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Internet Engineering Task Force (IETF) C. Newman
Request for Comments: 5802 Oracle
Category: Standards Track A. Menon-Sen
ISSN: 2070-1721 Oryx Mail Systems GmbH
A. Melnikov
Isode, Ltd.
N. Williams
Oracle
July 2010
Salted Challenge Response Authentication Mechanism (SCRAM)
SASL and GSS-API Mechanisms
Abstract
The secure authentication mechanism most widely deployed and used by
Internet application protocols is the transmission of clear-text
passwords over a channel protected by Transport Layer Security (TLS).
There are some significant security concerns with that mechanism,
which could be addressed by the use of a challenge response
authentication mechanism protected by TLS. Unfortunately, the
challenge response mechanisms presently on the standards track all
fail to meet requirements necessary for widespread deployment, and
have had success only in limited use.
This specification describes a family of Simple Authentication and
Security Layer (SASL; RFC 4422) authentication mechanisms called the
Salted Challenge Response Authentication Mechanism (SCRAM), which
addresses the security concerns and meets the deployability
requirements. When used in combination with TLS or an equivalent
security layer, a mechanism from this family could improve the status
quo for application protocol authentication and provide a suitable
choice for a mandatory-to-implement mechanism for future application
protocol standards.
Newman, et al. Standards Track [Page 1]
RFC 5802 SCRAM July 2010
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(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.
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 (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Newman, et al. Standards Track [Page 2]
RFC 5802 SCRAM July 2010
Table of Contents
1. Introduction ....................................................4
2. Conventions Used in This Document ...............................5
2.1. Terminology ................................................5
2.2. Notation ...................................................6
3. SCRAM Algorithm Overview ........................................7
4. SCRAM Mechanism Names ...........................................8
5. SCRAM Authentication Exchange ...................................9
5.1. SCRAM Attributes ..........................................10
5.2. Compliance with SASL Mechanism Requirements ...............13
6. Channel Binding ................................................14
6.1. Default Channel Binding ...................................15
7. Formal Syntax ..................................................15
8. SCRAM as a GSS-API Mechanism ...................................19
8.1. GSS-API Principal Name Types for SCRAM ....................19
8.2. GSS-API Per-Message Tokens for SCRAM ......................20
8.3. GSS_Pseudo_random() for SCRAM .............................20
9. Security Considerations ........................................20
10. IANA Considerations ...........................................22
11. Acknowledgements ..............................................23
12. References ....................................................24
12.1. Normative References .....................................24
12.2. Normative References for GSS-API Implementors ............24
12.3. Informative References ...................................25
Appendix A. Other Authentication Mechanisms .......................27
Appendix B. Design Motivations ....................................27
Newman, et al. Standards Track [Page 3]
RFC 5802 SCRAM July 2010
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.
Newman, et al. Standards Track [Page 4]
RFC 5802 SCRAM July 2010
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Formal syntax is defined by [RFC5234] including the core rules
defined in Appendix B of [RFC5234].
Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server. If a single "C:" or "S:" label
applies to multiple lines, then the line breaks between those lines
are for editorial clarity only, and are not part of the actual
protocol exchange.
2.1. Terminology
This document uses several terms defined in [RFC4949] ("Internet
Security Glossary") including the following: authentication,
authentication exchange, authentication information, brute force,
challenge-response, cryptographic hash function, dictionary attack,
eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
one-way encryption function, password, replay attack, and salt.
Readers not familiar with these terms should use that glossary as a
reference.
Some clarifications and additional definitions follow:
o Authentication information: Information used to verify an identity
claimed by a SCRAM client. The authentication information for a
SCRAM identity consists of salt, iteration count, "StoredKey" and
"ServerKey" (as defined in the algorithm overview) for each
supported cryptographic hash function.
o Authentication database: The database used to look up the
authentication information associated with a particular identity.
