draft-ietf-httpauth-scram-auth-15.txt   rfc7804.txt 
HTTPAUTH A. Melnikov Internet Engineering Task Force (IETF) A. Melnikov
Internet-Draft Isode Ltd Request for Comments: 7804 Isode Ltd
Intended status: Experimental December 16, 2015 Category: Experimental March 2016
Expires: June 18, 2016 ISSN: 2070-1721
Salted Challenge Response (SCRAM) HTTP Authentication Mechanism Salted Challenge Response HTTP Authentication Mechanism
draft-ietf-httpauth-scram-auth-15.txt
Abstract Abstract
This specification describes a family of HTTP authentication This specification describes a family of HTTP authentication
mechanisms called the Salted Challenge Response Authentication mechanisms called the Salted Challenge Response Authentication
Mechanism (SCRAM), which provides a more robust authentication Mechanism (SCRAM), which provides a more robust authentication
mechanism than a plaintext password protected by Transport Layer mechanism than a plaintext password protected by Transport Layer
Security (TLS) and avoids the deployment obstacles presented by Security (TLS) and avoids the deployment obstacles presented by
earlier TLS-protected challenge response authentication mechanisms. earlier TLS-protected challenge response authentication mechanisms.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for examination, experimental implementation, and
evaluation.
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 This document defines an Experimental Protocol for the Internet
and may be updated, replaced, or obsoleted by other documents at any community. This document is a product of the Internet Engineering
time. It is inappropriate to use Internet-Drafts as reference Task Force (IETF). It represents the consensus of the IETF
material or to cite them other than as "work in progress." community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
This Internet-Draft will expire on June 18, 2016. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7804.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . . . . 5 3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . . . . 6
4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . . 6 4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . . 7
5. SCRAM Authentication Exchange . . . . . . . . . . . . . . . . 7 5. SCRAM Authentication Exchange . . . . . . . . . . . . . . . . 7
5.1. One round trip reauthentication . . . . . . . . . . . . . . 10 5.1. One Round-Trip Reauthentication . . . . . . . . . . . . . 10
6. Use of Authentication-Info header field with SCRAM . . . . . 12 6. Use of the Authentication-Info Header Field with SCRAM . . . 12
7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 10. Design Motivations . . . . . . . . . . . . . . . . . . . . . 15
11. Design Motivations . . . . . . . . . . . . . . . . . . . . . 16 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . . 17 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
The authentication mechanism most widely deployed and used by The authentication mechanism most widely deployed and used by
Internet application protocols is the transmission of clear-text Internet application protocols is the transmission of clear-text
passwords over a channel protected by Transport Layer Security (TLS). passwords over a channel protected by Transport Layer Security (TLS).
There are some significant security concerns with that mechanism, There are some significant security concerns with that mechanism,
which could be addressed by the use of a challenge response which could be addressed by the use of a challenge response
authentication mechanism protected by TLS. Unfortunately, the HTTP authentication mechanism protected by TLS. Unfortunately, the HTTP
Digest challenge response mechanism presently on the standards track Digest challenge response mechanism presently on the Standards Track
failed widespread deployment, and has had success only in limited failed widespread deployment and has had only limited success.
use.
This specification describes a family of authentication mechanisms This specification describes a family of authentication mechanisms
called the Salted Challenge Response Authentication Mechanism (SCRAM) called the Salted Challenge Response Authentication Mechanism
which addresses the requirements necessary to deploy a challenge- (SCRAM), which addresses the requirements necessary to deploy a
response mechanism more widely than past attempts (see [RFC5802]). challenge response mechanism more widely than past attempts (see
In particular, it addresses some of the issues identified with HTTP [RFC5802]). In particular, it addresses some of the issues
Digest, as described in [RFC6331], such as complexity of implementing identified with HTTP Digest, as described in [RFC6331], such as the
and protection of the whole authentication exchange in order to complexity of implementation and protection of the whole
protect from certain man-in-the-middle attacks. authentication exchange in order to protect against certain man-in-
the-middle attacks.
HTTP SCRAM is an adoptation of [RFC5802] for use in HTTP. The SCRAM HTTP SCRAM is an adaptation of [RFC5802] for use in HTTP. The SCRAM
data exchanged is identical to what is defined in [RFC5802]. This data exchanged is identical to what is defined in [RFC5802]. This
document also adds a 1 round trip reauthentication mode. document also adds a 1 round-trip reauthentication mode.
HTTP SCRAM provides the following protocol features: HTTP SCRAM provides the following protocol features:
o The authentication information stored in the authentication o The authentication information stored in the authentication
database is not sufficient by itself (without a dictionary attack) database is not sufficient by itself (without a dictionary attack)
to impersonate the client. The information is salted to make it to impersonate the client. The information is salted to make it
harder to do a pre-stored dictionary attack if the database is harder to do a pre-stored dictionary attack if the database is
stolen. stolen.
o The server does not gain the ability to impersonate the client to o The server does not gain the ability to impersonate the client to
skipping to change at page 3, line 36 skipping to change at page 3, line 50
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Formal syntax is defined by [RFC5234] including the core rules Formal syntax is defined by [RFC5234] including the core rules
defined in Appendix B of [RFC5234]. defined in Appendix B of [RFC5234].
Example lines prefaced by "C:" are sent by the client and ones Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server. If a single "C:" or "S:" label prefaced by "S:" by the server. If a single "C:" or "S:" label
applies to multiple lines, then the line breaks between those lines applies to multiple lines, then the line breaks between those lines
are for editorial clarity only, and are not part of the actual are for editorial clarity only and are not part of the actual
protocol exchange. protocol exchange.
2.1. Terminology 2.1. Terminology
This document uses several terms defined in [RFC4949] ("Internet This document uses several terms defined in the "Internet Security
Security Glossary") including the following: authentication, Glossary" [RFC4949], including the following: authentication,
authentication exchange, authentication information, brute force, authentication exchange, authentication information, brute force,
challenge-response, cryptographic hash function, dictionary attack, challenge-response, cryptographic hash function, dictionary attack,
eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
one-way encryption function, password, replay attack and salt. one-way encryption function, password, replay attack, and salt.
