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NETWORK WORKING GROUP A. Menon-Sen
Internet-Draft Oryx Mail Systems GmbH
Intended status: Standards Track A. Melnikov
Expires: September 24, 2009 Isode Ltd
C. Newman
N. Williams
Sun Microsystems
March 23, 2009
Salted Challenge Response (SCRAM) SASL Mechanism
draft-newman-auth-scram-11.txt
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
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
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 24, 2009.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Menon-Sen, et al. Expires September 24, 2009 [Page 1]
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Abstract
The secure authentication mechanism most widely deployed and used by
Internet application protocols is the transmission of clear-text
passwords over a channel protected by Transport Layer Security (TLS).
There are some significant security concerns with that mechanism,
which could be addressed by the use of a challenge response
authentication mechanism protected by TLS. Unfortunately, the
challenge response mechanisms presently on the standards track all
fail to meet requirements necessary for widespread deployment, and
have had success only in limited use.
This specification describes a family of authentication mechanisms
called the Salted Challenge Response Authentication Mechanism
(SCRAM), which addresses the security concerns and meets the
deployability requirements. When used in combination with TLS or an
equivalent security layer, a mechanism from this family could improve
the status-quo for application protocol authentication and provide a
suitable choice for a mandatory-to-implement mechanism for future
application protocol standards.
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Table of Contents
1. Conventions Used in This Document . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 4
1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . 7
3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . 9
4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . 10
5. SCRAM Authentication Exchange . . . . . . . . . . . . 11
5.1. SCRAM Attributes . . . . . . . . . . . . . . . . . . . 12
6. Channel Binding . . . . . . . . . . . . . . . . . . . 15
6.1. Channel Binding to TLS Channels . . . . . . . . . . . 16
7. Formal Syntax . . . . . . . . . . . . . . . . . . . . 17
8. SCRAM as a GSS-API Mechanism . . . . . . . . . . . . . 20
8.1. GSS-API Principal Name Types for SCRAM . . . . . . . . 20
8.2. GSS-API Per-Message Tokens for SCRAM . . . . . . . . . 20
8.3. GSS_Pseudo_random() for SCRAM . . . . . . . . . . . . 21
9. Security Considerations . . . . . . . . . . . . . . . 22
10. IANA Considerations . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . 25
Appendix A. Other Authentication Mechanisms . . . . . . . . . . . 26
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . . 27
Appendix C. SCRAM Examples and Internet-Draft Change History . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . 31
12.2. Normative References for GSS-API implementors . . . . 31
12.3. Informative References . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . 34
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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 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.
o Octet string: A sequence of 8-bit bytes.
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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 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):
U0 := HMAC(str, salt + INT(1))
U1 := HMAC(str, U0)
U2 := HMAC(str, U1)
...
Ui-1 := HMAC(str, Ui-2)
Ui := HMAC(str, Ui-1)
Hi := U0 XOR U1 XOR U2 XOR ... XOR Ui
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where "i" is the iteration count, "+" is the string concatenation
operator and INT(g) is a four-octet encoding of the integer g,
most significant octet first.
o This 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. 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 mechanism does not presently include
negotiation of a security layer. It is intended to be used with an
external security layer such as that provided by TLS or SSH, 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" [ref-needed]. This
means that SCRAM is actually both, a GSS-API and SASL 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 A standard attribute is defined to enable storage of the
authentication information in LDAPv3 (see [RFC4510]).
o Mutual authentication is supported, but only the client is named
(i.e., the server has no name).
For an in-depth discussion of why other challenge response mechanisms
are not considered sufficient, see appendix A. For more information
about the motivations behind the design of this mechanism, see
appendix B.
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Comments regarding this draft may be sent either to the
ietf-sasl@imc.org mailing list or to the authors.
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3. SCRAM Algorithm Overview
Note that this section omits some details, such as client and server
nonces. See Section 5 for more details.
To begin with, the client is in possession of a username and
password. It sends the username to the server, which retrieves the
corresponding authentication information, i.e. a salt, StoredKey,
ServerKey and the iteration count i. (Note that a server
implementation may chose to use the same iteration count for all
account.) The server sends the salt and the iteration count to the
client, which then computes the following values and sends a
ClientProof to the server:
SaltedPassword := Hi(password, salt)
ClientKey := H(SaltedPassword)
StoredKey := H(ClientKey)
AuthMessage := client-first-message + "," +
server-first-message + "," +
client-final-message-without-proof
ClientSignature := HMAC(StoredKey, AuthMessage)
ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, salt)
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.