For application protocols, LDAPv3 (see [RFC4510]) is frequently
Newman, et al. Standards Track [Page 5]
RFC 5802 SCRAM July 2010
used as the authentication database. For network-level protocols
such as PPP or 802.11x, the use of RADIUS [RFC2865] is more
common.
o Base64: An encoding mechanism defined in [RFC4648] that converts
an octet string input to a textual output string that can be
easily displayed to a human. The use of base64 in SCRAM is
restricted to the canonical form with no whitespace.
o Octet: An 8-bit byte.
o Octet string: A sequence of 8-bit bytes.
o Salt: A random octet string that is combined with a password
before applying a one-way encryption function. This value is used
to protect passwords that are stored in an authentication
database.
2.2. Notation
The pseudocode description of the algorithm uses the following
notations:
o ":=": The variable on the left-hand side represents the octet
string resulting from the expression on the right-hand side.
o "+": Octet string concatenation.
o "[ ]": A portion of an expression enclosed in "[" and "]" may not
be included in the result under some circumstances. See the
associated text for a description of those circumstances.
o Normalize(str): Apply the SASLprep profile [RFC4013] of the
"stringprep" algorithm [RFC3454] as the normalization algorithm to
a UTF-8 [RFC3629] encoded "str". The resulting string is also in
UTF-8. When applying SASLprep, "str" is treated as a "stored
strings", which means that unassigned Unicode codepoints are
prohibited (see Section 7 of [RFC3454]). Note that
implementations MUST either implement SASLprep or disallow use of
non US-ASCII Unicode codepoints in "str".
o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in
[RFC2104]) using the octet string represented by "key" as the key
and the octet string "str" as the input string. The size of the
result is the hash result size for the hash function in use. For
example, it is 20 octets for SHA-1 (see [RFC3174]).
Newman, et al. Standards Track [Page 6]
RFC 5802 SCRAM July 2010
o H(str): Apply the cryptographic hash function to the octet string
"str", producing an octet string as a result. The size of the
result depends on the hash result size for the hash function in
use.
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
this operator. The length of the output and each of the two
inputs will be the same for this use.
o Hi(str, salt, i):
U1 := HMAC(str, salt + INT(1))
U2 := HMAC(str, U1)
...
Ui-1 := HMAC(str, Ui-2)
Ui := HMAC(str, Ui-1)
Hi := U1 XOR U2 XOR ... XOR Ui
where "i" is the iteration count, "+" is the string concatenation
operator, and INT(g) is a 4-octet encoding of the integer g, most
significant octet first.
Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the
pseudorandom function (PRF) and with dkLen == output length of
HMAC() == output length of H().
3. SCRAM Algorithm Overview
The following is a description of a full, uncompressed SASL SCRAM
authentication exchange. Nothing in SCRAM prevents either sending
the client-first message with the SASL authentication request defined
by an application protocol ("initial client response"), or sending
the server-final message as additional data of the SASL outcome of
authentication exchange defined by an application protocol. See
[RFC4422] for more details.
Note that this section omits some details, such as client and server
nonces. See Section 5 for more details.
To begin with, the SCRAM client is in possession of a username and
password (*) (or a ClientKey/ServerKey, or SaltedPassword). It sends
the username to the server, which retrieves the corresponding
authentication information, i.e., a salt, StoredKey, ServerKey, and
the iteration count i. (Note that a server implementation may choose
Newman, et al. Standards Track [Page 7]
RFC 5802 SCRAM July 2010
to use the same iteration count for all accounts.) The server sends
the salt and the iteration count to the client, which then computes
the following values and sends a ClientProof to the server:
(*) Note that both the username and the password MUST be encoded in
UTF-8 [RFC3629].
Informative Note: Implementors are encouraged to create test cases
that use both usernames and passwords with non-ASCII codepoints. In
particular, it's useful to test codepoints whose "Unicode
Normalization Form C" and "Unicode Normalization Form KC" are
different. Some examples of such codepoints include Vulgar Fraction
One Half (U+00BD) and Acute Accent (U+00B4).
SaltedPassword := Hi(Normalize(password), salt, i)
ClientKey := HMAC(SaltedPassword, "Client Key")
StoredKey := H(ClientKey)
AuthMessage := client-first-message-bare + "," +
server-first-message + "," +
client-final-message-without-proof
ClientSignature := HMAC(StoredKey, AuthMessage)
ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, "Server Key")
ServerSignature := HMAC(ServerKey, AuthMessage)
The server authenticates the client by computing the ClientSignature,
exclusive-ORing that with the ClientProof to recover the ClientKey
and verifying the correctness of the ClientKey by applying the hash
function and comparing the result to the StoredKey. If the ClientKey
is correct, this proves that the client has access to the user's
password.