Readers not familiar with these terms should use that glossary as a Readers not familiar with these terms should use that glossary as a
reference. reference.
Some clarifications and additional definitions follow: Some clarifications and additional definitions follow:
o Authentication information: Information used to verify an identity o Authentication information: Information used to verify an identity
claimed by a SCRAM client. The authentication information for a claimed by a SCRAM client. The authentication information for a
SCRAM identity consists of salt, iteration count, the "StoredKey" SCRAM identity consists of salt, iteration count, the StoredKey,
and "ServerKey" (as defined in the algorithm overview) for each and the ServerKey (as defined in the algorithm overview) for each
supported cryptographic hash function. supported cryptographic hash function.
o Authentication database: The database used to look up the o Authentication database: The database used to look up the
authentication information associated with a particular identity. authentication information associated with a particular identity.
For application protocols, LDAPv3 (see [RFC4510]) is frequently For application protocols, LDAPv3 (see [RFC4510]) is frequently
used as the authentication database. For lower-layer protocols used as the authentication database. For lower-layer protocols
such as PPP or 802.11x, the use of RADIUS [RFC2865] is more such as PPP or 802.11x, the use of RADIUS [RFC2865] is more
common. common.
o Base64: An encoding mechanism defined in Section 4 of [RFC4648] o Base64: An encoding mechanism defined in Section 4 of [RFC4648]
which converts an octet string input to a textual output string that converts an octet string input to a textual output string
which can be easily displayed to a human. The use of base64 in that can be easily displayed to a human. The use of base64 in
SCRAM is restricted to the canonical form with no whitespace. SCRAM is restricted to the canonical form with no whitespace.
o Octet: An 8-bit byte. o Octet: An 8-bit byte.
o Octet string: A sequence of 8-bit bytes. o Octet string: A sequence of 8-bit bytes.
o Salt: A random octet string that is combined with a password o Salt: A random octet string that is combined with a password
before applying a one-way encryption function. This value is used before applying a one-way encryption function. This value is used
to protect passwords that are stored in an authentication to protect passwords that are stored in an authentication
database. database.
2.2. Notation 2.2. Notation
The pseudocode description of the algorithm uses the following The pseudocode description of the algorithm uses the following
notations: notation:
o ":=": The variable on the left hand side represents the octet o ":=": The variable on the left-hand side represents the octet
string resulting from the expression on the right hand side. string resulting from the expression on the right-hand side.
o "+": Octet string concatenation. o "+": Octet string concatenation.
o "[ ]": A portion of an expression enclosed in "[" and "]" is o "[ ]": A portion of an expression enclosed in "[" and "]" is
optional in the result under some circumstances. See the optional in the result under some circumstances. See the
associated text for a description of those circumstances. associated text for a description of those circumstances.
o Normalize(str): Apply the Preparation and Enforcement steps o Normalize(str): Apply the Preparation and Enforcement steps
according to the OpaqueString profile (see [RFC7613]) to a UTF-8 according to the OpaqueString profile (see [RFC7613]) to a UTF-8
[RFC3629] encoded "str". The resulting string is also in UTF-8. [RFC3629] encoded "str". The resulting string is also in UTF-8.
Note that implementations MUST either implement OpaqueString Note that implementations MUST either implement OpaqueString
profile operations from [RFC7613], or disallow use of non US-ASCII profile operations from [RFC7613] or disallow the use of non
Unicode codepoints in "str". The latter is a particular case of US-ASCII Unicode codepoints in "str". The latter is a particular
compliance with [RFC7613]. case of compliance with [RFC7613].
o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in o HMAC(key, str): Apply the HMAC-keyed hash algorithm (defined in
[RFC2104]) using the octet string represented by "key" as the key [RFC2104]) using the octet string represented by "key" as the key
and the octet string "str" as the input string. The size of the and the octet string "str" as the input string. The size of the
result is the hash result size for the hash function in use. For result is the hash result size for the hash function in use. For
example, it is 32 octets for SHA-256 and 20 octets for SHA-1 (see example, it is 32 octets for SHA-256 and 20 octets for SHA-1 (see
[RFC6234]). [RFC6234]).
o H(str): Apply the cryptographic hash function to the octet string o H(str): Apply the cryptographic hash function to the octet string
"str", producing an octet string as a result. The size of the "str", producing an octet string as a result. The size of the
result depends on the hash result size for the hash function in result depends on the hash result size for the hash function in
use. use.
skipping to change at page 5, line 33 skipping to change at page 5, line 47
U1 := HMAC(str, salt + INT(1)) U1 := HMAC(str, salt + INT(1))
U2 := HMAC(str, U1) U2 := HMAC(str, U1)
... ...
Ui-1 := HMAC(str, Ui-2) Ui-1 := HMAC(str, Ui-2)
Ui := HMAC(str, Ui-1) Ui := HMAC(str, Ui-1)
Hi := U1 XOR U2 XOR ... XOR Ui Hi := U1 XOR U2 XOR ... XOR Ui
where "i" is the iteration count, "+" is the string concatenation where "i" is the iteration count, "+" is the string concatenation
operator and INT(g) is a four-octet encoding of the integer g, operator, and INT(g) is a four-octet encoding of the integer g,
most significant octet first. most significant octet first.
Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the
with dkLen == output length of HMAC() == output length of H(). Pseudorandom Function (PRF) and with dkLen == output length of
HMAC() == output length of H().
3. SCRAM Algorithm Overview 3. SCRAM Algorithm Overview
The following is a description of a full HTTP SCRAM authentication The following is a description of a full HTTP SCRAM authentication
exchange. Note that this section omits some details, such as client exchange. Note that this section omits some details, such as client
and server nonces. See Section 5 for more details. and server nonces. See Section 5 for more details.