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4. SCRAM Mechanism Names
A SCRAM mechanism name is a string "SCRAM-HMAC-" followed by the
uppercased name of the underlying hashed function taken from the IANA
"Hash Function Textual Names" registry (see http://www.iana.org),
optionally followed by the suffix "-PLUS" (see below)..
For interoperability, all SCRAM clients and servers MUST implement
the SCRAM-HMAC-SHA-1 authentication mechanism, i.e. an authentication
mechanism from the SCRAM family that uses the SHA-1 hash function as
defined in [RFC3174].
The "-PLUS" suffix is used only when the server supports channel
binding to the external channel. In this case the server will
advertise both, SCRAM-HMAC-SHA-1 and SCRAM-HMAC-SHA-1-PLUS, otherwise
the server will advertise only SCRAM-HMAC-SHA-1. The "-PLUS" exists
to allow negotiation of the use of channel binding. See Section 6.
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5. SCRAM Authentication Exchange
SCRAM is a text protocol where the client and server exchange
messages containing one or more attribute-value pairs separated by
commas. Each attribute has a one-letter name. The messages and
their attributes are described in Section 5.1, and defined in
Section 7.
This is a simple example of a SCRAM-HMAC-SHA-1 authentication
exchange:
C: n,n=Chris Newman,r=ClientNonce
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
C: r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
S: v=WxPv/siO5l+qxN4
With channel-binding data sent by the client this might look like
this:
C: p,n=Chris Newman,r=ClientNonce
S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
C: c=0123456789ABCDEF,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
S: v=WxPv/siO5l+qxN4
<<Note that the channel-bind data above, as well as all hashes are
fake>>
First, the client sends a message containing:
o a GS2 header consisting of a flag indicating whether channel
binding is supported-but-not-used, not supported, or used, and the
SASL authzid (optional);
o SCRAM username and client 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 the user's iteration count i, the
user's salt, and appends its own nonce to the client-specified one.
The client then responds with the same nonce and a ClientProof
computed using the selected hash function as explained earlier. In
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this step the client can also include an optional authorization
identity. The server verifies the nonce and the proof, verifies that
the authorization identity (if supplied by the client in the second
message) is authorized to act as the authentication identity, and,
finally, it responds with a ServerSignature, concluding the
authentication exchange. The client then authenticates the server by
computing the ServerSignature and comparing it to the value sent by
the server. If the two are different, the client MUST consider the
authentication exchange to be unsuccessful and it might have to drop
the connection.
5.1. SCRAM Attributes
This section describes the permissible attributes, their use, and the
format of their values. All attribute names are single US-ASCII
letters and are case-sensitive.
o a: This is an optional attribute, and is part of the GS2 [ref-
needed] bridge between the GSS-API and SASL. This attribute
specifies an authorization identity. A client may include it in
its second message to the server if it wants to authenticate as
one user, but subsequently act as a different user. This is
typically used by an administrator to perform some management task
on behalf of another user, or by a proxy in some situations.
Upon the receipt of this value the server verifies its
correctness according to the used SASL protocol profile.
Failed verification results in failed authentication exchange.
If this attribute is omitted (as it normally would be), or
specified with an empty value, the authorization identity is
assumed to be derived from the username specified with the
(required) "n" attribute.
The server always authenticates the user specified by the "n"
attribute. If the "a" attribute specifies a different user,
the 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 client must include it in its first
message to the server. If the "a" attribute is not specified
(which would normally be the case), this username is also the
identity which will be associated with the connection subsequent
to authentication and authorization.
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Before sending the username to the server, the client MUST
prepare the username using the "SASLPrep" profile [RFC4013] of
the "stringprep" algorithm [RFC3454]. If the preparation of
the username fails or results in an empty string, the client
SHOULD abort the authentication exchange (*).
(*) An interactive client can request a repeated entry of the
username value.
Upon receipt of the username by the server, the server SHOULD
prepare it using the "SASLPrep" profile [RFC4013] of the
"stringprep" algorithm [RFC3454]. If the preparation of the
username fails or results in an empty string, the server SHOULD
abort the authentication exchange.
The characters ',' or '=' in usernames are sent as '=2C' and
'=3D' respectively. If the server receives a username which
contains '=' not followed by either '2C' or '3D', then the
server MUST fail the authentication.
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
characters excluding ',' which forms the nonce used as input to
the hash function. No quoting is applied to this string (<<unless
the binding of SCRAM to a particular protocol states otherwise>>).