Similarly, the client authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If
the two are equal, it proves that the server had access to the user's
ServerKey.
The AuthMessage is computed by concatenating messages from the
authentication exchange. The format of these messages is defined in
Section 7.
4. SCRAM Mechanism Names
A SCRAM mechanism name is a string "SCRAM-" followed by the
uppercased name of the underlying hash function taken from the IANA
"Hash Function Textual Names" registry (see http://www.iana.org),
optionally followed by the suffix "-PLUS" (see below). Note that
SASL mechanism names are limited to 20 octets, which means that only
Newman, et al. Standards Track [Page 8]
RFC 5802 SCRAM July 2010
hash function names with lengths shorter or equal to 9 octets
(20-length("SCRAM-")-length("-PLUS") can be used. For cases when the
underlying hash function name is longer than 9 octets, an alternative
9-octet (or shorter) name can be used to construct the corresponding
SCRAM mechanism name, as long as this alternative name doesn't
conflict with any other hash function name from the IANA "Hash
Function Textual Names" registry. In order to prevent future
conflict, such alternative names SHOULD be registered in the IANA
"Hash Function Textual Names" registry.
For interoperability, all SCRAM clients and servers MUST implement
the SCRAM-SHA-1 authentication mechanism, i.e., an authentication
mechanism from the SCRAM family that uses the SHA-1 hash function as
defined in [RFC3174].
The "-PLUS" suffix is used only when 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
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
and SCRAM-SHA-1-PLUS). If the server does not support channel
binding, then it will advertise only the "bare" version of the
mechanism (e.g., only SCRAM-SHA-1). The "-PLUS" exists to allow
negotiation of the use of channel binding. See Section 6.
5. SCRAM Authentication Exchange
SCRAM is a SASL mechanism whose client response and server challenge
messages are text-based messages containing one or more attribute-
value pairs separated by commas. Each attribute has a one-letter
name. The messages and their attributes are described in
Section 5.1, and defined in Section 7.
SCRAM is a client-first SASL mechanism (see [RFC4422], Section 5,
item 2a), and returns additional data together with a server's
indication of a successful outcome.
This is a simple example of a SCRAM-SHA-1 authentication exchange
when the client doesn't support channel bindings (username 'user' and
password 'pencil' are used):
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=
Newman, et al. Standards Track [Page 9]
RFC 5802 SCRAM July 2010
First, the client sends the "client-first-message" containing:
o a GS2 header consisting of a flag indicating whether channel
binding is supported-but-not-used, not supported, or used, and an
optional SASL authorization identity;
o SCRAM username and a random, unique nonce attributes.
Note that the client's first message will always start with "n", "y",
or "p"; otherwise, the message is invalid and authentication MUST
fail. This is important, as it allows for GS2 extensibility (e.g.,
to add support for security layers).
In response, the server sends a "server-first-message" containing the
user's iteration count i and the user's salt, and appends its own
nonce to the client-specified one.
The client then responds by sending a "client-final-message" with the
same nonce and a ClientProof computed using the selected hash
function as explained earlier.
The server verifies the nonce and the proof, verifies that the
authorization identity (if supplied by the client in the first
message) is authorized to act as the authentication identity, and,
finally, it responds with a "server-final-message", concluding the
authentication exchange.
The client then authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If
the two are different, the client MUST consider the authentication
exchange to be unsuccessful, and it might have to drop the
connection.
5.1. SCRAM Attributes
This section describes the permissible attributes, their use, and the
format of their values. All attribute names are single US-ASCII
letters and are case-sensitive.