To begin with, the SCRAM client is in possession of a username and To begin with, the SCRAM client is in possession of a username and
password, both encoded in UTF-8 [RFC3629] (or a ClientKey/ServerKey, password, both encoded in UTF-8 [RFC3629] (or a ClientKey/ServerKey,
or SaltedPassword). It sends the username to the server, which or SaltedPassword). It sends the username to the server, which
retrieves the corresponding authentication information: a salt, a retrieves the corresponding authentication information: a salt, a
StoredKey, a ServerKey and an iteration count ("i"). (Note that a StoredKey, a ServerKey, and an iteration count ("i"). (Note that a
server implementation may choose to use the same iteration count for server implementation may choose to use the same iteration count for
all accounts.) The server sends the salt and the iteration count to all accounts.) The server sends the salt and the iteration count to
the client, which then computes the following values and sends a the client, which then computes the following values and sends a
ClientProof to the server: ClientProof to the server:
Informative Note: Implementors are encouraged to create test cases Informative Note: Implementors are encouraged to create test cases
that use both username passwords with non-ASCII codepoints. In that use both usernames and passwords with non-ASCII codepoints. In
particular, it is useful to test codepoints whose "Unicode particular, it is useful to test codepoints whose Unicode
Normalization Canonical Composition (NFC)" and "Unicode Normalization Normalization Canonical Composition (NFC) and Unicode Normalization
Form Compatibility Composition (NFKC)" are different. Some examples Form Compatibility Composition (NFKC) are different (see
of such codepoints include Vulgar Fraction One Half (U+00BD) and [Unicode-UAX15]). Some examples of such codepoints include Vulgar
Acute Accent (U+00B4). Fraction One Half (U+00BD) and Acute Accent (U+00B4).
SaltedPassword := Hi(Normalize(password), salt, i) SaltedPassword := Hi(Normalize(password), salt, i)
ClientKey := HMAC(SaltedPassword, "Client Key") ClientKey := HMAC(SaltedPassword, "Client Key")
StoredKey := H(ClientKey) StoredKey := H(ClientKey)
AuthMessage := client-first-message-bare + "," + AuthMessage := client-first-message-bare + "," +
server-first-message + "," + server-first-message + "," +
client-final-message-without-proof client-final-message-without-proof
ClientSignature := HMAC(StoredKey, AuthMessage) ClientSignature := HMAC(StoredKey, AuthMessage)
ClientProof := ClientKey XOR ClientSignature ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, "Server Key") ServerKey := HMAC(SaltedPassword, "Server Key")
ServerSignature := HMAC(ServerKey, AuthMessage) ServerSignature := HMAC(ServerKey, AuthMessage)
The server authenticates the client by computing the ClientSignature, The server authenticates the client by computing the ClientSignature,
exclusive-ORing that with the ClientProof to recover the ClientKey exclusive-ORing that with the ClientProof to recover the ClientKey,
and verifying the correctness of the ClientKey by applying the hash and verifying the correctness of the ClientKey by applying the hash
function and comparing the result to the StoredKey. If the ClientKey function and comparing the result to the StoredKey. If the ClientKey
is correct, this proves that the client has access to the user's is correct, this proves that the client has access to the user's
password. password.
Similarly, the client authenticates the server by computing the Similarly, the client authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If ServerSignature and comparing it to the value sent by the server. If
the two are equal, it proves that the server had access to the user's the two are equal, this proves that the server had access to the
ServerKey. user's ServerKey.
For initial authentication the AuthMessage is computed by For initial authentication, the AuthMessage is computed by
concatenating decoded "data" attribute values from the authentication concatenating decoded "data" attribute values from the authentication
exchange. The format of each of these 3 decoded "data" attributes is exchange. The format of each of these 3 decoded "data" attributes is
defined in [RFC5802]. defined in [RFC5802].
4. SCRAM Mechanism Names 4. SCRAM Mechanism Names
A SCRAM mechanism name (authentication scheme) is a string "SCRAM-" A SCRAM mechanism name (authentication scheme) is a string "SCRAM-"
followed by the uppercased name of the underlying hash function taken followed by the uppercased name of the underlying hash function taken
from the IANA "Hash Function Textual Names" registry (see from the IANA "Hash Function Textual Names" registry (see
http://www.iana.org/assignments/hash-function-text-names/) . <http://www.iana.org/assignments/hash-function-text-names>).
For interoperability, all HTTP clients and servers supporting SCRAM For interoperability, all HTTP clients and servers supporting SCRAM
MUST implement the SCRAM-SHA-256 authentication mechanism, i.e. an MUST implement the SCRAM-SHA-256 authentication mechanism, i.e., an
authentication mechanism from the SCRAM family that uses the SHA-256 authentication mechanism from the SCRAM family that uses the SHA-256
hash function as defined in [RFC7677]. hash function as defined in [RFC7677].
5. SCRAM Authentication Exchange 5. SCRAM Authentication Exchange
HTTP SCRAM is a HTTP Authentication mechanism whose client response HTTP SCRAM is an HTTP Authentication mechanism whose client response
(<credentials-scram>) and server challenge (<challenge-scram>) (<credentials-scram>) and server challenge (<challenge-scram>)
messages are text-based messages containing one or more attribute- messages are text-based messages containing one or more attribute-
value pairs separated by commas. The messages and their attributes value pairs separated by commas. The messages and their attributes
are described below and defined in Section 7. are described below and defined in Section 7.
challenge-scram = scram-name [1*SP 1#auth-param] challenge-scram = scram-name [1*SP 1#auth-param]
; Complies with <challenge> ABNF from RFC 7235. ; Complies with <challenge> ABNF from RFC 7235.
; Included in the WWW-Authenticate header field. ; Included in the WWW-Authenticate header field.
credentials-scram = scram-name [1*SP 1#auth-param] credentials-scram = scram-name [1*SP 1#auth-param]
; Complies with <credentials> from RFC 7235. ; Complies with <credentials> from RFC 7235.
