Salted Challenge Response (SCRAM) SASL Mechanism
draft-newman-auth-scram-11.txt

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Abstract

The secure authentication mechanism most widely deployed and used by Internet application protocols is the transmission of clear-text passwords over a channel protected by Transport Layer Security (TLS). There are some significant security concerns with that mechanism, which could be addressed by the use of a challenge response authentication mechanism protected by TLS. Unfortunately, the challenge response mechanisms presently on the standards track all fail to meet requirements necessary for widespread deployment, and have had success only in limited use.

This specification describes 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.

Table of Contents

1.  Conventions Used in This Document
1.1.  Terminology
1.2.  Notation
2.  Introduction
3.  SCRAM Algorithm Overview
4.  SCRAM Mechanism Names
5.  SCRAM Authentication Exchange
5.1.  SCRAM Attributes
6.  Channel Binding
6.1.  Channel Binding to TLS Channels
7.  Formal Syntax
8.  SCRAM as a GSS-API Mechanism
8.1.  GSS-API Principal Name Types for SCRAM
8.2.  GSS-API Per-Message Tokens for SCRAM
8.3.  GSS_Pseudo_random() for SCRAM
9.  Security Considerations
10.  IANA Considerations
11.  Acknowledgements
Appendix A.  Other Authentication Mechanisms
Appendix B.  Design Motivations
Appendix C.  SCRAM Examples and Internet-Draft Change History
12.  References
12.1.  Normative References
12.2.  Normative References for GSS-API implementors
12.3.  Informative References
§  Authors' Addresses

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

Formal syntax is defined by [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) including the core rules defined in Appendix B of [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).

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] (Shirey, R., “Internet Security Glossary, Version 2,” August 2007.) ("Internet Security Glossary") including the following: authentication, authentication exchange, authentication information, brute force, challenge-response, cryptographic hash function, dictionary attack, eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, one-way encryption function, password, replay attack and salt. Readers not familiar with these terms should use that glossary as a reference.

Some clarifications and additional definitions follow:

• Authentication information: Information used to verify an identity claimed by a SCRAM client. The authentication information for a SCRAM identity consists of salt, iteration count, the "StoredKey" and "ServerKey" (as defined in the algorithm overview) for each supported cryptographic hash function.
• Authentication database: The database used to look up the authentication information associated with a particular identity. For application protocols, LDAPv3 (see [RFC4510] (Zeilenga, K., “Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map,” June 2006.)) is frequently used as the authentication database. For network-level protocols such as PPP or 802.11x, the use of RADIUS is more common.
• Base64: An encoding mechanism defined in [RFC4648] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.) which converts an octet string input to a textual output string which can be easily displayed to a human. The use of base64 in SCRAM is restricted to the canonical form with no whitespace.
• Octet: An 8-bit byte.
• Octet string: A sequence of 8-bit bytes.
• Salt: A random octet string that is combined with a password before applying a one-way encryption function. This value is used to protect passwords that are stored in an authentication database.

1.2.  Notation

The pseudocode description of the algorithm uses the following notations:

• ":=": The variable on the left hand side represents the octet string resulting from the expression on the right hand side.
• "+": Octet string concatenation.
• "[ ]": A portion of an expression enclosed in "[" and "]" may not be included in the result under some circumstances. See the associated text for a description of those circumstances.
• HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in [RFC2104] (Krawczyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” February 1997.)) 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] (Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.)).
• H(str): Apply the cryptographic hash function to the octet string "str", producing an octet string as a result. The size of the result depends on the hash result size for the hash function in use.
• XOR: Apply the exclusive-or operation to combine the octet string on the left of this operator with the octet string on the right of this operator. The length of the output and each of the two inputs will be the same for this use.
• Hi(str, salt):

   U0   := HMAC(str, salt + INT(1))
   U1   := HMAC(str, U0)
   U2   := HMAC(str, U1)
   ...
   Ui-1 := HMAC(str, Ui-2)
   Ui   := HMAC(str, Ui-1)

   Hi := U0 XOR U1 XOR U2 XOR ... XOR Ui

where "i" is the iteration count, "+" is the string concatenation operator and INT(g) is a four-octet encoding of the integer g, most significant octet first.

• This is, essentially, PBKDF2 [RFC2898] (Kaliski, B., “PKCS #5: Password-Based Cryptography Specification Version 2.0,” September 2000.) with HMAC() as the PRF and with dkLen == output length of HMAC() == output length of H().