Note that the order of attributes in client or server messages is
fixed, with the exception of extension attributes (described by the
"extensions" ABNF production), which can appear in any order in the
designated positions. See Section 7 for authoritative reference.
o a: This is an optional attribute, and is part of the GS2 [RFC5801]
bridge between the GSS-API and SASL. This attribute specifies an
authorization identity. A client may include it in its first
message to the server if it wants to authenticate as one user, but
Newman, et al. Standards Track [Page 10]
RFC 5802 SCRAM July 2010
subsequently act as a different user. This is typically used by
an administrator to perform some management task on behalf of
another user, or by a proxy in some situations.
Upon the receipt of this value the server verifies its
correctness according to the used SASL protocol profile.
Failed verification results in failed authentication exchange.
If this attribute is omitted (as it normally would be), the
authorization identity is assumed to be derived from the
username specified with the (required) "n" attribute.
The server always authenticates the user specified by the "n"
attribute. If the "a" attribute specifies a different user,
the server associates that identity with the connection after
successful authentication and authorization checks.
The syntax of this field is the same as that of the "n" field
with respect to quoting of '=' and ','.
o n: This attribute specifies the name of the user whose password is
used for authentication (a.k.a. "authentication identity"
[RFC4422]). A client MUST include it in its first message to the
server. If the "a" attribute is not specified (which would
normally be the case), this username is also the identity that
will be associated with the connection subsequent to
authentication and authorization.
Before sending the username to the server, the client SHOULD
prepare the username using the "SASLprep" profile [RFC4013] of
the "stringprep" algorithm [RFC3454] treating it as a query
string (i.e., unassigned Unicode code points are allowed). If
the preparation of the username fails or results in an empty
string, the client SHOULD abort the authentication exchange
(*).
(*) An interactive client can request a repeated entry of the
username value.
Upon receipt of the username by the server, the server MUST
either prepare it using the "SASLprep" profile [RFC4013] of the
"stringprep" algorithm [RFC3454] treating it as a query string
(i.e., unassigned Unicode codepoints are allowed) or otherwise
be prepared to do SASLprep-aware string comparisons and/or
index lookups. If the preparation of the username fails or
results in an empty string, the server SHOULD abort the
Newman, et al. Standards Track [Page 11]
RFC 5802 SCRAM July 2010
authentication exchange. Whether or not the server prepares
the username using "SASLprep", it MUST use it as received in
hash calculations.
The characters ',' or '=' in usernames are sent as '=2C' and
'=3D' respectively. If the server receives a username that
contains '=' not followed by either '2C' or '3D', then the
server MUST fail the authentication.
o m: This attribute is reserved for future extensibility. In this
version of SCRAM, its presence in a client or a server message
MUST cause authentication failure when the attribute is parsed by
the other end.
o r: This attribute specifies a sequence of random printable ASCII
characters excluding ',' (which forms the nonce used as input to
the hash function). No quoting is applied to this string. As
described earlier, the client supplies an initial value in its
first message, and the server augments that value with its own
nonce in its first response. It is important that this value be
different for each authentication (see [RFC4086] for more details
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
nonce it initially specified. The server MUST verify that the
nonce sent by the client in the second message is the same as the
one sent by the server in its first message.
o c: This REQUIRED attribute specifies the base64-encoded GS2 header
and channel binding data. It is sent by the client in its second
authentication message. The attribute data consist of:
* the GS2 header from the client's first message (recall that the
GS2 header contains a channel binding flag and an optional
authzid). This header is going to include channel binding type
prefix (see [RFC5056]), if and only if the client is using
channel binding;
* followed by the external channel's channel binding data, if and
only if the client is using channel binding.
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
message to the client.
o i: This attribute specifies an iteration count for the selected
hash function and user, and MUST be sent by the server along with
the user's salt.
Newman, et al. Standards Track [Page 12]
RFC 5802 SCRAM July 2010
For the SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism, servers
SHOULD announce a hash iteration-count of at least 4096. Note
that a client implementation MAY cache ClientKey&ServerKey (or
just SaltedPassword) for later reauthentication to the same
service, as it is likely that the server is going to advertise
the same salt value upon reauthentication. This might be
useful for mobile clients where CPU usage is a concern.
o p: This attribute specifies a base64-encoded ClientProof. The
client computes this value as described in the overview and sends
it to the server.
o v: This attribute specifies a base64-encoded ServerSignature. It
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
authentication information. This value is computed as explained
in the overview.
