draft-ietf-sasl-scram-11.txt   rfc5802.txt 
NETWORK WORKING GROUP C. Newman Internet Engineering Task Force (IETF) C. Newman
Internet-Draft Sun Microsystems Request for Comments: 5802 Oracle
Intended status: Standards Track A. Menon-Sen Category: Standards Track A. Menon-Sen
Expires: August 12, 2010 Oryx Mail Systems GmbH ISSN: 2070-1721 Oryx Mail Systems GmbH
A. Melnikov A. Melnikov
Isode Ltd Isode, Ltd.
N. Williams N. Williams
Sun Microsystems Oracle
February 8, 2010 July 2010
Salted Challenge Response (SCRAM) SASL and GSS-API Mechanism Salted Challenge Response Authentication Mechanism (SCRAM)
draft-ietf-sasl-scram-11.txt SASL and GSS-API Mechanisms
Abstract Abstract
The secure authentication mechanism most widely deployed and used by The secure authentication mechanism most widely deployed and used by
Internet application protocols is the transmission of clear-text Internet application protocols is the transmission of clear-text
passwords over a channel protected by Transport Layer Security (TLS). passwords over a channel protected by Transport Layer Security (TLS).
There are some significant security concerns with that mechanism, There are some significant security concerns with that mechanism,
which could be addressed by the use of a challenge response which could be addressed by the use of a challenge response
authentication mechanism protected by TLS. Unfortunately, the authentication mechanism protected by TLS. Unfortunately, the
challenge response mechanisms presently on the standards track all challenge response mechanisms presently on the standards track all
fail to meet requirements necessary for widespread deployment, and fail to meet requirements necessary for widespread deployment, and
have had success only in limited use. have had success only in limited use.
This specification describes a family of Simple Authentication and This specification describes a family of Simple Authentication and
Security Layer (SASL, RFC 4422) authentication mechanisms called the Security Layer (SASL; RFC 4422) authentication mechanisms called the
Salted Challenge Response Authentication Mechanism (SCRAM), which Salted Challenge Response Authentication Mechanism (SCRAM), which
addresses the security concerns and meets the deployability addresses the security concerns and meets the deployability
requirements. When used in combination with TLS or an equivalent requirements. When used in combination with TLS or an equivalent
security layer, a mechanism from this family could improve the security layer, a mechanism from this family could improve the status
status-quo for application protocol authentication and provide a quo for application protocol authentication and provide a suitable
suitable choice for a mandatory-to-implement mechanism for future choice for a mandatory-to-implement mechanism for future application
application protocol standards. protocol standards.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Status of This Memo
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at This is an Internet Standards Track document.
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at This document is a product of the Internet Engineering Task Force
http://www.ietf.org/shadow.html. (IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on August 12, 2010. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5802.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
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described in the BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Conventions Used in This Document . . . . . . . . . . 4 1. Introduction ....................................................4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions Used in This Document ...............................5
1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Terminology ................................................5
2. Introduction . . . . . . . . . . . . . . . . . . . . . 7 2.2. Notation ...................................................6
3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . 9 3. SCRAM Algorithm Overview ........................................7
4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . 11 4. SCRAM Mechanism Names ...........................................8
5. SCRAM Authentication Exchange . . . . . . . . . . . . 12 5. SCRAM Authentication Exchange ...................................9
5.1. SCRAM Attributes . . . . . . . . . . . . . . . . . . . 13 5.1. SCRAM Attributes ..........................................10
5.2. Compliance with SASL mechanism requirements . . . . . 16 5.2. Compliance with SASL Mechanism Requirements ...............13
6. Channel Binding . . . . . . . . . . . . . . . . . . . 17 6. Channel Binding ................................................14
6.1. Default Channel Binding . . . . . . . . . . . . . . . 18 6.1. Default Channel Binding ...................................15
7. Formal Syntax . . . . . . . . . . . . . . . . . . . . 19 7. Formal Syntax ..................................................15
8. SCRAM as a GSS-API Mechanism . . . . . . . . . . . . . 23 8. SCRAM as a GSS-API Mechanism ...................................19
8.1. GSS-API Principal Name Types for SCRAM . . . . . . . . 23 8.1. GSS-API Principal Name Types for SCRAM ....................19
8.2. GSS-API Per-Message Tokens for SCRAM . . . . . . . . . 23 8.2. GSS-API Per-Message Tokens for SCRAM ......................20
8.3. GSS_Pseudo_random() for SCRAM . . . . . . . . . . . . 24 8.3. GSS_Pseudo_random() for SCRAM .............................20
9. Security Considerations . . . . . . . . . . . . . . . 25 9. Security Considerations ........................................20
10. IANA Considerations . . . . . . . . . . . . . . . . . 27 10. IANA Considerations ...........................................22
11. Acknowledgements . . . . . . . . . . . . . . . . . . . 29 11. Acknowledgements ..............................................23
Appendix A. Other Authentication Mechanisms . . . . . . . . . . . 30 12. References ....................................................24
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . . 31 12.1. Normative References .....................................24
Appendix C. Internet-Draft Change History . . . . . . . . . . . . 32 12.2. Normative References for GSS-API Implementors ............24
12. References . . . . . . . . . . . . . . . . . . . . . . 34 12.3. Informative References ...................................25
12.1. Normative References . . . . . . . . . . . . . . . . . 34 Appendix A. Other Authentication Mechanisms .......................27
12.2. Normative References for GSS-API implementors . . . . 34 Appendix B. Design Motivations ....................................27
12.3. Informative References . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . 37
1. Conventions Used in This Document 1. 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 (see Appendix A and
Appendix B). When used in combination with Transport Layer Security
(TLS; see [RFC5246]) 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 mechanisms does not presently include
negotiation of a security layer [RFC4422]. It is intended to be used
with an external security layer such as that provided by TLS or SSH,
with optional channel binding [RFC5056] to the external security
layer.
SCRAM is specified herein as a pure Simple Authentication and
Security Layer (SASL) [RFC4422] mechanism, but it conforms to the new
bridge between SASL and the Generic Security Service Application
Program Interface (GSS-API) called "GS2" [RFC5801]. This means that
this document defines both, a SASL mechanism and a GSS-API mechanism.
SCRAM provides the following protocol features:
o 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.
o The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies).
o The mechanism permits the use of a server-authorized proxy without
requiring that proxy to have super-user rights with the back-end
server.
o Mutual authentication is supported, but only the client is named
(i.e., the server has no name).
o When used as a SASL mechanism, SCRAM is capable of transporting
authorization identities (see [RFC4422], Section 2) from the
client to the server.
A separate document defines a standard LDAPv3 [RFC4510] attribute
that enables storage of the SCRAM authentication information in LDAP.
See [RFC5803].
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.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Formal syntax is defined by [RFC5234] including the core rules Formal syntax is defined by [RFC5234] including the core rules
defined in Appendix B of [RFC5234]. defined in Appendix B of [RFC5234].
Example lines prefaced by "C:" are sent by the client and ones 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 prefaced by "S:" by the server. If a single "C:" or "S:" label
applies to multiple lines, then the line breaks between those lines applies to multiple lines, then the line breaks between those lines
are for editorial clarity only, and are not part of the actual are for editorial clarity only, and are not part of the actual
protocol exchange. protocol exchange.
1.1. Terminology 2.1. Terminology
This document uses several terms defined in [RFC4949] ("Internet This document uses several terms defined in [RFC4949] ("Internet
Security Glossary") including the following: authentication, Security Glossary") including the following: authentication,
authentication exchange, authentication information, brute force, authentication exchange, authentication information, brute force,
challenge-response, cryptographic hash function, dictionary attack, challenge-response, cryptographic hash function, dictionary attack,
eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
one-way encryption function, password, replay attack and salt. one-way encryption function, password, replay attack, and salt.
Readers not familiar with these terms should use that glossary as a Readers not familiar with these terms should use that glossary as a
reference. reference.
Some clarifications and additional definitions follow: Some clarifications and additional definitions follow:
o Authentication information: Information used to verify an identity o Authentication information: Information used to verify an identity
claimed by a SCRAM client. The authentication information for a claimed by a SCRAM client. The authentication information for a
SCRAM identity consists of salt, iteration count, the "StoredKey" SCRAM identity consists of salt, iteration count, "StoredKey" and
and "ServerKey" (as defined in the algorithm overview) for each "ServerKey" (as defined in the algorithm overview) for each
supported cryptographic hash function. supported cryptographic hash function.
o Authentication database: The database used to look up the o Authentication database: The database used to look up the
authentication information associated with a particular identity. authentication information associated with a particular identity.
For application protocols, LDAPv3 (see [RFC4510]) is frequently For application protocols, LDAPv3 (see [RFC4510]) is frequently
used as the authentication database. For network-level protocols used as the authentication database. For network-level protocols
such as PPP or 802.11x, the use of RADIUS [RFC2865] is more such as PPP or 802.11x, the use of RADIUS [RFC2865] is more
common. common.
o Base64: An encoding mechanism defined in [RFC4648] which converts o Base64: An encoding mechanism defined in [RFC4648] that converts
an octet string input to a textual output string which can be an octet string input to a textual output string that can be
easily displayed to a human. The use of base64 in SCRAM is easily displayed to a human. The use of base64 in SCRAM is
restricted to the canonical form with no whitespace. restricted to the canonical form with no whitespace.
o Octet: An 8-bit byte. o Octet: An 8-bit byte.
o Octet string: A sequence of 8-bit bytes. o Octet string: A sequence of 8-bit bytes.
o Salt: A random octet string that is combined with a password o Salt: A random octet string that is combined with a password
before applying a one-way encryption function. This value is used before applying a one-way encryption function. This value is used
to protect passwords that are stored in an authentication to protect passwords that are stored in an authentication
database. database.