; Included in the Authorization header field. ; Included in the Authorization header field.
scram-name = "SCRAM-SHA-256" / "SCRAM-SHA-1" / other-scram-name scram-name = "SCRAM-SHA-256" / "SCRAM-SHA-1" / other-scram-name
; SCRAM-SHA-256 and SCRAM-SHA-1 are registered by this RFC. ; SCRAM-SHA-256 and SCRAM-SHA-1 are registered by this RFC.
; ;
; SCRAM-SHA-1 is registered for database compatibility ; SCRAM-SHA-1 is registered for database compatibility
; with implementations of RFC 5802 (such as IMAP or XMPP ; with implementations of RFC 5802 (such as IMAP or Extensible
; servers), but it is not recommended for new deployments. Messaging and Presence Protocol (XMPP)
; servers), but it is not recommended for new deployments.
other-scram-name = "SCRAM-" hash-name other-scram-name = "SCRAM-" hash-name
; hash-name is a capitalized form of names from IANA ; hash-name is a capitalized form of names from IANA.
; "Hash Function Textual Names" registry. ; "Hash Function Textual Names" registry.
; Additional SCRAM names must be registered in both ; Additional SCRAM names must be registered in both
; the IANA "SASL mechanisms" registry ; the IANA "SASL Mechanisms" registry
; and the IANA "authentication scheme" registry. ; and the IANA "HTTP Authentication Schemes" registry.
This is a simple example of a SCRAM-SHA-256 authentication exchange This is a simple example of a SCRAM-SHA-256 authentication exchange
(no support for channel bindings, as this feature is not currently (no support for channel bindings, as this feature is not currently
supported by HTTP). Username 'user' and password 'pencil' are used. supported by HTTP). Username 'user' and password 'pencil' are used.
Note that long lines are folded for readability. Note that long lines are folded for readability.
C: GET /resource HTTP/1.1 C: GET /resource HTTP/1.1
C: Host: server.example.com C: Host: server.example.com
C: [...] C: [...]
skipping to change at page 8, line 26 skipping to change at page 8, line 31
C: GET /resource HTTP/1.1 C: GET /resource HTTP/1.1
C: Host: server.example.com C: Host: server.example.com
C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com", C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",
data=biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8K data=biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8K
C: [...] C: [...]
S: HTTP/1.1 401 Unauthorized S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: SCRAM-SHA-256 S: WWW-Authenticate: SCRAM-SHA-256
sid=AAAABBBBCCCCDDDD, sid=AAAABBBBCCCCDDDD,
data=cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJbGo data=cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJ
paE5sRixzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTYK bGopaE5sRixzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTYK
S: [...] S: [...]
C: GET /resource HTTP/1.1 C: GET /resource HTTP/1.1
C: Host: server.example.com C: Host: server.example.com
C: Authorization: SCRAM-SHA-256 sid=AAAABBBBCCCCDDDD, C: Authorization: SCRAM-SHA-256 sid=AAAABBBBCCCCDDDD,
data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ0FmdXhG data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ
SWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empmTUhnc3FtbWl6 0FmdXhGSWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empm
N0FuZFZRPQo= TUhnc3FtbWl6N0FuZFZRPQo=
C: [...] C: [...]
S: HTTP/1.1 200 Ok S: HTTP/1.1 200 Ok
S: Authentication-Info: sid=AAAABBBBCCCCDDDD, S: Authentication-Info: sid=AAAABBBBCCCCDDDD,
data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5NUc0PQo= data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5N
Uc0PQo=
S: [...Other header fields and resource body...] S: [...Other header fields and resource body...]
In the above example, the first client request contains a "data"
attribute that base64 decodes as follows:
In the above example the first client request contains data attribute n,,n=user,r=rOprNGfwEbeRWgbNEkqO
which base64 decodes as follows: "n,,n=user,r=rOprNGfwEbeRWgbNEkqO"
(with no quotes). Server then responds with data attribute which
base64 decodes as follows: "r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxF
Ilj)hNlF,s=W22ZaJ0SNY7soEsUEjb6gQ==,i=4096". The next client request
contains data attribute which base64 decodes as follows: "c=biws,r=rO
prNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,p=dHzbZapWIk4jUhN+Ute9y
tag9zjfMHgsqmmiz7AndVQ=". The final server response contains a data
attribute which base64 decodes as follows:
"v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=". The server then responds with a "data" attribute that base64 decodes
as follows:
Note that in the example above the client can also initiate SCRAM r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,s=W22ZaJ0SNY7soE
sUEjb6gQ==,i=4096
The next client request contains a "data" attribute that base64
decodes as follows:
c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,p=dHzbZap
WIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=
The final server response contains a "data" attribute that base64
decodes as follows:
v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=
Note that in the example above, the client can also initiate SCRAM
authentication without first being prompted by the server. authentication without first being prompted by the server.
Initial "SCRAM-SHA-256" authentication starts with sending the Initial "SCRAM-SHA-256" authentication starts with sending the
"Authorization" request header field defined by HTTP/1.1, Part 7 Authorization request header field (defined by HTTP/1.1, Part 7
[RFC7235] containing "SCRAM-SHA-256" authentication scheme and the [RFC7235]) containing the "SCRAM-SHA-256" authentication scheme and
following attributes: the following attributes:
o A "realm" attribute MAY be included to indicate the scope of o A "realm" attribute MAY be included to indicate the scope of
protection in the manner described in HTTP/1.1, Part 7 [RFC7235]. protection in the manner described in HTTP/1.1, Part 7 [RFC7235].
