HTTPBis A. Melnikov
Internet-Draft Isode Ltd
Intended status: Standards Track June 11, 2012
Expires: December 13, 2012
Salted Challenge Response (SCRAM) HTTP Authentication Mechanism
draft-melnikov-httpbis-scram-auth-00.txt
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 HTTP
Digest challenge response mechanism presently on the standards track
failed widespread deployment, and have had success only in limited
use.
This specification describes a family of HTTP 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.
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). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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."
This Internet-Draft will expire on December 13, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Conventions Used in This Document . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6
3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . . . . 7
4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . 9
5. SCRAM Authentication Exchange . . . . . . . . . . . . . . . 10
5.1. SCRAM Attributes . . . . . . . . . . . . . . . . . . . . . . 12
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 21
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
10. Design Motivations . . . . . . . . . . . . . . . . . . . . . 23
11. Internet-Draft Change History . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . . . 25
Author's Address . . . . . . . . . . . . . . . . . . . . . . 27
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1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Formal syntax is defined by [RFC5234] including the core rules
defined in Appendix B of [RFC5234].
Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server. If a single "C:" or "S:" label
applies to multiple lines, then the line breaks between those lines
are for editorial clarity only, and are not part of the actual
protocol exchange.
1.1. Terminology
This document uses several terms defined in [RFC4949] ("Internet
Security Glossary") including the following: authentication,
authentication exchange, authentication information, brute force,
challenge-response, cryptographic hash function, dictionary attack,
eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
one-way encryption function, password, replay attack and salt.
Readers not familiar with these terms should use that glossary as a
reference.
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, the "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
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] which converts
an octet string input to a textual output string which can be
easily displayed to a human. The use of base64 in SCRAM is
restricted to the canonical form with no whitespace.
o Octet: An 8-bit byte.
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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.
1.2. 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]).
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):
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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 four-octet encoding of the integer g,
most significant octet first.
Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and
with dkLen == output length of HMAC() == output length of H().
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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 [RFC5802]).
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.
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).
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3. SCRAM Algorithm Overview
The following is a description of a full HTTP SCRAM authentication
exchange. 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 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 username 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
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ServerKey.
The AuthMessage is computed by concatenating messages from the
authentication exchange. The format of these messages is defined in
Section 6.
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4. SCRAM Mechanism Names
A SCRAM mechanism name (authentication scheme) 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) .
For interoperability, all HTTP clients and servers supporting SCRAM
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].
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5. SCRAM Authentication Exchange
SCRAM is a HTTP Authentication mechanism whose client response
(<credentials-scram>) and server challenge (<challenge-scram>)
messages are text-based messages containing one or more attribute-
value pairs separated by commas. Each attribute has a one-letter
name, with the exception of a couple of attributes which are generic
to HTTP authentication, such as "realm" (and "sid"). The messages
and their attributes are described in Section 5.1, and defined in
Section 6.
challenge-scram = "SCRAM-SHA-1" 1*SP 1#auth-param
; Complies with <challenge> ABNF from I-D.ietf-httpbis-p7-auth.
; Included in the WWW-Authenticate header field.
credentials-scram = "SCRAM-SHA-1" 1*SP 1#auth-param
; Complies with <credentials> from I-D.ietf-httpbis-p7-auth
; Included in the Authorization header field.
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):
[...The server might have returned 401 Unauthorized first...]
C: GET /resource HTTP/1.1
C: Host: server.example.com
C: Authorization: SCRAM-SHA-1 realm="testrealm@host.com",
g=n,n=user,r=fyko+d2lbbFgONRv9qkxdawL
C: [...]
S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: SCRAM-SHA-1
realm="testrealm@host.com",sid=AAAABBBBCCCCDDDD,
r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,
i=4096
S: [...]
C: GET /resource HTTP/1.1
C: Host: server.example.com
C: Authorization: SCRAM-SHA-1 sid=AAAABBBBCCCCDDDD,
c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
C: [...]
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S: HTTP/1.1 200 Ok
S: Authentication-Info: SCRAM-SHA-1
sid=AAAABBBBCCCCDDDD,
v=rmF9pqV8S7suAoZWja4dJRkFsKQ=
S: [...Other header fields and resource body...]
"SCRAM-SHA-1" authentication starts with sending the "Authorization"
request header field defined by HTTP/1.1, Part 7
[I-D.ietf-httpbis-p7-auth] containing "SCRAM-SHA-1" authentication
scheme and the following attributes:
o A "realm" attribute MAY be included to indicate the scope of
protection in the manner described in HTTP/1.1, Part 7
[I-D.ietf-httpbis-p7-auth]. As specified in
[I-D.ietf-httpbis-p7-auth], the "realm" attribute MUST NOT appear
more than once.
o The client also includes the "client-first-message" containing:
* a header ("g" attribute) consisting of a flag indicating
whether channel binding is supported-but-not-used, not
supported, or used ;
* 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.