1.2. Notation 2.2. Notation
The pseudocode description of the algorithm uses the following The pseudocode description of the algorithm uses the following
notations: notations:
o ":=": The variable on the left hand side represents the octet o ":=": The variable on the left-hand side represents the octet
string resulting from the expression on the right hand side. string resulting from the expression on the right-hand side.
o "+": Octet string concatenation. o "+": Octet string concatenation.
o "[ ]": A portion of an expression enclosed in "[" and "]" may not o "[ ]": A portion of an expression enclosed in "[" and "]" may not
be included in the result under some circumstances. See the be included in the result under some circumstances. See the
associated text for a description of those circumstances. associated text for a description of those circumstances.
o Normalize(str): Apply the SASLPrep profile [RFC4013] of the o Normalize(str): Apply the SASLprep profile [RFC4013] of the
"stringprep" algorithm [RFC3454] as the normalization algorithm to "stringprep" algorithm [RFC3454] as the normalization algorithm to
a UTF-8 [RFC3629] encoded "str". The resulting string is also in a UTF-8 [RFC3629] encoded "str". The resulting string is also in
UTF-8. When applying SASLPrep, "str" is treated as a "stored UTF-8. When applying SASLprep, "str" is treated as a "stored
strings", which means that unassigned Unicode codepoints are strings", which means that unassigned Unicode codepoints are
prohibited (see Section 7 of [RFC3454]). Note that prohibited (see Section 7 of [RFC3454]). Note that
implementations MUST either implement SASLPrep, or disallow use of implementations MUST either implement SASLprep or disallow use of
non US-ASCII Unicode codepoints in "str". non US-ASCII Unicode codepoints in "str".
o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in
[RFC2104]) using the octet string represented by "key" as the key [RFC2104]) using the octet string represented by "key" as the key
and the octet string "str" as the input string. The size of the 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 result is the hash result size for the hash function in use. For
example, it is 20 octets for SHA-1 (see [RFC3174]). example, it is 20 octets for SHA-1 (see [RFC3174]).
o H(str): Apply the cryptographic hash function to the octet string o H(str): Apply the cryptographic hash function to the octet string
"str", producing an octet string as a result. The size of the "str", producing an octet string as a result. The size of the
result depends on the hash result size for the hash function in result depends on the hash result size for the hash function in
use. use.
o XOR: Apply the exclusive-or operation to combine the octet string o 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 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 this operator. The length of the output and each of the two
inputs will be the same for this use. inputs will be the same for this use.
o Hi(str, salt, i): o Hi(str, salt, i):
U1 := HMAC(str, salt + INT(1)) U1 := HMAC(str, salt + INT(1))
U2 := HMAC(str, U1) U2 := HMAC(str, U1)
... ...
Ui-1 := HMAC(str, Ui-2) Ui-1 := HMAC(str, Ui-2)
Ui := HMAC(str, Ui-1) Ui := HMAC(str, Ui-1)
Hi := U1 XOR U2 XOR ... XOR Ui Hi := U1 XOR U2 XOR ... XOR Ui
where "i" is the iteration count, "+" is the string concatenation where "i" is the iteration count, "+" is the string concatenation
operator and INT(g) is a four-octet encoding of the integer g, operator, and INT(g) is a 4-octet encoding of the integer g, most
most significant octet first. significant octet first.
Hi() 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 (see Appendix A and
Appendix B). When used in combination with Transport Layer Security
(TLS, see [RFC5246]) 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 mechanisms does not presently include
negotiation of a security layer [RFC4422]. It is intended to be used
with an external security layer such as that provided by TLS or SSH,
with optional channel binding [RFC5056] to the external security
layer.
SCRAM is specified herein as a pure Simple Authentication and
Security Layer (SASL) [RFC4422] mechanism, but it conforms to the new
bridge between SASL and the Generic Security Services Application
Programming Interface (GSS-API) called "GS2" [I-D.ietf-sasl-gs2].
This means that this document defines both, a SASL mechanism and a
GSS-API mechanism.
SCRAM provides the following protocol features:
o 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.
o The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies).
o The mechanism permits the use of a server-authorized proxy without
requiring that proxy to have super-user rights with the back-end
server.
o Mutual authentication is supported, but only the client is named
(i.e., the server has no name).
o When used as a SASL mechanism, SCRAM is capable of transporting
authorization identities (see [RFC4422], Section 2) from the
client to the server.
A separate document defines a standard LDAPv3 [RFC4510] attribute
that enables storage of the SCRAM authentication information in LDAP.
See [I-D.melnikov-sasl-scram-ldap].
For an in-depth discussion of why other challenge response mechanisms Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the
are not considered sufficient, see appendix A. For more information pseudorandom function (PRF) and with dkLen == output length of
about the motivations behind the design of this mechanism, see HMAC() == output length of H().
appendix B.
3. SCRAM Algorithm Overview 3. SCRAM Algorithm Overview
The following is a description of a full, uncompressed SASL SCRAM The following is a description of a full, uncompressed SASL SCRAM
authentication exchange. Nothing in SCRAM prevents either sending authentication exchange. Nothing in SCRAM prevents either sending
the client-first message with the SASL authentication request defined the client-first message with the SASL authentication request defined
by an application protocol ("initial client response"), nor sending by an application protocol ("initial client response"), or sending
the server-final message as additional data of the SASL outcome of the server-final message as additional data of the SASL outcome of
authentication exchange defined by an application protocol. See authentication exchange defined by an application protocol. See
[RFC4422] for more details. [RFC4422] for more details.
Note that this section omits some details, such as client and server Note that this section omits some details, such as client and server
nonces. See Section 5 for more details. nonces. See Section 5 for more details.
To begin with, the SCRAM client is in possession of a username and To begin with, the SCRAM client is in possession of a username and
password (*) (or a ClientKey/ServerKey, or SaltedPassword). It sends password (*) (or a ClientKey/ServerKey, or SaltedPassword). It sends
the username to the server, which retrieves the corresponding the username to the server, which retrieves the corresponding
authentication information, i.e. a salt, StoredKey, ServerKey and the authentication information, i.e., a salt, StoredKey, ServerKey, and
iteration count i. (Note that a server implementation may choose to the iteration count i. (Note that a server implementation may choose
use the same iteration count for all accounts.) The server sends the to use the same iteration count for all accounts.) The server sends
salt and the iteration count to the client, which then computes the the salt and the iteration count to the client, which then computes
following values and sends a ClientProof to the server: the following values and sends a ClientProof to the server:
(*) - Note that both the username and the password MUST be encoded in (*) Note that both the username and the password MUST be encoded in
UTF-8 [RFC3629]. UTF-8 [RFC3629].
Informative Note: Implementors are encouraged to create test cases Informative Note: Implementors are encouraged to create test cases
that use both username passwords with non-ASCII codepoints. In that use both usernames and passwords with non-ASCII codepoints. In
particular, it's useful to test codepoints whose "Unicode particular, it's useful to test codepoints whose "Unicode
Normalization Form C" and "Unicode Normalization Form KC" are Normalization Form C" and "Unicode Normalization Form KC" are
different. Some examples of such codepoints include Vulgar Fraction different. Some examples of such codepoints include Vulgar Fraction
One Half (U+00BD) and Acute Accent (U+00B4). One Half (U+00BD) and Acute Accent (U+00B4).
SaltedPassword := Hi(Normalize(password), salt, i) SaltedPassword := Hi(Normalize(password), salt, i)
ClientKey := HMAC(SaltedPassword, "Client Key") ClientKey := HMAC(SaltedPassword, "Client Key")
StoredKey := H(ClientKey) StoredKey := H(ClientKey)
AuthMessage := client-first-message-bare + "," + AuthMessage := client-first-message-bare + "," +
server-first-message + "," + server-first-message + "," +
client-final-message-without-proof client-final-message-without-proof
ClientSignature := HMAC(StoredKey, AuthMessage) ClientSignature := HMAC(StoredKey, AuthMessage)
ClientProof := ClientKey XOR ClientSignature ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, "Server Key") ServerKey := HMAC(SaltedPassword, "Server Key")
ServerSignature := HMAC(ServerKey, AuthMessage) ServerSignature := HMAC(ServerKey, AuthMessage)
The server authenticates the client by computing the ClientSignature, The server authenticates the client by computing the ClientSignature,
exclusive-ORing that with the ClientProof to recover the ClientKey exclusive-ORing that with the ClientProof to recover the ClientKey
and verifying the correctness of the ClientKey by applying the hash and verifying the correctness of the ClientKey by applying the hash
function and comparing the result to the StoredKey. If the ClientKey function and comparing the result to the StoredKey. If the ClientKey
is correct, this proves that the client has access to the user's is correct, this proves that the client has access to the user's
password. password.