As specified in [RFC7235], the "realm" attribute MUST NOT appear As specified in [RFC7235], the "realm" attribute MUST NOT appear
more than once. The realm attribute only appears in the first more than once. The "realm" attribute only appears in the first
SCRAM message to the server and in the first SCRAM response from SCRAM message to the server and in the first SCRAM response from
the server. the server.
o The client also includes the data attribute that contains base64 o The client also includes the "data" attribute that contains the
encoded "client-first-message" [RFC5802] containing: base64-encoded "client-first-message" [RFC5802] containing:
* a header consisting of a flag indicating whether channel * a header consisting of a flag indicating whether channel
binding is supported-but-not-used, not supported, or used . binding is supported-but-not-used, not supported, or used.
Note that this version of SCRAM doesn't support HTTP channel Note that this version of SCRAM doesn't support HTTP channel
bindings, so this header always starts with "n"; otherwise the bindings, so this header always starts with "n"; otherwise, the
message is invalid and authentication MUST fail. message is invalid and authentication MUST fail.
* SCRAM username and a random, unique nonce attributes. * SCRAM username and a random, unique "nonce" attribute.
In HTTP response, the server sends WWW-Authenticate header field In an HTTP response, the server sends the WWW-Authenticate header
containing: a unique session identifier (the "sid" attribute) plus field containing a unique session identifier (the "sid" attribute)
the "data" attribute containing base64-encoded "server-first-message" plus the "data" attribute containing the base64-encoded "server-
[RFC5802]. The "server-first-message" contains the user's iteration first-message" [RFC5802]. The "server-first-message" contains the
count i, the user's salt, and the nonce with a concatenation of the user's iteration count i, the user's salt, and the nonce with a
client-specified one (taken from the "client-first-message") with a concatenation of the client-specified one (taken from the "client-
freshly generated server nonce. first-message") with a freshly generated server nonce.
The client then responds with another HTTP request with the The client then responds with another HTTP request with the
Authorization header field, which includes the "sid" attribute Authorization header field, which includes the "sid" attribute
received in the previous server response, together with the "data" received in the previous server response, together with the "data"
attribute containing base64-encoded "client-final-message" data. The attribute containing base64-encoded "client-final-message" data. The
latter has the same nonce as in "server-first-message" and a latter has the same nonce as in "server-first-message" and a
ClientProof computed using the selected hash function (e.g. SHA-256) ClientProof computed using the selected hash function (e.g., SHA-256)
as explained earlier. as explained earlier.
The server verifies the nonce and the proof, and, finally, it The server verifies the nonce and the proof, and, finally, it
responds with a 200 HTTP response with the Authentication-Info header responds with a 200 HTTP response with the Authentication-Info header
field [RFC7615] containing the "sid" attribute (as received from the field [RFC7615] containing the "sid" attribute (as received from the
client) and the "data" attribute containing base64-encoded "server- client) and the "data" attribute containing the base64-encoded
final-message", concluding the authentication exchange. "server-final-message", concluding the authentication exchange.
The client then authenticates the server by computing the The client then authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If ServerSignature and comparing it to the value sent by the server. If
the two are different, the client MUST consider the authentication the two are different, the client MUST consider the authentication
exchange to be unsuccessful and it might have to drop the connection. exchange to be unsuccessful, and it might have to drop the
connection.
5.1. One round trip reauthentication 5.1. One Round-Trip Reauthentication
If the server supports SCRAM reauthentication, the server sends in If the server supports SCRAM reauthentication, the server sends in
its initial HTTP response a WWW-Authenticate header field containing: its initial HTTP response a WWW-Authenticate header field containing
the "realm" attribute (as defined earlier), the "sr" attribute that the "realm" attribute (as defined earlier), the "sr" attribute that
contains the server part of the "r" attribute (see s-nonce in contains the server part of the "r" attribute (see s-nonce in
[RFC5802]) and optional "ttl" attribute (which contains the "sr" [RFC5802]), and an optional "ttl" attribute (which contains the "sr"
value validity in seconds). value validity in seconds).
If the client has authenticated to the same realm before (i.e. it If the client has authenticated to the same realm before (i.e., it
remembers "i" and "s" attributes for the user from earlier remembers "i" and "s" attributes for the user from earlier
authentication exchanges with the server), it can respond to that authentication exchanges with the server), it can respond to that
with "client-final-message". When constructing the "client-final- with "client-final-message". When constructing the "client-final-
message" the client constructs the c-nonce part of the "r" attribute message", the client constructs the c-nonce part of the "r" attribute
as on initial authentication and the s-nonce part as follows: s-nonce as on initial authentication and the s-nonce part as follows: s-nonce
is a concatenation of nonce-count and the "sr" attribute (in that is a concatenation of nonce-count and the "sr" attribute (in that
order). The nonce-count is a positive integer that that is equal to order). The nonce-count is a positive integer that is equal to the
the user's "i" attribute on first reauthentication and is incremented user's "i" attribute on first reauthentication and is incremented by
by 1 on each successful re-authentication. 1 on each successful reauthentication.
The purpose of the nonce-count is to allow the server to detect The purpose of the nonce-count is to allow the server to detect
request replays by maintaining its own copy of this count - if the request replays by maintaining its own copy of this count -- if
same nonce-count value is seen twice, then the request is a the same nonce-count value is seen twice, then the request is a
replay. replay.
If the server considers the s-nonce part of the nonce attribute (the If the server considers the s-nonce part of the "nonce" attribute
"r" attribute) to be still valid (i.e. the nonce-count part is as (the "r" attribute) to still be valid (i.e., the nonce-count part is
expected (see above) and the "sr" part is still fresh), it will as expected (see above) and the "sr" part is still fresh), it will
provide access to the requested resource (assuming the client hash provide access to the requested resource (assuming the client hash
verifies correctly, of course). However if the server considers that verifies correctly, of course). However, if the server considers
the server part of the nonce is stale (for example if the "sr" value that the server part of the nonce is stale (for example, if the "sr"
is used after the "ttl" seconds), the server returns "401 value is used after the "ttl" seconds), the server returns "401
Unauthorized" containing the SCRAM mechanism name with the following Unauthorized" containing the SCRAM mechanism name with the following
attributes: a new "sr", "stale=true" and an optional "ttl". The attributes: a new "sr", "stale=true", and an optional "ttl". The
"stale" attribute signals to the client that there is no need to ask "stale" attribute signals to the client that there is no need to ask
user for the password. the user for the password.