In response, the server sends WWW-Authenticate header field
containing: a unique session identifier (the "sid" attribute) plus
the "server-first-message" containing the user's iteration count i,
the user's salt, and the nonce with a concatenation of the client-
specified one with a server nonce.
The client then responds with another HTTP request with the
Authorization header field, which includes the "sid" attribute
received in the previous server response, together with "client-
final-message" data. The latter has the same nonce and a ClientProof
computed using the selected hash function (SHA-1) as explained
earlier.
The server verifies the nonce and the proof, and, finally, it
responds with a 200 HTTP response with the Authentication-Info header
field containing "server-final-message", concluding the
authentication exchange.
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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 the ABNF section for authoritative
reference.
o g: This attribute value consist of a flag indicating whether
channel binding is supported-but-not-used, not supported, or used
.
o n: This attribute specifies the name of the user whose password is
used for authentication. A client MUST include it in its first
message to the server.
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
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
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'=3D' respectively. If the server receives a username which
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 ). 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.
For SCRAM-SHA-1 authentication 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
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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.
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6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234]. "UTF8-2", "UTF8-3"
and "UTF8-4" non-terminal are defined in [RFC3629].
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.
cb-name = 1*(ALPHA / DIGIT / "." / "-")
;; See RFC 5056, Section 7.
;; E.g., "tls-server-end-point" or
;; "tls-unique".
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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 ","
;; GS2 header for SCRAM.
username = "n=" 1*(value-safe-char / "=2C" / "=3D")
;; Conforms to <value>.
;; 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 =
"g=" gs2-header client-first-message-bare
server-first-message =
[reserved-mext ","] nonce "," salt ","
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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
cbind-input = gs2-header [ cbind-data ]
;; cbind-data MUST be present for
;; gs2-cbind-flag of "p" and MUST be absent
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;; for "y" or "n".
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7. 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 which can be used to recover the
user's password. The amount of time necessary for this attack
depends on the cryptographic hash function selected, the strength of
the password and the iteration count supplied by the server. An
external security layer with strong encryption will prevent this
attack.
If the external security layer used to protect the SCRAM exchange
uses an anonymous key exchange, then the SCRAM channel binding
mechanism can be used to detect a man-in-the-middle attack on the
security layer and cause the authentication to fail as a result.
However, the man-in-the-middle attacker will have gained sufficient
information to mount an offline dictionary or brute-force attack.
For this reason, SCRAM allows to increase the iteration count over
time. (Note that a server that is only in posession of "StoredKey"
and "ServerKey" can't automatic increase the iteration count upon
successful authentication. Such increase would require resetting
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 which protect against this attack are
available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is
an example of such technology.
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 HTTP authentication 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 most
preferred of 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).
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SCRAM does not protect against downgrade attacks of channel binding
types. The complexities of negotiation a channel binding type, and
handling down-grade attacks in that negotiation, was 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.
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8. IANA Considerations
"IETF Review" [RFC5226] registration procedure MUST be used for
registering new mechanisms in the SCRAM- family. The Kitten WG
mailing list <kitten@ietf.org> (or a successor designated by the
responsible Security AD) MUST be used for soliciting reviews on such
registrations.
Note to future SCRAM- mechanism designers: each new SCRAM- HTTP
authentication mechanism MUST be explicitly registered with IANA and
MUST comply with SCRAM- mechanism naming convention defined in
Section 4 of this document.
IANA is requested to add the following entry to the Authentication
Scheme Registry defined in HTTP/1.1, Part 7
[I-D.ietf-httpbis-p7-auth]:
Authentication Scheme Name: SCRAM-SHA-1
Pointer to specification text: [[ this document ]]
Notes (optional): (none)
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9. Acknowledgements
This document benefited from discussions on the SASL and Kitten WG
mailing lists. The authors would like to specially thank co-authors
of [RFC5802] from which lots of text was copied.
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10. 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 HTTP authentication mechanism has all modern features: support
for internationalized usernames and passwords, 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 HTTP Digest in both clients and servers.
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11. Internet-Draft Change History
(RFC Editor: Please delete this section and all subsections.)
Changes since -00
o
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12. References
12.1. Normative References
[I-D.ietf-httpbis-p7-auth]
Fielding, R., Lafon, Y., and J. Reschke, "HTTP/1.1, part
7: Authentication", draft-ietf-httpbis-p7-auth-19 (work in
progress), March 2012.
[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.
[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. Informative References
[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
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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.
[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.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.
[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]
Zhu, L., "Registration of TLS server end-point channel
bindings", IANA http://www.iana.org/assignments/
channel-binding-types/tls-server-end-point, July 2008.
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Author's Address
Alexey Melnikov
Isode Ltd
Email: Alexey.Melnikov@isode.com
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