Similarly, the client authenticates the server by computing the Similarly, the client authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If ServerSignature and comparing it to the value sent by the server. If
skipping to change at page 11, line 12 skipping to change at page 9, line 4
authentication exchange. The format of these messages is defined in authentication exchange. The format of these messages is defined in
Section 7. Section 7.
4. SCRAM Mechanism Names 4. SCRAM Mechanism Names
A SCRAM mechanism name is a string "SCRAM-" followed by the A SCRAM mechanism name is a string "SCRAM-" followed by the
uppercased name of the underlying hash function taken from the IANA uppercased name of the underlying hash function taken from the IANA
"Hash Function Textual Names" registry (see http://www.iana.org), "Hash Function Textual Names" registry (see http://www.iana.org),
optionally followed by the suffix "-PLUS" (see below). Note that optionally followed by the suffix "-PLUS" (see below). Note that
SASL mechanism names are limited to 20 octets, which means that only SASL mechanism names are limited to 20 octets, which means that only
hash function names with lengths shorter or equal to 9 octets (20- hash function names with lengths shorter or equal to 9 octets
length("SCRAM-")-length("-PLUS") can be used. For cases when the (20-length("SCRAM-")-length("-PLUS") can be used. For cases when the
underlying hash function name is longer than 9 octets, an alternative underlying hash function name is longer than 9 octets, an alternative
9 octet (or shorter) name can be used to construct the corresponding 9-octet (or shorter) name can be used to construct the corresponding
SCRAM mechanism name, as long as this alternative name doesn't SCRAM mechanism name, as long as this alternative name doesn't
conflict with any other hash function name from the IANA "Hash conflict with any other hash function name from the IANA "Hash
Function Textual Names" registry. In order to prevent future Function Textual Names" registry. In order to prevent future
conflict, such alternative name SHOULD be registered in the IANA conflict, such alternative names SHOULD be registered in the IANA
"Hash Function Textual Names" registry. "Hash Function Textual Names" registry.
For interoperability, all SCRAM clients and servers MUST implement For interoperability, all SCRAM clients and servers MUST implement
the SCRAM-SHA-1 authentication mechanism, i.e. an authentication the SCRAM-SHA-1 authentication mechanism, i.e., an authentication
mechanism from the SCRAM family that uses the SHA-1 hash function as mechanism from the SCRAM family that uses the SHA-1 hash function as
defined in [RFC3174]. defined in [RFC3174].
The "-PLUS" suffix is used only when the server supports channel The "-PLUS" suffix is used only when the server supports channel
binding to the external channel. If the server supports channel binding to the external channel. If the server supports channel
binding, it will advertise both the "bare" and "plus" versions of binding, it will advertise both the "bare" and "plus" versions of
whatever mechanisms it supports (e.g., if the server supports only whatever mechanisms it supports (e.g., if the server supports only
SCRAM with SHA-1 then it will advertise support for both SCRAM-SHA-1 SCRAM with SHA-1, then it will advertise support for both SCRAM-SHA-1
and SCRAM-SHA-1-PLUS); if the server does not support channel and SCRAM-SHA-1-PLUS). If the server does not support channel
binding, then it will advertise only the "bare" version of the binding, then it will advertise only the "bare" version of the
mechanism (e.g., only SCRAM-SHA-1). The "-PLUS" exists to allow mechanism (e.g., only SCRAM-SHA-1). The "-PLUS" exists to allow
negotiation of the use of channel binding. See Section 6. negotiation of the use of channel binding. See Section 6.
5. SCRAM Authentication Exchange 5. SCRAM Authentication Exchange
SCRAM is a SASL mechanism whose client response and server challenge SCRAM is a SASL mechanism whose client response and server challenge
messages are text-based messages containing one or more attribute- messages are text-based messages containing one or more attribute-
value pairs separated by commas. Each attribute has a one-letter value pairs separated by commas. Each attribute has a one-letter
name. The messages and their attributes are described in name. The messages and their attributes are described in
Section 5.1, and defined in Section 7. Section 5.1, and defined in Section 7.
SCRAM is a client-first SASL mechanism (See [RFC4422], Section 5, SCRAM is a client-first SASL mechanism (see [RFC4422], Section 5,
item 2a), and returns additional data together with a server's item 2a), and returns additional data together with a server's
indication of a successful outcome. indication of a successful outcome.
This is a simple example of a SCRAM-SHA-1 authentication exchange This is a simple example of a SCRAM-SHA-1 authentication exchange
when the client doesn't support channel bindings: when the client doesn't support channel bindings (username 'user' and
password 'pencil' are used):
C: n,,n=Chris Newman,r=ClientNonce
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
C: c=biwsCg==,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
S: v=WxPv/siO5l+qxN4
[[anchor5: Note that the all hashes above are fake and will be fixed
during AUTH48.]]
With channel-binding data sent by the client this might look like
this (see [tls-server-end-point] for the definition of tls-server-
end-point TLS channel binding):
C: p=tls-server-end-point,,n=Chris Newman,r=ClientNonce
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
C: c=cD10bHMtc2VydmVyLWVuZC1wb2ludCwsy1hFtXOnZ+ySrQM6srFp
l/77uqvtxrg7nBY1BetEr/g=,r=ClientNonceServerNonce,p=Wx
Pv/siO5l+qxN4
S: v=WxPv/siO5l+qxN4
[[anchor6: Note that all hashes above are fake and will be fixed
during AUTH48.]]
C: n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL
S: r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,
i=4096
C: c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
S: v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
First, the client sends the "client-first-message" containing: First, the client sends the "client-first-message" containing:
o a GS2 header consisting of a flag indicating whether channel o a GS2 header consisting of a flag indicating whether channel
binding is supported-but-not-used, not supported, or used, and an binding is supported-but-not-used, not supported, or used, and an
optional SASL authorization identity; optional SASL authorization identity;
o SCRAM username and a random, unique nonce attributes. o SCRAM username and a random, unique nonce attributes.
Note that the client's first message will always start with "n", "y" Note that the client's first message will always start with "n", "y",
or "p", otherwise the message is invalid and authentication MUST or "p"; otherwise, the message is invalid and authentication MUST
fail. This is important, as it allows for GS2 extensibility (e.g., fail. This is important, as it allows for GS2 extensibility (e.g.,
to add support for security layers). to add support for security layers).
In response, the server sends a "server-first-message" containing the In response, the server sends a "server-first-message" containing the
user's iteration count i, the user's salt, and appends its own nonce user's iteration count i and the user's salt, and appends its own
to the client-specified one. nonce to the client-specified one.
The client then responds by sending "client-final-message" with the The client then responds by sending a "client-final-message" with the
same nonce and a ClientProof computed using the selected hash same nonce and a ClientProof computed using the selected hash
function as explained earlier. function as explained earlier.
The server verifies the nonce and the proof, verifies that the The server verifies the nonce and the proof, verifies that the
authorization identity (if supplied by the client in the first authorization identity (if supplied by the client in the first
message) is authorized to act as the authentication identity, and, message) is authorized to act as the authentication identity, and,
finally, it responds with a "server-final-message", concluding the finally, it responds with a "server-final-message", concluding the
authentication exchange. authentication exchange.
The client then authenticates the server by computing the The client then authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If ServerSignature and comparing it to the value sent by the server. If
the two are different, the client MUST consider the authentication the two are different, the client MUST consider the authentication
exchange to be unsuccessful and it might have to drop the connection. exchange to be unsuccessful, and it might have to drop the
connection.
5.1. SCRAM Attributes 5.1. SCRAM Attributes
This section describes the permissible attributes, their use, and the This section describes the permissible attributes, their use, and the
format of their values. All attribute names are single US-ASCII format of their values. All attribute names are single US-ASCII
letters and are case-sensitive. letters and are case-sensitive.
Note that the order of attributes in client or server messages is Note that the order of attributes in client or server messages is
fixed, with the exception of extension attributes (described by the fixed, with the exception of extension attributes (described by the
"extensions" ABNF production), which can appear in any order in the "extensions" ABNF production), which can appear in any order in the
designated positions. See the ABNF section for authoritative designated positions. See Section 7 for authoritative reference.
reference.
o a: This is an optional attribute, and is part of the GS2 o a: This is an optional attribute, and is part of the GS2 [RFC5801]
[I-D.ietf-sasl-gs2] bridge between the GSS-API and SASL. This bridge between the GSS-API and SASL. This attribute specifies an
attribute specifies an authorization identity. A client may authorization identity. A client may include it in its first
include it in its first message to the server if it wants to message to the server if it wants to authenticate as one user, but
authenticate as one user, but subsequently act as a different subsequently act as a different user. This is typically used by
user. This is typically used by an administrator to perform some an administrator to perform some management task on behalf of
management task on behalf of another user, or by a proxy in some another user, or by a proxy in some situations.
situations.
Upon the receipt of this value the server verifies its Upon the receipt of this value the server verifies its
correctness according to the used SASL protocol profile. correctness according to the used SASL protocol profile.
Failed verification results in failed authentication exchange. Failed verification results in failed authentication exchange.
If this attribute is omitted (as it normally would be), the If this attribute is omitted (as it normally would be), the
authorization identity is assumed to be derived from the authorization identity is assumed to be derived from the
username specified with the (required) "n" attribute. username specified with the (required) "n" attribute.