As described earlier, the client supplies an initial value in its
first message, and the server augments that value with its own
nonce in its first response. It is important that this be value
different for each authentication. The client MUST verify that
the initial part of the nonce used in subsequent messages is the
same as the nonce it initially specified. The server MUST verify
that 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 base64-encoded of a header
and the channel-binding data. It is sent by the client in its
second authentication message. The header consist of:
* the GS2 header from the client's first message (recall: a
channel binding flag and an optional authzid);
* followed by the external channel's channel binding type prefix
(see [RFC5056], if and only if the client is using channel
binding;
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* 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-HMAC-SHA-1 SASL mechanism servers SHOULD announce a
hash iteration-count of at least 128.
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. 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 The server advertises support for channel binding by advertising
both, SCRAM-HMAC-<hash-function> and SCRAM-HMAC-<hash-function>-
PLUS.
o If the client negotiates mechanisms then client MUST select SCRAM-
HMAC-<hash-function>-PLUS if offered by the server. Otherwise, if
the client does not negotiate mechanisms then it MUST select only
SCRAM-HMAC-<hash-function> (not suffixed with "-PLUS").
o If the client and server both support channel binding, or if the
client wishes to use channel binding but the client does not
negotiate mechanisms, the client MUST set the GS2 channel binding
flag to "p" and MUST include 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
then the client MUST 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
MUST set the GS2 channel binding flag to "n" 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 server receives a client first message with the GS2 channel
binding flag set to "y" and the server supports channel binding
the server MUST fail authentication. This is because if the
client sets the GS2 channel binding flag set 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)).
The server MUST always validate the client's "c=" field. The server
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does this by constructing the value of the "c=" attribute and then
checking that it matches the client's c= attribute value.
6.1. Channel Binding to TLS Channels
If an external TLS channel is to be bound into the SCRAM
authentication, and if the channel was established using a server
certificate to authenticate the server, then the SCRAM client and
server MUST use the 'tls-server-end-point' channel binding type. See
the IANA Channel Binding Types registry.
If an external TLS channel is to be bound into the SCRAM
authentication, and if the channel was established without the use of
any server certificate to authenticate the server, then the SCRAM
client and server MUST use the 'tls-unique' channel binding type.
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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].
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)>
generic-message = attr-val *("," attr-val)
;; Generic syntax of any server challenge
;; or client response
attr-val = ALPHA "=" value
value = *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 / "="
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.
gs2-cbind-flag = "n" / "y" / "p"
;; "n" -> client doesn't support channel binding
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;; "y" -> client does support channel binding
;; but thinks the server does not.
;; "p" -> client requires channel binding
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 signalling mandatory extensions.
;; The exact syntax will be defined in
;; the future.
;;cbind-type = value
;;cbind-input = gs2-header [ value ":" cbind-data ]
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 = value
s-nonce = value
salt = "s=" base64
verifier = "v=" base64
;; base-64 encoded ServerSignature.
iteration-count = "i=" posit-number
;; A positive number
client-first-message =
gs2-header [reserved-mext ","]
username "," nonce ["," extensions]
server-first-message =
[reserved-mext ","] nonce "," salt ","
iteration-count ["," extensions]
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client-final-message-without-proof =
[channel-binding ","] nonce [","
extensions]
client-final-message =
client-final-message-without-proof "," proof
gss-server-error = "e=" value
server-final-message = gss-server-error /
verifier ["," 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)
;; All extensions are optional,
;; i.e. unrecognized attributes
;; not defined in this document
;; MUST be ignored.
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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 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 is <TBD> (see Section 10).
8.1. GSS-API Principal Name Types for SCRAM
SCRAM does not name acceptors. Therefore only GSS_C_NO_NAME and
names of type GSS_C_NT_ANONYMOUS shall be allowed as the target name
input of GSS_Init_sec_context() when using a SCRAM mechanism.
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 syntax for SCRAM principal names
is the same: there is no syntax -- SCRAM principal names are free-
form. (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], using
the Kerberos V "aes128-cts-hmac-sha1-96" enctype [RFC3962].
The 128-bit session key SHALL be derived by using the least
significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API session
key" || ClientKey || AuthMessage).
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.
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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().
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9. Security Considerations
If the authentication exchange is performed without a strong security
layer, then a passive eavesdropper can gain sufficient information to
mount an offline dictionary or brute-force attack which can be used
to recover the user's password. The amount of time necessary for
this attack depends on the cryptographic hash function selected, the
strength of the password and the iteration count supplied by the
server. An external security layer with strong encryption will
prevent this attack.
If the external security layer used to protect the SCRAM exchange
uses an anonymous key exchange, then the SCRAM channel binding
mechanism can be used to detect a man-in-the-middle attack on the
security layer and cause the authentication to fail as a result.
However, the man-in-the-middle attacker will have gained sufficient
information to mount an offline dictionary or brute-force attack.