2.  Introduction

This specification describes a family of authentication mechanisms called the Salted Challenge Response Authentication Mechanism (SCRAM) which addresses the requirements necessary to deploy a challenge-response mechanism more widely than past attempts. When used in combination with Transport Layer Security (TLS, see [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.)) 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] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.) to the external security layer.

SCRAM is specified herein as a pure Simple Authentication and Security Layer (SASL) [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.) 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:

• The authentication information stored in the authentication database is not sufficient by itself to impersonate the client. The information is salted to prevent a pre-stored dictionary attack if the database is stolen.
• The server does not gain the ability to impersonate the client to other servers (with an exception for server-authorized proxies).
• The mechanism permits the use of a server-authorized proxy without requiring that proxy to have super-user rights with the back-end server.
• A standard attribute is defined to enable storage of the authentication information in LDAPv3 (see [RFC4510] (Zeilenga, K., “Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map,” June 2006.)).
• 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.

Comments regarding this draft may be sent either to the ietf-sasl@imc.org mailing list or to the authors.

3.  SCRAM Algorithm Overview

Note that this section omits some details, such as client and server nonces. See Section 5 (SCRAM Authentication Exchange) 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 (Formal Syntax).

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] (Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.).

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 (Channel Binding).

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 (SCRAM Attributes), and defined in Section 7 (Formal Syntax).

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:

• 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);
• 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 this step the client can also include an optional authorization identity. The server verifies the nonce and the proof, verifies that the authorization identity (if supplied by the client in the second message) is authorized to act as the authentication identity, and, finally, it responds with a ServerSignature, concluding the authentication exchange. The client then authenticates the server by computing the ServerSignature and comparing it to the value sent by the server. If the two are different, the client MUST consider the authentication exchange to be unsuccessful and it might have to drop the connection.

5.1.  SCRAM Attributes

This section describes the permissible attributes, their use, and the format of their values. All attribute names are single US-ASCII letters and are case-sensitive.

• a: This 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 ','.

• n: This attribute specifies the name of the user whose password is used for authentication. A client must include it in its first message to the server. If the "a" attribute is not specified (which would normally be the case), this username is also the identity which will be associated with the connection subsequent to authentication and authorization.

  Before sending the username to the server, the client MUST prepare the username using the "SASLPrep" profile [RFC4013] (Zeilenga, K., “SASLprep: Stringprep Profile for User Names and Passwords,” February 2005.) of the "stringprep" algorithm [RFC3454] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.). 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] (Zeilenga, K., “SASLprep: Stringprep Profile for User Names and Passwords,” February 2005.) of the "stringprep" algorithm [RFC3454] (Hoffman, P. and M. Blanchet, “Preparation of Internationalized Strings ("stringprep"),” December 2002.). 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.

• m: This attribute is reserved for future extensibility. In this version of SCRAM, its presence in a client or a server message MUST cause authentication failure when the attribute is parsed by the other end.
• r: This attribute specifies a sequence of random printable characters excluding ',' which forms the nonce used as input to the hash function. No quoting is applied to this string (<<unless the binding of SCRAM to a particular protocol states otherwise>>). As described earlier, the client supplies an initial value in its first message, and the server augments that value with its own nonce in its first response. It is important that this be value different for each authentication. The client MUST verify that the initial part of the nonce used in subsequent messages is the same as the nonce it initially specified. The server MUST verify that the nonce sent by the client in the second message is the same as the one sent by the server in its first message.
• c: This 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] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.), 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.
• s: This attribute specifies the base64-encoded salt used by the server for this user. It is sent by the server in its first message to the client.
• i: This attribute specifies an iteration count for the selected hash function and user, and must be sent by the server along with the user's salt.

  For SCRAM-HMAC-SHA-1 SASL mechanism servers SHOULD announce a hash iteration-count of at least 128.

• p: This attribute specifies a base64-encoded ClientProof. The client computes this value as described in the overview and sends it to the server.
• v: This attribute specifies a base64-encoded ServerSignature. It is sent by the server in its final message, and 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.

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:

• The server advertises support for channel binding by advertising both, SCRAM-HMAC-<hash-function> and SCRAM-HMAC-<hash-function>-PLUS.
• 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").
• 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 (SCRAM Attributes)).
• 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 (SCRAM Attributes)).
• 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 (SCRAM Attributes)).
• 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 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.

7.  Formal Syntax

The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.). "UTF8-2", "UTF8-3" and "UTF8-4" non-terminal are defined in [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.).

   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
                     ;; "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]

   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.

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 (IANA Considerations)).

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 (SCRAM Attributes).