The server always authenticates the user specified by the "n" The server always authenticates the user specified by the "n"
skipping to change at page 14, line 21 skipping to change at page 11, line 28
the server associates that identity with the connection after the server associates that identity with the connection after
successful authentication and authorization checks. successful authentication and authorization checks.
The syntax of this field is the same as that of the "n" field The syntax of this field is the same as that of the "n" field
with respect to quoting of '=' and ','. with respect to quoting of '=' and ','.
o n: This attribute specifies the name of the user whose password is o n: This attribute specifies the name of the user whose password is
used for authentication (a.k.a. "authentication identity" used for authentication (a.k.a. "authentication identity"
[RFC4422]). A client MUST include it in its first message to the [RFC4422]). A client MUST include it in its first message to the
server. If the "a" attribute is not specified (which would server. If the "a" attribute is not specified (which would
normally be the case), this username is also the identity which normally be the case), this username is also the identity that
will be associated with the connection subsequent to will be associated with the connection subsequent to
authentication and authorization. authentication and authorization.
Before sending the username to the server, the client SHOULD Before sending the username to the server, the client SHOULD
prepare the username using the "SASLPrep" profile [RFC4013] of prepare the username using the "SASLprep" profile [RFC4013] of
the "stringprep" algorithm [RFC3454] treating it as a query the "stringprep" algorithm [RFC3454] treating it as a query
string (i.e., unassigned Unicode code points are allowed). If string (i.e., unassigned Unicode code points are allowed). If
the preparation of the username fails or results in an empty the preparation of the username fails or results in an empty
string, the client SHOULD abort the authentication exchange string, the client SHOULD abort the authentication exchange
(*). (*).
(*) An interactive client can request a repeated entry of the (*) An interactive client can request a repeated entry of the
username value. username value.
Upon receipt of the username by the server, the server MUST Upon receipt of the username by the server, the server MUST
either prepare it using the "SASLPrep" profile [RFC4013] of the either prepare it using the "SASLprep" profile [RFC4013] of the
"stringprep" algorithm [RFC3454] treating it as a query string "stringprep" algorithm [RFC3454] treating it as a query string
(i.e., unassigned Unicode codepoints are allowed) or otherwise (i.e., unassigned Unicode codepoints are allowed) or otherwise
be prepared to do SASLprep-aware string comparisons and/or be prepared to do SASLprep-aware string comparisons and/or
index lookups. If the preparation of the username fails or index lookups. If the preparation of the username fails or
results in an empty string, the server SHOULD abort the results in an empty string, the server SHOULD abort the
authentication exchange. Whether or not the server prepares authentication exchange. Whether or not the server prepares
the username using "SASLPrep", it MUST use it as received in the username using "SASLprep", it MUST use it as received in
hash calculations. hash calculations.
The characters ',' or '=' in usernames are sent as '=2C' and The characters ',' or '=' in usernames are sent as '=2C' and
'=3D' respectively. If the server receives a username which '=3D' respectively. If the server receives a username that
contains '=' not followed by either '2C' or '3D', then the contains '=' not followed by either '2C' or '3D', then the
server MUST fail the authentication. server MUST fail the authentication.
o m: This attribute is reserved for future extensibility. In this o m: This attribute is reserved for future extensibility. In this
version of SCRAM, its presence in a client or a server message version of SCRAM, its presence in a client or a server message
MUST cause authentication failure when the attribute is parsed by MUST cause authentication failure when the attribute is parsed by
the other end. the other end.
o r: This attribute specifies a sequence of random printable ASCII o r: This attribute specifies a sequence of random printable ASCII
characters excluding ',' which forms the nonce used as input to characters excluding ',' (which forms the nonce used as input to
the hash function. No quoting is applied to this string. As the hash function). No quoting is applied to this string. As
described earlier, the client supplies an initial value in its described earlier, the client supplies an initial value in its
first message, and the server augments that value with its own first message, and the server augments that value with its own
nonce in its first response. It is important that this value be nonce in its first response. It is important that this value be
different for each authentication (see [RFC4086] for more details different for each authentication (see [RFC4086] for more details
on how to achieve this). The client MUST verify that the initial on how to achieve this). The client MUST verify that the initial
part of the nonce used in subsequent messages is the same as the part of the nonce used in subsequent messages is the same as the
nonce it initially specified. The server MUST verify that 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 nonce sent by the client in the second message is the same as the
one sent by the server in its first message. one sent by the server in its first message.
o c: This REQUIRED attribute specifies the base64-encoded GS2 header o c: This REQUIRED attribute specifies the base64-encoded GS2 header
and channel-binding data. It is sent by the client in its second and channel binding data. It is sent by the client in its second
authentication message. The attribute data consist of: authentication message. The attribute data consist of:
* the GS2 header from the client's first message (recall that the * the GS2 header from the client's first message (recall that the
GS2 header contains a channel binding flag and an optional GS2 header contains a channel binding flag and an optional
authzid). This header is going to include channel binding type authzid). This header is going to include channel binding type
prefix (see [RFC5056]), if and only if the client is using prefix (see [RFC5056]), if and only if the client is using
channel binding; channel binding;
* followed by the external channel's channel binding data, if and * followed by the external channel's channel binding data, if and
only if the client is using channel binding. only if the client is using channel binding.
o s: This attribute specifies the base64-encoded salt used by the o s: This attribute specifies the base64-encoded salt used by the
server for this user. It is sent by the server in its first server for this user. It is sent by the server in its first
message to the client. message to the client.
o i: This attribute specifies an iteration count for the selected o i: This attribute specifies an iteration count for the selected
hash function and user, and MUST be sent by the server along with hash function and user, and MUST be sent by the server along with
the user's salt. the user's salt.
For SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism servers SHOULD For the SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism, servers
announce a hash iteration-count of at least 4096. Note that a SHOULD announce a hash iteration-count of at least 4096. Note
client implementation MAY cache ClientKey&ServerKey (or just that a client implementation MAY cache ClientKey&ServerKey (or
SaltedPassword) for later reauthentication to the same service, just SaltedPassword) for later reauthentication to the same
as it is likely that the server is going to advertise the same service, as it is likely that the server is going to advertise
salt value upon reauthentication. This might be useful for the same salt value upon reauthentication. This might be
mobile clients where CPU usage is a concern. useful for mobile clients where CPU usage is a concern.
o p: This attribute specifies a base64-encoded ClientProof. The o p: This attribute specifies a base64-encoded ClientProof. The
client computes this value as described in the overview and sends client computes this value as described in the overview and sends
it to the server. it to the server.
o v: This attribute specifies a base64-encoded ServerSignature. It o v: This attribute specifies a base64-encoded ServerSignature. It
is sent by the server in its final message, and is used by the is sent by the server in its final message, and is used by the
client to verify that the server has access to the user's client to verify that the server has access to the user's
authentication information. This value is computed as explained authentication information. This value is computed as explained
in the overview. in the overview.
5.2. Compliance with SASL mechanism requirements o e: This attribute specifies an error that occurred during
authentication exchange. It is sent by the server in its final
message and can help diagnose the reason for the authentication
exchange failure. On failed authentication, the entire server-
final-message is OPTIONAL; specifically, a server implementation
MAY conclude the SASL exchange with a failure without sending the
server-final-message. This results in an application-level error
response without an extra round-trip. If the server-final-message
is sent on authentication failure, then the "e" attribute MUST be
included.
o As-yet unspecified mandatory and optional extensions. Mandatory
extensions are encoded as values of the 'm' attribute (see ABNF
for reserved-mext in section 7). Optional extensions use as-yet
unassigned attribute names.
Mandatory extensions sent by one peer but not understood by the
other MUST cause authentication failure (the server SHOULD send
the "extensions-not-supported" server-error-value).
Unknown optional extensions MUST be ignored upon receipt.
5.2. Compliance with SASL Mechanism Requirements
This section describes compliance with SASL mechanism requirements This section describes compliance with SASL mechanism requirements
specified in Section 5 of [RFC4422]. specified in Section 5 of [RFC4422].
1) "SCRAM-SHA-1" and "SCRAM-SHA-1-PLUS". 1) "SCRAM-SHA-1" and "SCRAM-SHA-1-PLUS".
2a) SCRAM is a client-first mechanism. 2a) SCRAM is a client-first mechanism.
2b) SCRAM sends additional data with success. 2b) SCRAM sends additional data with success.
3) SCRAM is capable of transferring authorization identities from the 3) SCRAM is capable of transferring authorization identities from
client to the server. the client to the server.
4) SCRAM does not offer any security layers (SCRAM offers channel 4) SCRAM does not offer any security layers (SCRAM offers channel
binding instead). binding instead).
5) SCRAM has a hash protecting the authorization identity. 5) SCRAM has a hash protecting the authorization identity.
6. Channel Binding 6. Channel Binding
SCRAM supports channel binding to external secure channels, such as SCRAM supports channel binding to external secure channels, such as
TLS. Clients and servers may or may not support channel binding, TLS. Clients and servers may or may not support channel binding,
therefore the use of channel binding is negotiable. SCRAM does not therefore the use of channel binding is negotiable. SCRAM does not
provide security layers, however, therefore it is imperative that provide security layers, however, therefore it is imperative that
SCRAM provide integrity protection for the negotiation of channel SCRAM provide integrity protection for the negotiation of channel
binding. binding.