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
specified in Section 5 of [RFC4422].
1) "SCRAM-SHA-1" and "SCRAM-SHA-1-PLUS".
2a) SCRAM is a client-first mechanism.
Newman, et al. Standards Track [Page 13]
RFC 5802 SCRAM July 2010
2b) SCRAM sends additional data with success.
3) SCRAM is capable of transferring authorization identities from
the client to the server.
4) SCRAM does not offer any security layers (SCRAM offers channel
binding instead).
5) SCRAM has a hash protecting the authorization identity.
6. Channel Binding
SCRAM supports channel binding to external secure channels, such as
TLS. Clients and servers may or may not support channel binding,
therefore the use of channel binding is negotiable. SCRAM does not
provide security layers, however, therefore it is imperative that
SCRAM provide integrity protection for the negotiation of channel
binding.
Use of channel binding is negotiated as follows:
o Servers that support the use of channel binding SHOULD advertise
both the non-PLUS (SCRAM-<hash-function>) and PLUS-variant (SCRAM-
<hash-function>-PLUS) mechanism name. If the server cannot
support channel binding, it SHOULD advertise only the non-PLUS-
variant. If the server would never succeed in the authentication
of the non-PLUS-variant due to policy reasons, it MUST advertise
only the PLUS-variant.
o If the client supports channel binding and the server does not
appear to (i.e., the client did not see the -PLUS name advertised
by the server), then the client MUST NOT use an "n" gs2-cbind-
flag.
o Clients that support mechanism negotiation and channel binding
MUST use a "p" gs2-cbind-flag when the server offers the PLUS-
variant of the desired GS2 mechanism.
o If the client does not support channel binding, then it MUST use
an "n" gs2-cbind-flag. Conversely, if the client requires the use
of channel binding then it MUST use a "p" gs2-cbind-flag. Clients
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
channel binding flag (gs2-cbind-flag).
Newman, et al. Standards Track [Page 14]
RFC 5802 SCRAM July 2010
* If the flag is set to "y" and the server supports channel
binding, the server MUST fail authentication. This is because
if the client sets the channel binding flag to "y", then the
client must have believed that the server did not support
channel binding -- if the server did in fact support channel
binding, then this is an indication that there has been a
downgrade attack (e.g., an attacker changed the server's
mechanism list to exclude the -PLUS suffixed SCRAM mechanism
name(s)).
* If the channel binding flag was "p" and the server does not
support the indicated channel binding type, then the server
MUST fail authentication.
The server MUST always validate the client's "c=" field. The server
does this by constructing the value of the "c=" attribute and then
checking that it matches the client's c= attribute value.
For more discussions of channel bindings, and the syntax of channel
binding data for various security protocols, see [RFC5056].
6.1. Default Channel Binding
A default channel binding type agreement process for all SASL
application protocols that do not provide their own channel binding
type agreement is provided as follows.
'tls-unique' is the default channel binding type for any application
that doesn't specify one.
Servers MUST implement the "tls-unique" [RFC5929] channel binding
type, if they implement any channel binding. Clients SHOULD
implement the "tls-unique" [RFC5929] channel binding type, if they
implement any channel binding. Clients and servers SHOULD choose the
highest-layer/innermost end-to-end TLS channel as the channel to
which to bind.
Servers MUST choose the channel binding type indicated by the client,
or fail authentication if they don't support it.
7. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3",
and "UTF8-4" non-terminal are defined in [RFC3629].
Newman, et al. Standards Track [Page 15]
RFC 5802 SCRAM July 2010
ALPHA = <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)>
UTF8-3 = <as defined in RFC 3629 (STD 63)>
UTF8-4 = <as defined in RFC 3629 (STD 63)>
attr-val = ALPHA "=" value
;; Generic syntax of any attribute sent
;; by server or client
value = 1*value-char
value-safe-char = %x01-2B / %x2D-3C / %x3E-7F /
UTF8-2 / UTF8-3 / UTF8-4
;; UTF8-char except NUL, "=", and ",".
value-char = value-safe-char / "="
printable = %x21-2B / %x2D-7E
;; Printable ASCII except ",".