Formally, the "stale" attribute is defined as follows: A flag, Formally, the "stale" attribute is defined as a flag, indicating
indicating that the previous request from the client was rejected that the previous request from the client was rejected because the
because the nonce value was stale. If stale is TRUE (case- nonce value was stale. If stale is TRUE (case-insensitive), the
insensitive), the client may wish to simply retry the request with client may wish to simply retry the request with a new encrypted
a new encrypted response, without reprompting the user for a new response without reprompting the user for a new username and
username and password. The server should only set stale to TRUE password. The server should only set stale to TRUE if it receives
if it receives a request for which the nonce is invalid but with a a request for which the nonce is invalid but with a valid digest
valid digest for that nonce (indicating that the client knows the for that nonce (indicating that the client knows the correct
correct username/password). If stale is FALSE, or anything other username/password). If stale is FALSE or anything other than
than TRUE, or the stale directive is not present, the username TRUE, or the stale directive is not present, the username and/or
and/or password are invalid, and new values must be obtained. password are invalid, and new values must be obtained.
When constructing AuthMessage Section 3 to be used for calculating When constructing AuthMessage (see Section 3) to be used for
client and server proofs, "client-first-message-bare" and "server- calculating client and server proofs, "client-first-message-bare" and
first-message" are reconstructed from data known to the client and "server-first-message" are reconstructed from data known to the
the server. client and the server.
Reauthentication can look like this: Reauthentication can look like this:
C: GET /resource HTTP/1.1 C: GET /resource HTTP/1.1
C: Host: server.example.com C: Host: server.example.com
C: [...] C: [...]
S: HTTP/1.1 401 Unauthorized S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: Digest realm="realm1@example.com", S: WWW-Authenticate: Digest realm="realm1@example.com",
Digest realm="realm2@example.com", Digest realm="realm2@example.com",
Digest realm="realm3@example.com", Digest realm="realm3@example.com",
SCRAM-SHA-256 realm="realm3@example.com", SCRAM-SHA-256 realm="realm3@example.com",
SCRAM-SHA-256 realm="testrealm@example.com", sr=%hvYDpWUa2RaTCAfuxFIlj)hNlF SCRAM-SHA-256 realm="testrealm@example.com", sr=%hvYDpWUa2RaTC
SCRAM-SHA-256 realm="testrealm2@example.com", sr=AAABBBCCCDDD, ttl=120 AfuxFIlj)hNlF
SCRAM-SHA-256 realm="testrealm2@example.com", sr=AAABBBCCCDDD,
ttl=120
S: [...] S: [...]
[Client authenticates as usual to realm "testrealm@example.com"] [The client authenticates as usual to realm "testrealm@example.com"]
[Some time later, client decides to reauthenticate.
[Some time later client decides to reauthenticate. It will use the cached "i" (4096) and "s" (W22ZaJ0SNY7soEsUEjb6gQ==)
It will use the cached "i" (4096) and "s" (W22ZaJ0SNY7soEsUEjb6gQ==) from earlier exchanges. It will use the nonce-value of 4096 together
from earlier exchanges. It will use the nonce-value of 4096 together with the server advertised "sr" value as the server part of the "r".]
with the server advertised "sr" value as the server part of the "r".]
C: GET /resource HTTP/1.1 C: GET /resource HTTP/1.1
C: Host: server.example.com C: Host: server.example.com
C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com", C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",
data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU80MDk2JWh2WURwV1VhMlJhVENB data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU80MDk2JWh2WURwV1VhM
ZnV4RklsailoTmxGLHA9ZEh6YlphcFdJazRqVWhOK1V0ZTl5dGFnOXpqZk1IZ3Nx lJhVENBZnV4RklsailoTmxGLHA9ZEh6YlphcFdJazRqVWhOK1V0ZTl5dGFnOX
bW1pejdBbmRWUT0K pqZk1IZ3NxbW1pejdBbmRWUT0K
C: [...] C: [...]
S: HTTP/1.1 200 Ok S: HTTP/1.1 200 Ok
S: Authentication-Info: sid=AAAABBBBCCCCDDDD, S: Authentication-Info: sid=AAAABBBBCCCCDDDD,
data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5NUc0PQo= data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5N
Uc0PQo=
S: [...Other header fields and resource body...] S: [...Other header fields and resource body...]
6. Use of Authentication-Info header field with SCRAM 6. Use of the Authentication-Info Header Field with SCRAM
When used with SCRAM, the Authentication-Info header field is allowed When used with SCRAM, the Authentication-Info header field is allowed
in the trailer of an HTTP message transferred via chunked transfer- in the trailer of an HTTP message transferred via chunked transfer-
coding. coding.
7. Formal Syntax 7. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234]. Form (ABNF) notation as specified in [RFC5234].
ALPHA = <as defined in RFC 5234 appendix B.1> ALPHA = <as defined in RFC 5234 Appendix B.1>
DIGIT = <as defined in RFC 5234 appendix B.1> DIGIT = <as defined in RFC 5234 Appendix B.1>
base64-char = ALPHA / DIGIT / "/" / "+" base64-char = ALPHA / DIGIT / "/" / "+"
base64-4 = 4base64-char base64-4 = 4base64-char
base64-3 = 3base64-char "=" base64-3 = 3base64-char "="
base64-2 = 2base64-char "==" base64-2 = 2base64-char "=="
base64 = *base64-4 [base64-3 / base64-2] base64 = *base64-4 [base64-3 / base64-2]
sr = "sr=" s-nonce sr = "sr=" s-nonce
;; s-nonce is defined in RFC 5802. ;; s-nonce is defined in RFC 5802.
data = "data=" base64 data = "data=" base64
;; The data attribute value is base64 encoded ;; The "data" attribute value is base64-encoded
;; SCRAM challenge or response defined in ;; SCRAM challenge or response defined in
;; RFC 5802. ;; RFC 5802.
ttl = "ttl" = 1*DIGIT ttl = "ttl=" 1*DIGIT
;; "sr" value validity in seconds. ;; "sr" value validity in seconds.