Use of channel binding is negotiated as follows: Use of channel binding is negotiated as follows:
o Servers SHOULD advertise both non-PLUS (SCRAM-<hash-function>) and o Servers that support the use of channel binding SHOULD advertise
the PLUS-variant (SCRAM-<hash-function>-PLUS) SASL mechanism both the non-PLUS (SCRAM-<hash-function>) and PLUS-variant (SCRAM-
names. If the server cannot support channel binding, it MAY <hash-function>-PLUS) mechanism name. If the server cannot
advertise only the non-PLUS variant. If the server would never support channel binding, it SHOULD advertise only the non-PLUS-
succeed authentication of the non-PLUS variant due to policy variant. If the server would never succeed in the authentication
reasons, it MAY advertise only the PLUS-variant. of the non-PLUS-variant due to policy reasons, it MUST advertise
only the PLUS-variant.
o If the client negotiates mechanisms then the client MUST select
SCRAM-<hash-function>-PLUS if offered by the server and the client
wants to select SCRAM with the given hash function. Otherwise
(the client does not negotiate mechanisms), if the client has no
prior knowledge about mechanisms supported by the server and
wasn't explicitly configured to use a particular variant of the
SCRAM mechanism, then it MUST select only SCRAM-<hash-function>
(not suffixed with "-PLUS").
o If the client supports channel binding and the server appears to o If the client supports channel binding and the server does not
support it (i.e., the client sees SCRAM-<hash-function>-PLUS), or appear to (i.e., the client did not see the -PLUS name advertised
if the client wishes to use channel binding but the client does by the server), then the client MUST NOT use an "n" gs2-cbind-
not negotiate mechanisms, then the client MUST set the GS2 channel flag.
binding flag to "p" in order to indicate the channel binding type
it is using and it MUST include the channel binding data for the
external channel in the computation of the "c=" attribute (see
Section 5.1).
o If the client supports channel binding but the server does not o Clients that support mechanism negotiation and channel binding
appear to (i.e., the client did not see SCRAM-<hash-function>- MUST use a "p" gs2-cbind-flag when the server offers the PLUS-
PLUS) then the client MUST either fail authentication or it MUST variant of the desired GS2 mechanism.
choose the non-PLUS mechanism and set the GS2 channel binding flag
to "y" and MUST NOT include channel binding data for the external
channel in the computation of the "c=" attribute (see
Section 5.1).
o If the client does not support channel binding then the client o If the client does not support channel binding, then it MUST use
MUST set the GS2 channel binding flag to "n" and MUST NOT include an "n" gs2-cbind-flag. Conversely, if the client requires the use
channel binding data for the external channel in the computation of channel binding then it MUST use a "p" gs2-cbind-flag. Clients
of the "c=" attribute (see Section 5.1). that do not support mechanism negotiation never use a "y" gs2-
cbind-flag, they use either "p" or "n" according to whether they
require and support the use of channel binding or whether they do
not, respectively.
o Upon receipt of the client first message the server checks the GS2 o Upon receipt of the client-first message, the server checks the
channel binding flag (gs2-cb-flag). channel binding flag (gs2-cbind-flag).
* If the flag is set to "y" and the server supports channel * If the flag is set to "y" and the server supports channel
binding the server MUST fail authentication. This is because binding, the server MUST fail authentication. This is because
if the client sets the GS2 channel binding flag set to "y" then if the client sets the channel binding flag to "y", then the
the client must have believed that the server did not support client must have believed that the server did not support
channel binding -- if the server did in fact support channel channel binding -- if the server did in fact support channel
binding then this is an indication that there has been a binding, then this is an indication that there has been a
downgrade attack (e.g., an attacker changed the server's downgrade attack (e.g., an attacker changed the server's
mechanism list to exclude the -PLUS suffixed SCRAM mechanism mechanism list to exclude the -PLUS suffixed SCRAM mechanism
name(s)). name(s)).
* If the channel binding flag was "p" and the server does not * If the channel binding flag was "p" and the server does not
support the indicated channel binding type then the server MUST support the indicated channel binding type, then the server
fail authentication. MUST fail authentication.
The server MUST always validate the client's "c=" field. The server The server MUST always validate the client's "c=" field. The server
does this by constructing the value of the "c=" attribute and then does this by constructing the value of the "c=" attribute and then
checking that it matches the client's c= attribute value. checking that it matches the client's c= attribute value.
For more discussions of channel bindings, and the syntax of the For more discussions of channel bindings, and the syntax of channel
channel binding data for various security protocols, see [RFC5056]. binding data for various security protocols, see [RFC5056].
6.1. Default Channel Binding 6.1. Default Channel Binding
A default channel binding type agreement process for all SASL A default channel binding type agreement process for all SASL
application protocols that do not provide their own channel binding application protocols that do not provide their own channel binding
type agreement is provided as follows. type agreement is provided as follows.
'tls-unique' is the default channel binding type for any application 'tls-unique' is the default channel binding type for any application
that doesn't specify one. that doesn't specify one.
Servers MUST implement the "tls-unique" [tls-unique] Servers MUST implement the "tls-unique" [RFC5929] channel binding
[I-D.altman-tls-channel-bindings] channel binding type, if they type, if they implement any channel binding. Clients SHOULD
implement any channel binding. Clients SHOULD implement the "tls- implement the "tls-unique" [RFC5929] channel binding type, if they
unique" [tls-unique] [I-D.altman-tls-channel-bindings] channel implement any channel binding. Clients and servers SHOULD choose the
binding type, if they implement any channel binding. Clients and highest-layer/innermost end-to-end TLS channel as the channel to
servers SHOULD choose the highest- layer/innermost end-to-end TLS which to bind.
channel as the channel to bind to.
Servers MUST choose the channel binding type indicated by the client, Servers MUST choose the channel binding type indicated by the client,
or fail authentication if they don't support it. or fail authentication if they don't support it.
7. Formal Syntax 7. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3" form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3",
and "UTF8-4" non-terminal are defined in [RFC3629]. and "UTF8-4" non-terminal are defined in [RFC3629].
ALPHA = <as defined in RFC 5234 appendix B.1> ALPHA = <as defined in RFC 5234 appendix B.1>
DIGIT = <as defined in RFC 5234 appendix B.1> DIGIT = <as defined in RFC 5234 appendix B.1>
UTF8-2 = <as defined in RFC 3629 (STD 63)> UTF8-2 = <as defined in RFC 3629 (STD 63)>
UTF8-3 = <as defined in RFC 3629 (STD 63)> UTF8-3 = <as defined in RFC 3629 (STD 63)>
UTF8-4 = <as defined in RFC 3629 (STD 63)> UTF8-4 = <as defined in RFC 3629 (STD 63)>
attr-val = ALPHA "=" value attr-val = ALPHA "=" value
;; Generic syntax of any attribute sent ;; Generic syntax of any attribute sent
skipping to change at page 19, line 45 skipping to change at page 16, line 39
base64-4 = 4base64-char base64-4 = 4base64-char
base64-3 = 3base64-char "=" base64-3 = 3base64-char "="
base64-2 = 2base64-char "==" base64-2 = 2base64-char "=="
base64 = *base64-4 [base64-3 / base64-2] base64 = *base64-4 [base64-3 / base64-2]
posit-number = %x31-39 *DIGIT posit-number = %x31-39 *DIGIT
;; A positive number ;; A positive number.
saslname = 1*(value-safe-char / "=2C" / "=3D") saslname = 1*(value-safe-char / "=2C" / "=3D")
;; Conforms to <value> ;; Conforms to <value>.
authzid = "a=" saslname authzid = "a=" saslname
;; Protocol specific. ;; Protocol specific.
cb-name = 1*(ALPHA / DIGIT / "." / "-") cb-name = 1*(ALPHA / DIGIT / "." / "-")
;; See RFC 5056 section 7. ;; See RFC 5056, Section 7.
;; E.g. "tls-server-end-point" or ;; E.g., "tls-server-end-point" or
;; "tls-unique" ;; "tls-unique".
gs2-cbind-flag = "p=" cb-name / "n" / "y" gs2-cbind-flag = ("p=" cb-name) / "n" / "y"
;; "n" -> client doesn't support channel binding ;; "n" -> client doesn't support channel binding.
;; "y" -> client does support channel binding ;; "y" -> client does support channel binding
;; but thinks the server does not. ;; but thinks the server does not.
;; "p" -> client requires channel binding. ;; "p" -> client requires channel binding.
;; The selected channel binding follows "p=". ;; The selected channel binding follows "p=".
gs2-header = gs2-cbind-flag "," [ authzid ] "," gs2-header = gs2-cbind-flag "," [ authzid ] ","
;; GS2 header for SCRAM ;; GS2 header for SCRAM
;; (the actual GS2 header includes an optional ;; (the actual GS2 header includes an optional
;; flag to indicate that the GSS mechanism is not ;; flag to indicate that the GSS mechanism is not
;; "standard" but since SCRAM is "standard" we ;; "standard", but since SCRAM is "standard", we
;; don't include that flag). ;; don't include that flag).
username = "n=" saslname username = "n=" saslname
;; Usernames are prepared using SASLPrep. ;; Usernames are prepared using SASLprep.
reserved-mext = "m=" 1*(value-char) reserved-mext = "m=" 1*(value-char)
;; Reserved for signalling mandatory extensions. ;; Reserved for signaling mandatory extensions.