;; Note that any "printable" is also
;; a valid "value".
base64-char = ALPHA / DIGIT / "/" / "+"
base64-4 = 4base64-char
base64-3 = 3base64-char "="
base64-2 = 2base64-char "=="
base64 = *base64-4 [base64-3 / base64-2]
posit-number = %x31-39 *DIGIT
;; A positive number.
saslname = 1*(value-safe-char / "=2C" / "=3D")
;; Conforms to <value>.
authzid = "a=" saslname
;; Protocol specific.
cb-name = 1*(ALPHA / DIGIT / "." / "-")
;; See RFC 5056, Section 7.
;; E.g., "tls-server-end-point" or
;; "tls-unique".
Newman, et al. Standards Track [Page 16]
RFC 5802 SCRAM July 2010
gs2-cbind-flag = ("p=" cb-name) / "n" / "y"
;; "n" -> client doesn't support channel binding.
;; "y" -> client does support channel binding
;; but thinks the server does not.
;; "p" -> client requires channel binding.
;; The selected channel binding follows "p=".
gs2-header = gs2-cbind-flag "," [ authzid ] ","
;; GS2 header for SCRAM
;; (the actual GS2 header includes an optional
;; flag to indicate that the GSS mechanism is not
;; "standard", but since SCRAM is "standard", we
;; don't include that flag).
username = "n=" saslname
;; Usernames are prepared using SASLprep.
reserved-mext = "m=" 1*(value-char)
;; Reserved for signaling mandatory extensions.
;; The exact syntax will be defined in
;; the future.
channel-binding = "c=" base64
;; base64 encoding of cbind-input.
proof = "p=" base64
nonce = "r=" c-nonce [s-nonce]
;; Second part provided by server.
c-nonce = printable
s-nonce = printable
salt = "s=" base64
verifier = "v=" base64
;; base-64 encoded ServerSignature.
iteration-count = "i=" posit-number
;; A positive number.
client-first-message-bare =
[reserved-mext ","]
username "," nonce ["," extensions]
client-first-message =
gs2-header client-first-message-bare
Newman, et al. Standards Track [Page 17]
RFC 5802 SCRAM July 2010
server-first-message =
[reserved-mext ","] nonce "," salt ","
iteration-count ["," extensions]
client-final-message-without-proof =
channel-binding "," nonce [","
extensions]
client-final-message =
client-final-message-without-proof "," proof
server-error = "e=" server-error-value
server-error-value = "invalid-encoding" /
"extensions-not-supported" / ; unrecognized 'm' value
"invalid-proof" /
"channel-bindings-dont-match" /
"server-does-support-channel-binding" /
; server does not support channel binding
"channel-binding-not-supported" /
"unsupported-channel-binding-type" /
"unknown-user" /
"invalid-username-encoding" /
; invalid username encoding (invalid UTF-8 or
; SASLprep failed)
"no-resources" /
"other-error" /
server-error-value-ext
; Unrecognized errors should be treated as "other-error".
; In order to prevent information disclosure, the server
; may substitute the real reason with "other-error".
server-error-value-ext = value
; Additional error reasons added by extensions
; to this document.
server-final-message = (server-error / verifier)
["," extensions]
extensions = attr-val *("," attr-val)
;; All extensions are optional,
;; i.e., unrecognized attributes
;; not defined in this document
;; MUST be ignored.
cbind-data = 1*OCTET
Newman, et al. Standards Track [Page 18]
RFC 5802 SCRAM July 2010
cbind-input = gs2-header [ cbind-data ]
;; cbind-data MUST be present for
;; gs2-cbind-flag of "p" and MUST be absent
;; for "y" or "n".
8. SCRAM as a GSS-API Mechanism
This section and its sub-sections and all normative references of it
not referenced elsewhere in this document are INFORMATIONAL for SASL
implementors, but they are NORMATIVE for GSS-API implementors.
SCRAM is actually also a GSS-API mechanism. The messages are the
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,
and b) the RFC2743 section 3.1 initial context token header is
prefixed to the client's first authentication message (context
token).
