Internet DRAFT - draft-ietf-httpauth-scram-auth
draft-ietf-httpauth-scram-auth
HTTPAUTH A. Melnikov
Internet-Draft Isode Ltd
Intended status: Experimental December 16, 2015
Expires: June 18, 2016
Salted Challenge Response (SCRAM) HTTP Authentication Mechanism
draft-ietf-httpauth-scram-auth-15.txt
Abstract
This specification describes a family of HTTP authentication
mechanisms called the Salted Challenge Response Authentication
Mechanism (SCRAM), which provides a more robust authentication
mechanism than a plaintext password protected by Transport Layer
Security (TLS) and avoids the deployment obstacles presented by
earlier TLS-protected challenge response authentication mechanisms.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 18, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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carefully, as they describe your rights and restrictions with respect
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . . . . 5
4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . . 6
5. SCRAM Authentication Exchange . . . . . . . . . . . . . . . . 7
5.1. One round trip reauthentication . . . . . . . . . . . . . . 10
6. Use of Authentication-Info header field with SCRAM . . . . . 12
7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
11. Design Motivations . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
The authentication mechanism most widely deployed and used by
Internet application protocols is the transmission of clear-text
passwords over a channel protected by Transport Layer Security (TLS).
There are some significant security concerns with that mechanism,
which could be addressed by the use of a challenge response
authentication mechanism protected by TLS. Unfortunately, the HTTP
Digest challenge response mechanism presently on the standards track
failed widespread deployment, and has 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 requirements necessary to deploy a challenge-
response mechanism more widely than past attempts (see [RFC5802]).
In particular, it addresses some of the issues identified with HTTP
Digest, as described in [RFC6331], such as complexity of implementing
and protection of the whole authentication exchange in order to
protect from certain man-in-the-middle attacks.
HTTP SCRAM is an adoptation of [RFC5802] for use in HTTP. The SCRAM
data exchanged is identical to what is defined in [RFC5802]. This
document also adds a 1 round trip reauthentication mode.
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HTTP SCRAM provides the following protocol features:
o The authentication information stored in the authentication
database is not sufficient by itself (without a dictionary attack)
to impersonate the client. The information is salted to make it
harder to do a pre-stored dictionary attack if the database is
stolen.
o The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies),
unless it performs a dictionary attack.
o The mechanism permits the use of a server-authorized proxy without
requiring that proxy to have super-user rights with the back-end
server.
o Mutual authentication is supported, but only the client is named
(i.e., the server has no name).
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Formal syntax is defined by [RFC5234] including the core rules
defined in Appendix B of [RFC5234].
Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server. If a single "C:" or "S:" label
applies to multiple lines, then the line breaks between those lines
are for editorial clarity only, and are not part of the actual
protocol exchange.
2.1. Terminology
This document uses several terms defined in [RFC4949] ("Internet
Security Glossary") including the following: authentication,
authentication exchange, authentication information, brute force,
challenge-response, cryptographic hash function, dictionary attack,
eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
one-way encryption function, password, replay attack and salt.
Readers not familiar with these terms should use that glossary as a
reference.
Some clarifications and additional definitions follow:
o Authentication information: Information used to verify an identity
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claimed by a SCRAM client. The authentication information for a
SCRAM identity consists of salt, iteration count, the "StoredKey"
and "ServerKey" (as defined in the algorithm overview) for each
supported cryptographic hash function.
o Authentication database: The database used to look up the
authentication information associated with a particular identity.
For application protocols, LDAPv3 (see [RFC4510]) is frequently
used as the authentication database. For lower-layer protocols
such as PPP or 802.11x, the use of RADIUS [RFC2865] is more
common.
o Base64: An encoding mechanism defined in Section 4 of [RFC4648]
which converts an octet string input to a textual output string
which can be easily displayed to a human. The use of base64 in
SCRAM is restricted to the canonical form with no whitespace.
o Octet: An 8-bit byte.
o Octet string: A sequence of 8-bit bytes.
o Salt: A random octet string that is combined with a password
before applying a one-way encryption function. This value is used
to protect passwords that are stored in an authentication
database.
2.2. Notation
The pseudocode description of the algorithm uses the following
notations:
o ":=": The variable on the left hand side represents the octet
string resulting from the expression on the right hand side.
o "+": Octet string concatenation.
o "[ ]": A portion of an expression enclosed in "[" and "]" is
optional in the result under some circumstances. See the
associated text for a description of those circumstances.
o Normalize(str): Apply the Preparation and Enforcement steps
according to the OpaqueString profile (see [RFC7613]) to a UTF-8
[RFC3629] encoded "str". The resulting string is also in UTF-8.
Note that implementations MUST either implement OpaqueString
profile operations from [RFC7613], or disallow use of non US-ASCII
Unicode codepoints in "str". The latter is a particular case of
compliance with [RFC7613].