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] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.) 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] (Zhu, L., Jaganathan, K., and S. Hartman, “The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2,” July 2005.), using the Kerberos V "aes128-cts-hmac-sha1-96" enctype [RFC3962] (Raeburn, K., “Advanced Encryption Standard (AES) Encryption for Kerberos 5,” February 2005.).

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.

8.3.  GSS_Pseudo_random() for SCRAM

The GSS_Pseudo_random() [RFC4401] (Williams, N., “A Pseudo-Random Function (PRF) API Extension for the Generic Security Service Application Program Interface (GSS-API),” February 2006.) for SCRAM SHALL be the same as for the Kerberos V GSS-API mechanism [RFC4402] (Williams, N., “A Pseudo-Random Function (PRF) for the Kerberos V Generic Security Service Application Program Interface (GSS-API) Mechanism,” February 2006.). 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().

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 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.

10.  IANA Considerations

IANA is requested to add the following entries to the SASL Mechanism registry established by [RFC4422] (Melnikov, A. and K. Zeilenga, “Simple Authentication and Security Layer (SASL),” June 2006.):

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 (SCRAM Mechanism Names) of this document.

We hereby request that IANA assign a GSS-API mechanism OID for SCRAM.

11.  Acknowledgements

The authors would like to thank Dave Cridland for his contributions to this document.

Appendix A.  Other Authentication Mechanisms

The DIGEST-MD5 [I‑D.ietf‑sasl‑digest‑to‑historic] (Melnikov, A., “Moving DIGEST-MD5 to Historic,” July 2008.) 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] (Melnikov, A., “Moving DIGEST-MD5 to Historic,” July 2008.).

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] (Zeilenga, K., “CRAM-MD5 to Historic,” November 2008.).

The PLAIN [RFC4616] (Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” August 2006.) 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 DIGEST-MD5 [I‑D.ietf‑sasl‑digest‑to‑historic] (Melnikov, A., “Moving DIGEST-MD5 to Historic,” July 2008.) 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] (Melnikov, A., “Moving DIGEST-MD5 to Historic,” July 2008.).

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] (Zeilenga, K., “CRAM-MD5 to Historic,” November 2008.).

The PLAIN [RFC4616] (Zeilenga, K., “The PLAIN Simple Authentication and Security Layer (SASL) Mechanism,” August 2006.) 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 C.  SCRAM Examples and Internet-Draft Change History

<<To be written.>>

(RFC Editor: Please delete everything after this point)

Open Issues

• The appendices need to be written.
• Should the server send a base64-encoded ServerSignature for the value of the "v" attribute, or should it compute a ServerProof the way the client computes a ClientProof?

Changes since -10

• Converted the source for this I-D to XML.
• Added text to make SCRAM compliant with the new GS2 design.
• Added text on channel binding negotiation.
• Added text on channel binding, including a reference to RFC5056.
• 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

• Updated References.
• Clarified purpose of the m= attribute.
• Fixed a problem with authentication/authorization identity's ABNF not allowing for some characters.
• Updated ABNF for nonce to show client-generated and server-generated parts.
• Only register SCRAM-HMAC-SHA-1 with IANA and require explicit registrations of all other SCRAM-HMAC- mechanisms.

Changes since -06

• Removed hash negotiation from SCRAM and turned it into a family of SASL mechanisms.
• Start using "Hash Function Textual Names" IANA registry for SCRAM mechanism naming.
• Fixed definition of Hi(str, salt) to be consistent with [RFC2898].
• Clarified extensibility of SCRAM: added m= attribute (for future mandatory extensions) and specified that all unrecognized attributes must be ignored.

Changes since -05

• Changed the mandatory to implement hash algorithm to SHA-1 (as per WG consensus).
• Added text about use of SASLPrep for username canonicalization/validation.
• Clarified that authorization identity is canonicalized/verified according to SASL protocol profile.
• Clarified that iteration count is per-user.
• Clarified how clients select the authentication function.
• Added IANA registration for the new mechanism.
• Added missing normative references (UTF-8, SASLPrep).
• Various editorial changes based on comments from Hallvard B Furuseth, Nico William and Simon Josefsson.

Changes since -04

• Update Base64 and Security Glossary references.
• Add Formal Syntax section.
• Don't bother with "v=".
• Make MD5 mandatory to implement. Suggest i=128.

Changes since -03

• 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.
• Be hash agnostic, so MD5 can be replaced more easily.
• General simplification.

12.  References

12.1. Normative References

12.2. Normative References for GSS-API implementors

12.3. Informative References

Authors' Addresses