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
SCRAM does not explicitly name acceptor principals. However, the use
of acceptor principal names to find or prompt for passwords is
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:
GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for
SCRAM.
There is no name canonicalization procedure for SCRAM beyond applying
SASLprep as described in Section 5.1.
The query, display, and exported name syntaxes for SCRAM principal
names are all the same. There are no SCRAM-specific name syntaxes
(SCRAM initiator principal names are free-form); -- applications
should use generic GSS-API name types such as GSS_C_NT_USER_NAME and
Newman, et al. Standards Track [Page 19]
RFC 5802 SCRAM July 2010
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
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
Section 4.2 and sub-sections), using the Kerberos V "aes128-cts-hmac-
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
least significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API
session key" || ClientKey || AuthMessage). "Specific keys" are then
derived as usual as described in Section 2 of [RFC4121], [RFC3961],
and [RFC3962].
The terms "protocol key" and "specific key" are Kerberos V5 terms
[RFC3961].
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
context token.
8.3. GSS_Pseudo_random() for SCRAM
The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for
the Kerberos V GSS-API mechanism [RFC4402]. There is no acceptor-
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().
The protocol key to be used for the GSS_Pseudo_random() SHALL be the
same as the key defined in Section 8.2.
9. Security Considerations
If the authentication exchange is performed without a strong security
layer (such as TLS with data confidentiality), then a passive
eavesdropper can gain sufficient information to mount an offline
dictionary or brute-force attack that can be used to recover the
user's password. The amount of time necessary for this attack
depends on the cryptographic hash function selected, the strength of
the password, and the iteration count supplied by the server. An
external security layer with strong encryption will prevent this
attack.
Newman, et al. Standards Track [Page 20]
RFC 5802 SCRAM July 2010
If the external security layer used to protect the SCRAM exchange
uses an anonymous key exchange, then the SCRAM channel binding
mechanism can be used to detect a man-in-the-middle attack on the
security layer and cause the authentication to fail as a result.
However, the man-in-the-middle attacker will have gained sufficient
information to mount an offline dictionary or brute-force attack.
For this reason, SCRAM allows to increase the iteration count over
time. (Note that a server that is only in possession of "StoredKey"
and "ServerKey" can't automatically increase the iteration count upon
successful authentication. Such an increase would require resetting
the user's password.)
If the authentication information is stolen from the authentication
database, then an offline dictionary or brute-force attack can be
used to recover the user's password. The use of salt mitigates this
attack somewhat by requiring a separate attack on each password.
Authentication mechanisms that protect against this attack are
available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is
an example of such technology. The WG elected not to use EKE like
mechanisms as a basis for SCRAM.
If an attacker obtains the authentication information from the
authentication repository and either eavesdrops on one authentication
exchange or impersonates a server, the attacker gains the ability to
impersonate that user to all servers providing SCRAM access using the
same hash function, password, iteration count, and salt. For this
reason, it is important to use randomly generated salt values.
SCRAM does not negotiate a hash function to use. Hash function
negotiation is left to the SASL mechanism negotiation. It is
important that clients be able to sort a locally available list of
mechanisms by preference so that the client may pick the appropriate
mechanism to use from a server's advertised mechanism list. This
preference order is not specified here as it is a local matter. The
preference order should include objective and subjective notions of
mechanism cryptographic strength (e.g., SCRAM with a successor to
SHA-1 may be preferred over SCRAM with SHA-1).
Note that to protect the SASL mechanism negotiation applications
normally must list the server mechanisms twice: once before and once
after authentication, the latter using security layers. Since SCRAM
does not provide security layers, the only ways to protect the
mechanism negotiation are a) use channel binding to an external
channel, or b) use an external channel that authenticates a user-
provided server name.
Newman, et al. Standards Track [Page 21]
RFC 5802 SCRAM July 2010
SCRAM does not protect against downgrade attacks of channel binding
types. The complexities of negotiating a channel binding type, and
handling down-grade attacks in that negotiation, were intentionally
left out of scope for this document.
A hostile server can perform a computational denial-of-service attack
on clients by sending a big iteration count value.
See [RFC4086] for more information about generating randomness.