;; No leading 0s. ;; No leading 0s.
reauth-s-nonce = nonce-count s-nonce reauth-s-nonce = nonce-count s-nonce
nonce-count = posit-number nonce-count = posit-number
;; posit-number is defined in RFC 5802. ;; posit-number is defined in RFC 5802.
;; The initial value is taken from the "i" ;; The initial value is taken from the "i"
;; attribute for the user and is incremented ;; attribute for the user and is incremented
;; by 1 on each successful re-authentication. ;; by 1 on each successful reauthentication.
sid = "sid=" token sid = "sid=" token
;; See token definition in RFC 7235. ;; See token definition in RFC 7235.
stale = "stale=" ( "true" / "false" ) stale = "stale=" ( "true" / "false" )
realm = "realm=" <as defined in RFC 7235> realm = "realm=" <as defined in RFC 7235>
8. Security Considerations 8. Security Considerations
If the authentication exchange is performed without a strong session If the authentication exchange is performed without a strong session
encryption (such as TLS with data confidentiality), then a passive encryption (such as TLS with data confidentiality), then a passive
eavesdropper can gain sufficient information to mount an offline eavesdropper can gain sufficient information to mount an offline
dictionary or brute-force attack which can be used to recover the dictionary or brute-force attack that can be used to recover the
user's password. The amount of time necessary for this attack user's password. The amount of time necessary for this attack
depends on the cryptographic hash function selected, the strength of depends on the cryptographic hash function selected, the strength of
the password and the iteration count supplied by the server. SCRAM the password, and the iteration count supplied by the server. SCRAM
allows the server/server administrator to increase the iteration allows the server/server administrator to increase the iteration
count over time in order to slow down the above attacks. (Note that count over time in order to slow down the above attacks. (Note that
a server that is only in posession of "StoredKey" and "ServerKey" a server that is only in possession of StoredKey and ServerKey can't
can't automatic increase the iteration count upon successful automatically increase the iteration count upon successful
authentication. Such increase would require resetting user's authentication. Such an increase would require resetting the user's
password.) An external security layer with strong encryption will password.) An external security layer with strong encryption will
prevent these attack. prevent these attacks.
If the authentication information is stolen from the authentication If the authentication information is stolen from the authentication
database, then an offline dictionary or brute-force attack can be database, then an offline dictionary or brute-force attack can be
used to recover the user's password. The use of salt mitigates this used to recover the user's password. The use of salt mitigates this
attack somewhat by requiring a separate attack on each password. attack somewhat by requiring a separate attack on each password.
Authentication mechanisms which protect against this attack are Authentication mechanisms that protect against this attack are
available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is available (e.g., the Encrypted Key Exchange (EKE) class of
an example of such technology. mechanisms). RFC 2945 [RFC2945] is an example of such technology.
If an attacker obtains the authentication information from the If an attacker obtains the authentication information from the
authentication repository and either eavesdrops on one authentication authentication repository and either eavesdrops on one authentication
exchange or impersonates a server, the attacker gains the ability to exchange or impersonates a server, the attacker gains the ability to
impersonate that user to all servers providing SCRAM access using the impersonate that user to all servers providing SCRAM access using the
same hash function, password, iteration count and salt. For this same hash function, password, iteration count, and salt. For this
reason, it is important to use randomly-generated salt values. reason, it is important to use randomly generated salt values.
SCRAM does not negotiate a hash function to use. Hash function SCRAM does not negotiate which hash function to use. Hash function
negotiation is left to the HTTP authentication mechanism negotiation. negotiation is left to the HTTP authentication mechanism negotiation.
It is important that clients be able to sort a locally available list 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 of mechanisms by preference so that the client may pick the most
preferred of a server's advertised mechanism list. This preference preferred of a server's advertised mechanism list. This preference
order is not specified here as it is a local matter. The preference order is not specified here as it is a local matter. The preference
order should include objective and subjective notions of mechanism order should include objective and subjective notions of mechanism
cryptographic strength (e.g., SCRAM with SHA-256 should be preferred cryptographic strength (e.g., SCRAM with SHA-256 should be preferred
over SCRAM with SHA-1). over SCRAM with SHA-1).
This document recommends use of SCRAM with SHA-256 hash. SCRAM-SHA-1 This document recommends use of SCRAM with SHA-256 hash. SCRAM-SHA-1
is registered for database compatibility with implementations of RFC is registered for database compatibility with implementations of RFC
5802 (such as IMAP or XMPP servers) which want to also expose HTTP 5802 (such as IMAP or XMPP servers) that want to also expose HTTP
access to a related service, but it is not recommended for new access to a related service, but it is not recommended for new
deployments. deployments.
A hostile server can perform a computational denial-of-service attack A hostile server can perform a computational denial-of-service attack
on clients by sending a big iteration count value. In order to on clients by sending a big iteration count value. In order to
defend against that, a client implementation can pick a maximum defend against that, a client implementation can pick a maximum
iteration count that it is willing to use, and that it rejects any iteration count that it is willing to use and reject any values that
values that exceed that threshold (in such cases the client, of exceed that threshold (in such cases, the client, of course, has to
course, has to fail the authentication). fail the authentication).
See [RFC4086] for more information about generating randomness. See [RFC4086] for more information about generating randomness.
9. IANA Considerations 9. IANA Considerations
New mechanisms in the SCRAM- family are registered according to the New mechanisms in the SCRAM family are registered according to the
IANA procedure specified in [RFC5802]. IANA procedure specified in [RFC5802].