For this reason, SCRAM includes the ability to increase the iteration
count over time.
If the authentication information is stolen from the authentication
database, then an offline dictionary or brute-force attack can be
used to recover the user's password. The use of salt mitigates this
attack somewhat by requiring a separate attack on each password.
Authentication mechanisms which protect against this attack are
available (e.g., the EKE class of mechanisms), but the patent
situation is presently unclear.
If an attacker obtains the authentication information from the
authentication repository and either eavesdrops on one authentication
exchange or impersonates a server, the attacker gains the ability to
impersonate that user to all servers providing SCRAM access using the
same hash function, password, iteration count and salt. For this
reason, it is important to use randomly-generated salt values.
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 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).
Note that to protect the SASL mechanism negotiation applications
normally must list the server mechs twice: once before and once after
authentication, the latter using security layers. Since SCRAM does
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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.
A hostile server can perform a computational denial-of-service attack
on clients by sending a big iteration count value.
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10. IANA Considerations
IANA is requested to add the following entries to the SASL Mechanism
registry established by [RFC4422]:
To: iana@iana.org
Subject: Registration of a new SASL mechanism SCRAM-HMAC-SHA-1
SASL mechanism name (or prefix for the family): SCRAM-HMAC-SHA-1
Security considerations: Section 7 of [RFCXXXX]
Published specification (optional, recommended): [RFCXXXX]
Person & email address to contact for further information:
IETF SASL WG <ietf-sasl@imc.org>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note:
To: iana@iana.org
Subject: Registration of a new SASL mechanism SCRAM-HMAC-SHA-1-PLUS
SASL mechanism name (or prefix for the family): SCRAM-HMAC-SHA-1-PLUS
Security considerations: Section 7 of [RFCXXXX]
Published specification (optional, recommended): [RFCXXXX]
Person & email address to contact for further information:
IETF SASL WG <ietf-sasl@imc.org>
Intended usage: COMMON
Owner/Change controller: IESG <iesg@ietf.org>
Note:
Note that even though this document defines a family of SCRAM-HMAC
mechanisms, it doesn't register a family of SCRAM-HMAC mechanisms in
the SASL Mechanisms registry. IANA is requested to prevent future
registrations of SASL mechanisms starting with SCRAM-HMAC- without
consulting the SASL mailing list <ietf-sasl@imc.org> first.
Note to future SCRAM-HMAC mechanism designers: each new SCRAM-HMAC
SASL mechanism MUST be explicitly registered with IANA and MUST
comply with SCRAM-HMAC mechanism naming convention defined in
Section 4 of this document.
We hereby request that IANA assign a GSS-API mechanism OID for SCRAM.
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11. Acknowledgements
The authors would like to thank Dave Cridland for his contributions
to this document.
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Appendix A. Other Authentication Mechanisms
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.
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Appendix B. Design Motivations
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.
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Appendix C. SCRAM Examples and Internet-Draft Change History
<<To be written.>>
(RFC Editor: Please delete everything after this point)
Open Issues
o The appendices need to be written.
o Should the server send a base64-encoded ServerSignature for the
value of the "v" attribute, or should it compute a ServerProof the
way the client computes a ClientProof?
Changes since -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-HMAC-SHA-1 with IANA and require explicit
registrations of all other SCRAM-HMAC- mechanisms.
Changes since -06
o Removed hash negotiation from SCRAM and turned it into a family of
SASL mechanisms.
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o Start using "Hash Function Textual Names" IANA registry for SCRAM
mechanism naming.
o Fixed definition of Hi(str, salt) 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.
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o General simplification.
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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.
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.
[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.
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[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.
12.3. Informative References
[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.
[I-D.ietf-sasl-digest-to-historic]
Melnikov, A., "Moving DIGEST-MD5 to Historic",
draft-ietf-sasl-digest-to-historic-00 (work in progress),
July 2008.
[I-D.ietf-sasl-rfc2831bis]
Melnikov, A., "Using Digest Authentication as a SASL
Mechanism", draft-ietf-sasl-rfc2831bis-12 (work in
progress), March 2007.
[RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
AUTHorize Extension for Simple Challenge/Response",
RFC 2195, September 1997.
[RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and HMAC-
SHA-1", RFC 2202, September 1997.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, 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.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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Authors' Addresses
Abhijit Menon-Sen
Oryx Mail Systems GmbH
Email: ams@oryx.com
Alexey Melnikov
Isode Ltd
Email: Alexey.Melnikov@isode.com
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
USA
Email: chris.newman@sun.com
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
USA
Email: Nicolas.Williams@sun.com
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