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o HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in
[RFC2104]) using the octet string represented by "key" as the key
and the octet string "str" as the input string. The size of the
result is the hash result size for the hash function in use. For
example, it is 32 octets for SHA-256 and 20 octets for SHA-1 (see
[RFC6234]).
o H(str): Apply the cryptographic hash function to the octet string
"str", producing an octet string as a result. The size of the
result depends on the hash result size for the hash function in
use.
o XOR: Apply the exclusive-or operation to combine the octet string
on the left of this operator with the octet string on the right of
this operator. The length of the output and each of the two
inputs will be the same for this use.
o Hi(str, salt, i):
U1 := HMAC(str, salt + INT(1))
U2 := HMAC(str, U1)
...
Ui-1 := HMAC(str, Ui-2)
Ui := HMAC(str, Ui-1)
Hi := U1 XOR U2 XOR ... XOR Ui
where "i" is the iteration count, "+" is the string concatenation
operator and INT(g) is a four-octet encoding of the integer g,
most significant octet first.
Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and
with dkLen == output length of HMAC() == output length of H().
3. SCRAM Algorithm Overview
The following is a description of a full HTTP SCRAM authentication
exchange. Note that this section omits some details, such as client
and server nonces. See Section 5 for more details.
To begin with, the SCRAM client is in possession of a username and
password, both encoded in UTF-8 [RFC3629] (or a ClientKey/ServerKey,
or SaltedPassword). It sends the username to the server, which
retrieves the corresponding authentication information: a salt, a
StoredKey, a ServerKey and an iteration count ("i"). (Note that a
server implementation may choose to use the same iteration count for
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all accounts.) The server sends the salt and the iteration count to
the client, which then computes the following values and sends a
ClientProof to the server:
Informative Note: Implementors are encouraged to create test cases
that use both username passwords with non-ASCII codepoints. In
particular, it is useful to test codepoints whose "Unicode
Normalization Canonical Composition (NFC)" and "Unicode Normalization
Form Compatibility Composition (NFKC)" are different. Some examples
of such codepoints include Vulgar Fraction One Half (U+00BD) and
Acute Accent (U+00B4).
SaltedPassword := Hi(Normalize(password), salt, i)
ClientKey := HMAC(SaltedPassword, "Client Key")
StoredKey := H(ClientKey)
AuthMessage := client-first-message-bare + "," +
server-first-message + "," +
client-final-message-without-proof
ClientSignature := HMAC(StoredKey, AuthMessage)
ClientProof := ClientKey XOR ClientSignature
ServerKey := HMAC(SaltedPassword, "Server Key")
ServerSignature := HMAC(ServerKey, AuthMessage)
The server authenticates the client by computing the ClientSignature,
exclusive-ORing that with the ClientProof to recover the ClientKey
and verifying the correctness of the ClientKey by applying the hash
function and comparing the result to the StoredKey. If the ClientKey
is correct, this proves that the client has access to the user's
password.
Similarly, the client authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If
the two are equal, it proves that the server had access to the user's
ServerKey.
For initial authentication the AuthMessage is computed by
concatenating decoded "data" attribute values from the authentication
exchange. The format of each of these 3 decoded "data" attributes is
defined in [RFC5802].
4. SCRAM Mechanism Names
A SCRAM mechanism name (authentication scheme) is a string "SCRAM-"
followed by the uppercased name of the underlying hash function taken
from the IANA "Hash Function Textual Names" registry (see
http://www.iana.org/assignments/hash-function-text-names/) .
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For interoperability, all HTTP clients and servers supporting SCRAM
MUST implement the SCRAM-SHA-256 authentication mechanism, i.e. an
authentication mechanism from the SCRAM family that uses the SHA-256
hash function as defined in [RFC7677].
5. SCRAM Authentication Exchange
HTTP SCRAM is a HTTP Authentication mechanism whose client response
(<credentials-scram>) and server challenge (<challenge-scram>)
messages are text-based messages containing one or more attribute-
value pairs separated by commas. The messages and their attributes
are described below and defined in Section 7.
challenge-scram = scram-name [1*SP 1#auth-param]
; Complies with <challenge> ABNF from RFC 7235.
; Included in the WWW-Authenticate header field.
credentials-scram = scram-name [1*SP 1#auth-param]
; Complies with <credentials> from RFC 7235.
; Included in the Authorization header field.
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-1 is registered for database compatibility
; with implementations of RFC 5802 (such as IMAP or XMPP
; servers), but it is not recommended for new deployments.
other-scram-name = "SCRAM-" hash-name
; hash-name is a capitalized form of names from IANA
; "Hash Function Textual Names" registry.
; Additional SCRAM names must be registered in both
; the IANA "SASL mechanisms" registry
; and the IANA "authentication scheme" registry.
This is a simple example of a SCRAM-SHA-256 authentication exchange
(no support for channel bindings, as this feature is not currently
supported by HTTP). Username 'user' and password 'pencil' are used.
Note that long lines are folded for readability.
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C: GET /resource HTTP/1.1
C: Host: server.example.com
C: [...]