;; The exact syntax will be defined in ;; The exact syntax will be defined in
;; the future. ;; the future.
channel-binding = "c=" base64 channel-binding = "c=" base64
;; base64 encoding of cbind-input ;; base64 encoding of cbind-input.
proof = "p=" base64 proof = "p=" base64
nonce = "r=" c-nonce [s-nonce] nonce = "r=" c-nonce [s-nonce]
;; Second part provided by server. ;; Second part provided by server.
c-nonce = printable c-nonce = printable
s-nonce = printable s-nonce = printable
salt = "s=" base64 salt = "s=" base64
verifier = "v=" base64 verifier = "v=" base64
;; base-64 encoded ServerSignature. ;; base-64 encoded ServerSignature.
iteration-count = "i=" posit-number iteration-count = "i=" posit-number
;; A positive number ;; A positive number.
client-first-message-bare = client-first-message-bare =
[reserved-mext ","] [reserved-mext ","]
username "," nonce ["," extensions] username "," nonce ["," extensions]
client-first-message = client-first-message =
gs2-header client-first-message-bare gs2-header client-first-message-bare
server-first-message = server-first-message =
[reserved-mext ","] nonce "," salt "," [reserved-mext ","] nonce "," salt ","
iteration-count ["," extensions] iteration-count ["," extensions]
skipping to change at page 21, line 40 skipping to change at page 18, line 34
"channel-binding-not-supported" / "channel-binding-not-supported" /
"unsupported-channel-binding-type" / "unsupported-channel-binding-type" /
"unknown-user" / "unknown-user" /
"invalid-username-encoding" / "invalid-username-encoding" /
; invalid username encoding (invalid UTF-8 or ; invalid username encoding (invalid UTF-8 or
; SASLprep failed) ; SASLprep failed)
"no-resources" / "no-resources" /
"other-error" / "other-error" /
server-error-value-ext server-error-value-ext
; Unrecognized errors should be treated as "other-error". ; Unrecognized errors should be treated as "other-error".
; In order to prevent information disclosure the server ; In order to prevent information disclosure, the server
; may substitute the real reason with "other-error". ; may substitute the real reason with "other-error".
server-error-value-ext = value server-error-value-ext = value
; Additional error reasons added by extensions ; Additional error reasons added by extensions
; to this document. ; to this document.
server-final-message = (server-error / verifier) server-final-message = (server-error / verifier)
["," extensions] ["," extensions]
;; The error message is only for the GSS-API
;; form of SCRAM, and it is OPTIONAL to
;; implement it.
extensions = attr-val *("," attr-val) extensions = attr-val *("," attr-val)
;; All extensions are optional, ;; All extensions are optional,
;; i.e. unrecognized attributes ;; i.e., unrecognized attributes
;; not defined in this document ;; not defined in this document
;; MUST be ignored. ;; MUST be ignored.
cbind-data = 1*OCTET cbind-data = 1*OCTET
cbind-input = gs2-header [ cbind-data ] cbind-input = gs2-header [ cbind-data ]
;; cbind-data MUST be present for ;; cbind-data MUST be present for
;; gs2-cbind-flag of "p" and MUST be absent ;; gs2-cbind-flag of "p" and MUST be absent
;; for "y" or "n". ;; for "y" or "n".
8. SCRAM as a GSS-API Mechanism 8. SCRAM as a GSS-API Mechanism
This section and its sub-sections and all normative references of it This section and its sub-sections and all normative references of it
not referenced elsewhere in this document are INFORMATIONAL for SASL not referenced elsewhere in this document are INFORMATIONAL for SASL
implementors, but they are NORMATIVE for GSS-API implementors. implementors, but they are NORMATIVE for GSS-API implementors.
skipping to change at page 23, line 11 skipping to change at page 19, line 15
;; cbind-data MUST be present for ;; cbind-data MUST be present for
;; gs2-cbind-flag of "p" and MUST be absent ;; gs2-cbind-flag of "p" and MUST be absent
;; for "y" or "n". ;; for "y" or "n".
8. SCRAM as a GSS-API Mechanism 8. SCRAM as a GSS-API Mechanism
This section and its sub-sections and all normative references of it This section and its sub-sections and all normative references of it
not referenced elsewhere in this document are INFORMATIONAL for SASL not referenced elsewhere in this document are INFORMATIONAL for SASL
implementors, but they are NORMATIVE for GSS-API implementors. implementors, but they are NORMATIVE for GSS-API implementors.
SCRAM is actually also GSS-API mechanism. The messages are the same, SCRAM is actually also a GSS-API mechanism. The messages are the
but a) the GS2 header on the client's first message and channel same, but a) the GS2 header on the client's first message and channel
binding data is excluded when SCRAM is used as a GSS-API mechanism, binding data is excluded when SCRAM is used as a GSS-API mechanism,
and b) the RFC2743 section 3.1 initial context token header is and b) the RFC2743 section 3.1 initial context token header is
prefixed to the client's first authentication message (context prefixed to the client's first authentication message (context
token). token).
The GSS-API mechanism OID for SCRAM-SHA-1 is <TBD> (see Section 10). The GSS-API mechanism OID for SCRAM-SHA-1 is 1.3.6.1.5.5.14 (see
Section 10).
SCRAM security contexts always have the mutual_state flag
(GSS_C_MUTUAL_FLAG) set to TRUE. SCRAM does not support credential
delegation, therefore SCRAM security contexts alway have the
deleg_state flag (GSS_C_DELEG_FLAG) set to FALSE.
8.1. GSS-API Principal Name Types for SCRAM 8.1. GSS-API Principal Name Types for SCRAM
SCRAM does not name acceptors. Therefore only GSS_C_NO_NAME and SCRAM does not explicitly name acceptor principals. However, the use
names of type GSS_C_NT_ANONYMOUS shall be allowed as the target name of acceptor principal names to find or prompt for passwords is
input of GSS_Init_sec_context() when using a SCRAM mechanism. useful. Therefore, SCRAM supports standard generic name syntaxes for
acceptors such as GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section
4.1). Implementations should use the target name passed to
GSS_Init_sec_context(), if any, to help retrieve or prompt for SCRAM
passwords.
SCRAM supports only a single name type for initiators: SCRAM supports only a single name type for initiators:
GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for
SCRAM. SCRAM.
There is no name canonicalization procedure for SCRAM beyond applying There is no name canonicalization procedure for SCRAM beyond applying
SASLprep as described in Section 5.1. SASLprep as described in Section 5.1.
The query, display and exported name syntax for SCRAM principal names The query, display, and exported name syntaxes for SCRAM principal
is the same: there is no syntax -- SCRAM principal names are free- names are all the same. There are no SCRAM-specific name syntaxes
form. (The exported name token does, of course, conform to [RFC2743] (SCRAM initiator principal names are free-form); -- applications
section 3.2, but the "NAME" part of the token is just a SCRAM user should use generic GSS-API name types such as GSS_C_NT_USER_NAME and
name.) GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4). The exported
name token does, of course, conform to [RFC2743], Section 3.2, but
the "NAME" part of the token is just a SCRAM user name.
8.2. GSS-API Per-Message Tokens for SCRAM 8.2. GSS-API Per-Message Tokens for SCRAM
The per-message tokens for SCRAM as a GSS-API mechanism SHALL be the The per-message tokens for SCRAM as a GSS-API mechanism SHALL be the
same as those for the Kerberos V GSS-API mechanism [RFC4121] (see same as those for the Kerberos V GSS-API mechanism [RFC4121] (see
Section 4.2 and sub-sections), using the Kerberos V "aes128-cts-hmac- Section 4.2 and sub-sections), using the Kerberos V "aes128-cts-hmac-
sha1-96" enctype [RFC3962]. sha1-96" enctype [RFC3962].
The replay_det_state (GSS_C_REPLAY_FLAG), sequence_state
(GSS_C_SEQUENCE_FLAG), conf_avail (GSS_C_CONF_FLAG) and integ_avail
(GSS_C_CONF_FLAG) security context flags are always set to TRUE.
The 128-bit session "protocol key" SHALL be derived by using the The 128-bit session "protocol key" SHALL be derived by using the
least significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API least significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API
session key" || ClientKey || AuthMessage). "Specific keys" are then session key" || ClientKey || AuthMessage). "Specific keys" are then
derived as usual as described in Section 2 of [RFC4121], [RFC3961] derived as usual as described in Section 2 of [RFC4121], [RFC3961],
and [RFC3962]. and [RFC3962].
The terms "protocol key" and "specific key" are Kerberos V5 terms The terms "protocol key" and "specific key" are Kerberos V5 terms
[RFC3961]. [RFC3961].
SCRAM does support PROT_READY, and is PROT_READY on the initiator SCRAM does support PROT_READY, and is PROT_READY on the initiator
side first upon receipt of the server's reply to the initial security side first upon receipt of the server's reply to the initial security
context token. context token.
