel-binding here, but the GS2
;; spec says it MUST be NULL>>
scram-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.
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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.
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
None.
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
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.
[SASL-GS2] Josefsson, S., "Using GSS-API Mechanisms in SASL: The GS2
Mechanism Family", work in progress, draft-ietf-sasl-
gs2-10.txt, July 2008. <<Can we avoid making this a
normative reference?>>
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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-
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
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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
Simon Josefsson
Email: simon@josefsson.org
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
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
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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.
On the wire compatibility with GS2 [SASL-GS2].
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?
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