10. IANA Considerations
IANA has added the following family of SASL mechanisms to the SASL
Mechanism registry established by [RFC4422]:
To: iana@iana.org
Subject: Registration of a new SASL family SCRAM
SASL mechanism name (or prefix for the family): SCRAM-*
Security considerations: Section 7 of [RFC5802]
Published specification (optional, recommended): [RFC5802]
Person & email address to contact for further information:
IETF SASL WG <sasl@ietf.org>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note: Members of this family MUST be explicitly registered
using the "IETF Review" [RFC5226] registration procedure.
Reviews MUST be requested on the SASL mailing list
<sasl@ietf.org> (or a successor designated by the responsible
Security AD).
Note to future SCRAM-mechanism designers: each new SCRAM-SASL
mechanism MUST be explicitly registered with IANA and MUST comply
with SCRAM-mechanism naming convention defined in Section 4 of this
document.
Newman, et al. Standards Track [Page 22]
RFC 5802 SCRAM July 2010
IANA has added the following entries to the SASL Mechanism registry
established by [RFC4422]:
To: iana@iana.org
Subject: Registration of a new SASL mechanism SCRAM-SHA-1
SASL mechanism name (or prefix for the family): SCRAM-SHA-1
Security considerations: Section 7 of [RFC5802]
Published specification (optional, recommended): [RFC5802]
Person & email address to contact for further information:
IETF SASL WG <sasl@ietf.org>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note:
To: iana@iana.org
Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS
SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS
Security considerations: Section 7 of [RFC5802]
Published specification (optional, recommended): [RFC5802]
Person & email address to contact for further information:
IETF SASL WG <sasl@ietf.org>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note:
Per this document, IANA has assigned a GSS-API mechanism OID for
SCRAM-SHA-1 from the iso.org.dod.internet.security.mechanisms prefix
(see "SMI Security for Mechanism Codes" registry).
11. Acknowledgements
This document benefited from discussions on the SASL WG mailing list.
The authors would like to specially thank Dave Cridland, Simon
Josefsson, Jeffrey Hutzelman, Kurt Zeilenga, Pasi Eronen, Ben
Campbell, Peter Saint-Andre, and Tobias Markmann for their
contributions to this document. A special thank you to Simon
Josefsson for shepherding this document and for doing one of the
first implementations of this specification.
Newman, et al. Standards Track [Page 23]
RFC 5802 SCRAM July 2010
12. References
12.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
12.2. Normative References for GSS-API Implementors
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
Newman, et al. Standards Track [Page 24]
RFC 5802 SCRAM July 2010
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006.
[RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the
Kerberos V Generic Security Service Application Program
Interface (GSS-API) Mechanism", RFC 4402, February 2006.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, July 2010.
12.3. Informative References
[CRAMHISTORIC]
Zeilenga, K., "CRAM-MD5 to Historic", Work in Progress,
November 2008.
[DIGESTHISTORIC]
Melnikov, A., "Moving DIGEST-MD5 to Historic", Work
in Progress, July 2008.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
[RFC2945] Wu, T., "The SRP Authentication and Key Exchange System",
RFC 2945, September 2000.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510,
June 2006.
Newman, et al. Standards Track [Page 25]
RFC 5802 SCRAM July 2010
[RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616, August 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(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]
IANA, "Registration of TLS server end-point channel
bindings", available from http://www.iana.org, June 2008.
Newman, et al. Standards Track [Page 26]
RFC 5802 SCRAM July 2010
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.
Newman, et al. Standards Track [Page 27]
RFC 5802 SCRAM July 2010
Authors' Addresses
Chris Newman
Oracle
800 Royal Oaks
Monrovia, CA 91016
USA
EMail: chris.newman@oracle.com
Abhijit Menon-Sen
Oryx Mail Systems GmbH
EMail: ams@toroid.org
Alexey Melnikov
Isode, Ltd.
EMail: Alexey.Melnikov@isode.com
Nicolas Williams
Oracle
5300 Riata Trace Ct
Austin, TX 78727
USA
EMail: Nicolas.Williams@oracle.com
Newman, et al. Standards Track [Page 28]