8.3. GSS_Pseudo_random() for SCRAM 8.3. GSS_Pseudo_random() for SCRAM
The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for
the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor- the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor-
skipping to change at page 25, line 10 skipping to change at page 20, line 46
asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and
GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random(). GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random().
The protocol key to be used for the GSS_Pseudo_random() SHALL be the The protocol key to be used for the GSS_Pseudo_random() SHALL be the
same as the key defined in Section 8.2. same as the key defined in Section 8.2.
9. Security Considerations 9. Security Considerations
If the authentication exchange is performed without a strong security If the authentication exchange is performed without a strong security
layer (such as TLS with data confidentiality), then a passive layer (such as TLS with data confidentiality), then a passive
eavesdropper can gain sufficient information to mount an offline eavesdropper can gain sufficient information to mount an offline
dictionary or brute-force attack which can be used to recover the dictionary or brute-force attack that can be used to recover the
user's password. The amount of time necessary for this attack user's password. The amount of time necessary for this attack
depends on the cryptographic hash function selected, the strength of depends on the cryptographic hash function selected, the strength of
the password and the iteration count supplied by the server. An the password, and the iteration count supplied by the server. An
external security layer with strong encryption will prevent this external security layer with strong encryption will prevent this
attack. attack.
If the external security layer used to protect the SCRAM exchange If the external security layer used to protect the SCRAM exchange
uses an anonymous key exchange, then the SCRAM channel binding uses an anonymous key exchange, then the SCRAM channel binding
mechanism can be used to detect a man-in-the-middle attack on the 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. security layer and cause the authentication to fail as a result.
However, the man-in-the-middle attacker will have gained sufficient However, the man-in-the-middle attacker will have gained sufficient
information to mount an offline dictionary or brute-force attack. information to mount an offline dictionary or brute-force attack.
For this reason, SCRAM allows to increase the iteration count over For this reason, SCRAM allows to increase the iteration count over
time. (Note that a server that is only in posession of "StoredKey" time. (Note that a server that is only in possession of "StoredKey"
and "ServerKey" can't automatic increase the iteration count upon and "ServerKey" can't automatically increase the iteration count upon
successful authentication. Such increase would require resetting successful authentication. Such an increase would require resetting
user's password.) the user's password.)
If the authentication information is stolen from the authentication If the authentication information is stolen from the authentication
database, then an offline dictionary or brute-force attack can be database, then an offline dictionary or brute-force attack can be
used to recover the user's password. The use of salt mitigates this used to recover the user's password. The use of salt mitigates this
attack somewhat by requiring a separate attack on each password. attack somewhat by requiring a separate attack on each password.
Authentication mechanisms which protect against this attack are Authentication mechanisms that protect against this attack are
available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is
an example of such technology. The WG selected not to use EKE like an example of such technology. The WG elected not to use EKE like
mechanisms as basis for SCRAM. mechanisms as a basis for SCRAM.
If an attacker obtains the authentication information from the If an attacker obtains the authentication information from the
authentication repository and either eavesdrops on one authentication authentication repository and either eavesdrops on one authentication
exchange or impersonates a server, the attacker gains the ability to exchange or impersonates a server, the attacker gains the ability to
impersonate that user to all servers providing SCRAM access using the impersonate that user to all servers providing SCRAM access using the
same hash function, password, iteration count and salt. For this same hash function, password, iteration count, and salt. For this
reason, it is important to use randomly-generated salt values. reason, it is important to use randomly generated salt values.
SCRAM does not negotiate a hash function to use. Hash function SCRAM does not negotiate a hash function to use. Hash function
negotiation is left to the SASL mechanism negotiation. It is negotiation is left to the SASL mechanism negotiation. It is
important that clients be able to sort a locally available list of important that clients be able to sort a locally available list of
mechanisms by preference so that the client may pick the most mechanisms by preference so that the client may pick the appropriate
preferred of a server's advertised mechanism list. This preference mechanism to use from a server's advertised mechanism list. This
order is not specified here as it is a local matter. The preference preference order is not specified here as it is a local matter. The
order should include objective and subjective notions of mechanism preference order should include objective and subjective notions of
cryptographic strength (e.g., SCRAM with a successor to SHA-1 may be mechanism cryptographic strength (e.g., SCRAM with a successor to
preferred over SCRAM with SHA-1). SHA-1 may be preferred over SCRAM with SHA-1).
Note that to protect the SASL mechanism negotiation applications Note that to protect the SASL mechanism negotiation applications
normally must list the server mechs twice: once before and once after normally must list the server mechanisms twice: once before and once
authentication, the latter using security layers. Since SCRAM does after authentication, the latter using security layers. Since SCRAM
not provide security layers the only ways to protect the mechanism does not provide security layers, the only ways to protect the
negotiation are: a) use channel binding to an external channel, or b) mechanism negotiation are a) use channel binding to an external
use an external channel that authenticates a user-provided server channel, or b) use an external channel that authenticates a user-
name. provided server name.
SCRAM does not protect against downgrade attacks of channel binding SCRAM does not protect against downgrade attacks of channel binding
types. The complexities of negotiation a channel binding type, and types. The complexities of negotiating a channel binding type, and
handling down-grade attacks in that negotiation, was intentionally handling down-grade attacks in that negotiation, were intentionally
left out of scope for this document. left out of scope for this document.
A hostile server can perform a computational denial-of-service attack A hostile server can perform a computational denial-of-service attack
on clients by sending a big iteration count value. on clients by sending a big iteration count value.
See [RFC4086] for more information about generating randomness. See [RFC4086] for more information about generating randomness.
10. IANA Considerations 10. IANA Considerations
IANA is requested to add the following family of SASL mechanisms to IANA has added the following family of SASL mechanisms to the SASL
the SASL Mechanism registry established by [RFC4422]: Mechanism registry established by [RFC4422]:
To: iana@iana.org To: iana@iana.org
Subject: Registration of a new SASL family SCRAM Subject: Registration of a new SASL family SCRAM
SASL mechanism name (or prefix for the family): SCRAM-* SASL mechanism name (or prefix for the family): SCRAM-*
Security considerations: Section 7 of [RFCXXXX] Security considerations: Section 7 of [RFC5802]
Published specification (optional, recommended): [RFCXXXX] Published specification (optional, recommended): [RFC5802]
Person & email address to contact for further information: Person & email address to contact for further information:
IETF SASL WG <sasl@ietf.org> IETF SASL WG <sasl@ietf.org>
Intended usage: COMMON Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org> Owner/Change controller: IESG <iesg@ietf.org>
Note: Members of this family must be explicitly registered Note: Members of this family MUST be explicitly registered
using the "IETF Review" [RFC5226] registration procedure. using the "IETF Review" [RFC5226] registration procedure.
Reviews must be requested on the SASL WG mailing list. Reviews MUST be requested on the SASL mailing list
"IETF Review" [RFC5226] registration procedure MUST be used for
registering new mechanisms in this family. The SASL mailing list
<sasl@ietf.org> (or a successor designated by the responsible <sasl@ietf.org> (or a successor designated by the responsible
Security AD) MUST be used for soliciting reviews on such Security AD).
registrations.
Note to future SCRAM- mechanism designers: each new SCRAM- SASL Note to future SCRAM-mechanism designers: each new SCRAM-SASL
mechanism MUST be explicitly registered with IANA and MUST comply mechanism MUST be explicitly registered with IANA and MUST comply
with SCRAM- mechanism naming convention defined in Section 4 of this with SCRAM-mechanism naming convention defined in Section 4 of this
document. document.
IANA is requested to add the following entries to the SASL Mechanism IANA has added the following entries to the SASL Mechanism registry
registry established by [RFC4422]: established by [RFC4422]:
To: iana@iana.org To: iana@iana.org
Subject: Registration of a new SASL mechanism SCRAM-SHA-1 Subject: Registration of a new SASL mechanism SCRAM-SHA-1
SASL mechanism name (or prefix for the family): SCRAM-SHA-1 SASL mechanism name (or prefix for the family): SCRAM-SHA-1
Security considerations: Section 7 of [RFCXXXX] Security considerations: Section 7 of [RFC5802]
Published specification (optional, recommended): [RFCXXXX] Published specification (optional, recommended): [RFC5802]
Person & email address to contact for further information: Person & email address to contact for further information:
IETF SASL WG <sasl@ietf.org> IETF SASL WG <sasl@ietf.org>
Intended usage: COMMON Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org> Owner/Change controller: IESG <iesg@ietf.org>
Note: Note:
To: iana@iana.org To: iana@iana.org
Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS
SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS
Security considerations: Section 7 of [RFCXXXX] Security considerations: Section 7 of [RFC5802]
Published specification (optional, recommended): [RFCXXXX] Published specification (optional, recommended): [RFC5802]
Person & email address to contact for further information: Person & email address to contact for further information:
IETF SASL WG <sasl@ietf.org> IETF SASL WG <sasl@ietf.org>
Intended usage: COMMON Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org> Owner/Change controller: IESG <iesg@ietf.org>
Note: Note:
This document also requests IANA to assign a GSS-API mechanism OID Per this document, IANA has assigned a GSS-API mechanism OID for
for SCRAM-SHA-1 from the iso.org.dod.internet.security.mechanisms SCRAM-SHA-1 from the iso.org.dod.internet.security.mechanisms prefix
prefix (see "SMI Security for Mechanism Codes" registry). (see "SMI Security for Mechanism Codes" registry).