Note to future SCRAM- mechanism designers: each new SCRAM- HTTP Note to future "SCRAM-" mechanism designers: Each new "SCRAM-" HTTP
authentication mechanism MUST be explicitly registered with IANA and authentication mechanism MUST be explicitly registered with IANA and
MUST comply with SCRAM- mechanism naming convention defined in MUST comply with "SCRAM-" mechanism naming convention defined in
Section 4 of this document. Section 4 of this document.
IANA is requested to add the following entry to the Authentication IANA has added the following entries to the "HTTP Authentication
Scheme Registry defined in HTTP/1.1, Part 7 [RFC7235]: Schemes" registry defined in HTTP/1.1, Part 7 [RFC7235]:
Authentication Scheme Name: SCRAM-SHA-256
Pointer to specification text: [[ this document ]]
Notes (optional): (none)
Authentication Scheme Name: SCRAM-SHA-1
Pointer to specification text: [[ this document ]]
Notes (optional): (none)
10. Acknowledgements
This document benefited from discussions on the HTTPAuth, SASL and
Kitten WG mailing lists. The authors would like to specially thank
co-authors of [RFC5802] from which lots of text was copied.
Thank you to Martin Thomson for the idea of adding "ttl" attribute.
Thank you to Julian F. Reschke for corrections regarding use of
Authentication-Info header field.
Special thank you to Tony Hansen for doing an early implementation Authentication Scheme Name: SCRAM-SHA-256
and providing extensive comments on the draft. Pointer to specification text: RFC 7804
Notes (optional): (none)
Thank you to Russ Housley, Stephen Farrell, Barry Leiba and Tim Chown Authentication Scheme Name: SCRAM-SHA-1
for doing detailed reviews of the document. Pointer to specification text: RFC 7804
Notes (optional): (none)
11. Design Motivations 10. Design Motivations
The following design goals shaped this document. Note that some of The following design goals shaped this document. Note that some of
the goals have changed since the initial version of the document. the goals have changed since the initial draft version of the
document.
o The HTTP authentication mechanism has all modern features: support o The HTTP authentication mechanism has all modern features: support
for internationalized usernames and passwords. for internationalized usernames and passwords.
o The protocol supports mutual authentication. o The protocol supports mutual authentication.
o The authentication information stored in the authentication o The authentication information stored in the authentication
database is not sufficient by itself to impersonate the client. database is not sufficient by itself to impersonate the client.
o The server does not gain the ability to impersonate the client to o The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies), other servers (with an exception for server-authorized proxies),
unless such other servers allow SCRAM authentication and use the unless such other servers allow SCRAM authentication and use the
same salt and iteration count for the user. same salt and iteration count for the user.
o The mechanism is extensible, but [hopefully] not overengineered in o The mechanism is extensible, but (hopefully) not over-engineered
this respect. in this respect.
o Easier to implement than HTTP Digest in both clients and servers. o The mechanism is easier to implement than HTTP Digest in both
clients and servers.
o The protocol supports 1 round trip reauthentication. o The protocol supports 1 round-trip reauthentication.
12. References 11. References
12.1. Normative References 11.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997, DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>. <http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 17, line 42 skipping to change at page 17, line 26
[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy- [RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-
Authentication-Info Response Header Fields", RFC 7615, Authentication-Info Response Header Fields", RFC 7615,
DOI 10.17487/RFC7615, September 2015, DOI 10.17487/RFC7615, September 2015,
<http://www.rfc-editor.org/info/rfc7615>. <http://www.rfc-editor.org/info/rfc7615>.
[RFC7677] Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple [RFC7677] Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
Authentication and Security Layer (SASL) Mechanisms", Authentication and Security Layer (SASL) Mechanisms",
RFC 7677, DOI 10.17487/RFC7677, November 2015, RFC 7677, DOI 10.17487/RFC7677, November 2015,
<http://www.rfc-editor.org/info/rfc7677>. <http://www.rfc-editor.org/info/rfc7677>.
12.2. Informative References 11.2. Informative References
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", "Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000, RFC 2865, DOI 10.17487/RFC2865, June 2000,
<http://www.rfc-editor.org/info/rfc2865>. <http://www.rfc-editor.org/info/rfc2865>.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, Specification Version 2.0", RFC 2898,
DOI 10.17487/RFC2898, September 2000, DOI 10.17487/RFC2898, September 2000,
<http://www.rfc-editor.org/info/rfc2898>. <http://www.rfc-editor.org/info/rfc2898>.
skipping to change at page 18, line 27 skipping to change at page 18, line 13
<http://www.rfc-editor.org/info/rfc4510>. <http://www.rfc-editor.org/info/rfc4510>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>. <http://www.rfc-editor.org/info/rfc4949>.
[RFC6331] Melnikov, A., "Moving DIGEST-MD5 to Historic", RFC 6331, [RFC6331] Melnikov, A., "Moving DIGEST-MD5 to Historic", RFC 6331,
DOI 10.17487/RFC6331, July 2011, DOI 10.17487/RFC6331, July 2011,
<http://www.rfc-editor.org/info/rfc6331>. <http://www.rfc-editor.org/info/rfc6331>.
[Unicode-UAX15]
The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", June 2015,
<http://www.unicode.org/reports/tr15/>.
Acknowledgements
This document benefited from discussions on the mailing lists for the
HTTPAuth, SASL, and Kitten working groups. The author would like to
specially thank the co-authors of [RFC5802] from which lots of text
was copied.
Thank you to Martin Thomson for the idea of adding the "ttl"
attribute.
Thank you to Julian F. Reschke for corrections regarding use of the
Authentication-Info header field.
A special thank you to Tony Hansen for doing an early implementation
and providing extensive comments on the document.
Thank you to Russ Housley, Stephen Farrell, Barry Leiba, and Tim
Chown for doing detailed reviews of the document.
Author's Address Author's Address
Alexey Melnikov Alexey Melnikov
Isode Ltd Isode Ltd
Email: Alexey.Melnikov@isode.com Email: Alexey.Melnikov@isode.com
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