11. Acknowledgements 11. Acknowledgements
This document benefited from discussions on the SASL WG mailing list. This document benefited from discussions on the SASL WG mailing list.
The authors would like to specially thank Dave Cridland, Simon The authors would like to specially thank Dave Cridland, Simon
Josefsson, Jeffrey Hutzelman, Kurt Zeilenga, Pasi Eronen, Ben Josefsson, Jeffrey Hutzelman, Kurt Zeilenga, Pasi Eronen, Ben
Campbell and Peter Saint-Andre for their contributions to this Campbell, Peter Saint-Andre, and Tobias Markmann for their
document. contributions to this document. A special thank you to Simon
Josefsson for shepherding this document and for doing one of the
Appendix A. Other Authentication Mechanisms first implementations of this specification.
The DIGEST-MD5 [I-D.ietf-sasl-digest-to-historic] 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 [I-D.ietf-sasl-digest-to-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 [I-D.ietf-sasl-crammd5-to-historic].
The PLAIN [RFC4616] 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.
o The SASL mechanism has all modern SASL features: support for
internationalized usernames and passwords, support for passing of
authorization identity, support for channel bindings.
o The protocol supports mutual authentication.
o The authentication information stored in the authentication
database is not sufficient by itself to impersonate the client.
o The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies),
unless such other servers allow SCRAM authentication and use the
same salt and iteration count for the user.
o The mechanism is extensible, but [hopefully] not overengineered in
this respect.
o Easier to implement than DIGEST-MD5 in both clients and servers.
Appendix C. Internet-Draft Change History
(RFC Editor: Please delete this section and all subsections.)
Changes since -10
o Converted the source for this I-D to XML.
o Added text to make SCRAM compliant with the new GS2 design.
o Added text on channel binding negotiation.
o Added text on channel binding, including a reference to RFC5056.
o Added text on SCRAM as a GSS-API mechanism. This noted as not
relevant to SASL-only implementors -- the normative references for
SCRAM as a GSS-API mechanism are segregated as well.
Changes since -07
o Updated References.
o Clarified purpose of the m= attribute.
o Fixed a problem with authentication/authorization identity's ABNF
not allowing for some characters.
o Updated ABNF for nonce to show client-generated and server-
generated parts.
o Only register SCRAM-SHA-1 with IANA and require explicit
registrations of all other SCRAM- mechanisms.
Changes since -06
o Removed hash negotiation from SCRAM and turned it into a family of
SASL mechanisms.
o Start using "Hash Function Textual Names" IANA registry for SCRAM
mechanism naming.
o Fixed definition of Hi(str, salt, i) to be consistent with
[RFC2898].
o Clarified extensibility of SCRAM: added m= attribute (for future
mandatory extensions) and specified that all unrecognized
attributes must be ignored.
Changes since -05
o Changed the mandatory to implement hash algorithm to SHA-1 (as per
WG consensus).
o Added text about use of SASLPrep for username canonicalization/
validation.
o Clarified that authorization identity is canonicalized/verified
according to SASL protocol profile.
o Clarified that iteration count is per-user.
o Clarified how clients select the authentication function.
o Added IANA registration for the new mechanism.
o Added missing normative references (UTF-8, SASLPrep).
o Various editorial changes based on comments from Hallvard B
Furuseth, Nico William and Simon Josefsson.
Changes since -04
o Update Base64 and Security Glossary references.
o Add Formal Syntax section.
o Don't bother with "v=".
o Make MD5 mandatory to implement. Suggest i=128.
Changes since -03
o 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.
o Be hash agnostic, so MD5 can be replaced more easily.
o General simplification.
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, Hashing for Message Authentication", RFC 2104,
February 1997. February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
skipping to change at page 34, line 41 skipping to change at page 24, line 41
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006. Encodings", RFC 4648, October 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007. Channels", RFC 5056, November 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
12.2. Normative References for GSS-API implementors [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
[I-D.ietf-sasl-gs2] 12.2. Normative References for GSS-API Implementors
Josefsson, S. and N. Williams, "Using GSS-API Mechanisms
in SASL: The GS2 Mechanism Family", draft-ietf-sasl-gs2-12
(work in progress), April 2009.
[RFC2743] Linn, J., "Generic Security Service Application Program [RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000. Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005. Kerberos 5", RFC 3961, February 2005.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005. Encryption for Kerberos 5", RFC 3962, February 2005.
skipping to change at page 35, line 21 skipping to change at page 25, line 21
July 2005. July 2005.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006. Program Interface (GSS-API)", RFC 4401, February 2006.
[RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the [RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the
Kerberos V Generic Security Service Application Program Kerberos V Generic Security Service Application Program
Interface (GSS-API) Mechanism", RFC 4402, February 2006. Interface (GSS-API) Mechanism", RFC 4402, February 2006.
[tls-unique] [RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Zhu, L., "Registration of TLS unique channel binding Service Application Program Interface (GSS-API) Mechanisms
(generic)", IANA http://www.iana.org/assignments/ in Simple Authentication and Security Layer (SASL): The
channel-binding-types/tls-unique, July 2008. GS2 Mechanism Family", RFC 5801, July 2010.
12.3. Informative References 12.3. Informative References
[I-D.altman-tls-channel-bindings] [CRAMHISTORIC]
Altman, J., Williams, N., and L. Zhu, "Channel Bindings Zeilenga, K., "CRAM-MD5 to Historic", Work in Progress,
for TLS", draft-altman-tls-channel-bindings-07 (work in
progress), October 2009.
[I-D.ietf-sasl-crammd5-to-historic]
Zeilenga, K., "CRAM-MD5 to Historic",
draft-ietf-sasl-crammd5-to-historic-00 (work in progress),
November 2008. November 2008.
[I-D.ietf-sasl-digest-to-historic] [DIGESTHISTORIC]
Melnikov, A., "Moving DIGEST-MD5 to Historic", Melnikov, A., "Moving DIGEST-MD5 to Historic", Work
draft-ietf-sasl-digest-to-historic-00 (work in progress), in Progress, July 2008.
July 2008.
[I-D.melnikov-sasl-scram-ldap]
Melnikov, A., "LDAP schema for storing SCRAM secrets",
draft-melnikov-sasl-scram-ldap-02 (work in progress),
July 2009.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", "Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000. RFC 2865, June 2000.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000. Specification Version 2.0", RFC 2898, September 2000.
[RFC2945] Wu, T., "The SRP Authentication and Key Exchange System", [RFC2945] Wu, T., "The SRP Authentication and Key Exchange System",
RFC 2945, September 2000. RFC 2945, September 2000.
skipping to change at page 36, line 29 skipping to change at page 26, line 18
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007. RFC 4949, August 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5803] Melnikov, A., "Lightweight Directory Access Protocol
(LDAP) Schema for Storing Salted Challenge Response
Authentication Mechanism (SCRAM) Secrets", RFC 5803,
July 2010.
[tls-server-end-point] [tls-server-end-point]
Zhu, L., "Registration of TLS server end-point channel IANA, "Registration of TLS server end-point channel
bindings", IANA http://www.iana.org/assignments/ bindings", available from http://www.iana.org, June 2008.
channel-binding-types/tls-server-end-point, July 2008.
Appendix A. Other Authentication Mechanisms
The DIGEST-MD5 [DIGESTHISTORIC] 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 that led to the creation of SCRAM, see [DIGESTHISTORIC].
The CRAM-MD5 SASL mechanism, while widely deployed, also has 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, and support for
channel bindings. It also doesn't support server authentication.
For a more complete list of problems with CRAM-MD5, see
[CRAMHISTORIC].
The PLAIN [RFC4616] 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.
o The SASL mechanism has all modern SASL features: support for
internationalized usernames and passwords, support for passing of
authorization identity, and support for channel bindings.
o The protocol supports mutual authentication.
o The authentication information stored in the authentication
database is not sufficient by itself to impersonate the client.
o The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies),
unless such other servers allow SCRAM authentication and use the
same salt and iteration count for the user.
o The mechanism is extensible, but (hopefully) not over-engineered
in this respect.
o The mechanism is easier to implement than DIGEST-MD5 in both
clients and servers.
Authors' Addresses Authors' Addresses
Chris Newman Chris Newman
Sun Microsystems Oracle
1050 Lakes Drive 800 Royal Oaks
West Covina, CA 91790 Monrovia, CA 91016
USA USA
Email: chris.newman@sun.com EMail: chris.newman@oracle.com
Abhijit Menon-Sen Abhijit Menon-Sen
Oryx Mail Systems GmbH Oryx Mail Systems GmbH
Email: ams@toroid.org EMail: ams@toroid.org
Alexey Melnikov Alexey Melnikov
Isode Ltd Isode, Ltd.
Email: Alexey.Melnikov@isode.com EMail: Alexey.Melnikov@isode.com
Nicolas Williams Nicolas Williams
Sun Microsystems Oracle
5300 Riata Trace Ct 5300 Riata Trace Ct
Austin, TX 78727 Austin, TX 78727
USA USA
Email: Nicolas.Williams@sun.com EMail: Nicolas.Williams@oracle.com
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