S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: Digest realm="realm1@example.com",
Digest realm="realm2@example.com",
Digest realm="realm3@example.com",
SCRAM-SHA-256 realm="realm3@example.com",
SCRAM-SHA-256 realm="testrealm@example.com"
S: [...]
C: GET /resource HTTP/1.1
C: Host: server.example.com
C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",
data=biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8K
C: [...]
S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: SCRAM-SHA-256
sid=AAAABBBBCCCCDDDD,
data=cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJbGo
paE5sRixzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTYK
S: [...]
C: GET /resource HTTP/1.1
C: Host: server.example.com
C: Authorization: SCRAM-SHA-256 sid=AAAABBBBCCCCDDDD,
data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ0FmdXhG
SWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empmTUhnc3FtbWl6
N0FuZFZRPQo=
C: [...]
S: HTTP/1.1 200 Ok
S: Authentication-Info: sid=AAAABBBBCCCCDDDD,
data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5NUc0PQo=
S: [...Other header fields and resource body...]
In the above example the first client request contains data attribute
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:
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"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.
Initial "SCRAM-SHA-256" authentication starts with sending the
"Authorization" request header field defined by HTTP/1.1, Part 7
[RFC7235] containing "SCRAM-SHA-256" authentication scheme and the
following attributes:
o A "realm" attribute MAY be included to indicate the scope of
protection in the manner described in HTTP/1.1, Part 7 [RFC7235].
As specified in [RFC7235], the "realm" attribute MUST NOT appear
more than once. The realm attribute only appears in the first
SCRAM message to the server and in the first SCRAM response from
the server.
o The client also includes the data attribute that contains base64
encoded "client-first-message" [RFC5802] containing:
* a header consisting of a flag indicating whether channel
binding is supported-but-not-used, not supported, or used .
Note that this version of SCRAM doesn't support HTTP channel
bindings, so this header always starts with "n"; otherwise the
message is invalid and authentication MUST fail.
* SCRAM username and a random, unique nonce attributes.
In HTTP response, the server sends WWW-Authenticate header field
containing: a unique session identifier (the "sid" attribute) plus
the "data" attribute containing base64-encoded "server-first-message"
[RFC5802]. The "server-first-message" contains the user's iteration
count i, the user's salt, and the nonce with a concatenation of the
client-specified one (taken from the "client-first-message") with a
freshly generated server nonce.
The client then responds with another HTTP request with the
Authorization header field, which includes the "sid" attribute
received in the previous server response, together with the "data"
attribute containing base64-encoded "client-final-message" data. The
latter has the same nonce as in "server-first-message" and a
ClientProof computed using the selected hash function (e.g. SHA-256)
as explained earlier.
The server verifies the nonce and the proof, and, finally, it
responds with a 200 HTTP response with the Authentication-Info header
field [RFC7615] containing the "sid" attribute (as received from the
client) and the "data" attribute containing base64-encoded "server-
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final-message", concluding the authentication exchange.
The client then authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If
the two are different, the client MUST consider the authentication
exchange to be unsuccessful and it might have to drop the connection.
5.1. One round trip reauthentication
If the server supports SCRAM reauthentication, the server sends in
its initial HTTP response a WWW-Authenticate header field containing:
the "realm" attribute (as defined earlier), the "sr" attribute that
contains the server part of the "r" attribute (see s-nonce in
[RFC5802]) and optional "ttl" attribute (which contains the "sr"
value validity in seconds).
If the client has authenticated to the same realm before (i.e. it
remembers "i" and "s" attributes for the user from earlier
authentication exchanges with the server), it can respond to that
with "client-final-message". When constructing the "client-final-
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
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
the user's "i" attribute on first reauthentication and is incremented
by 1 on each successful re-authentication.
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
same nonce-count value is seen twice, then the request is a
replay.
If the server considers the s-nonce part of the nonce attribute (the
"r" attribute) to be still valid (i.e. the nonce-count part is as
expected (see above) and the "sr" part is still fresh), it will
provide access to the requested resource (assuming the client hash
verifies correctly, of course). However if the server considers that
the server part of the nonce is stale (for example if the "sr" value
is used after the "ttl" seconds), the server returns "401
Unauthorized" containing the SCRAM mechanism name with the following
attributes: a new "sr", "stale=true" and an optional "ttl". The
"stale" attribute signals to the client that there is no need to ask
user for the password.
Formally, the "stale" attribute is defined as follows: A flag,
indicating that the previous request from the client was rejected
because the nonce value was stale. If stale is TRUE (case-
insensitive), the client may wish to simply retry the request with
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a new encrypted response, without reprompting the user for a new
username and password. The server should only set stale to TRUE
if it receives a request for which the nonce is invalid but with a
valid digest for that nonce (indicating that the client knows the
correct username/password). If stale is FALSE, or anything other
than TRUE, or the stale directive is not present, the username
and/or password are invalid, and new values must be obtained.
When constructing AuthMessage Section 3 to be used for calculating
client and server proofs, "client-first-message-bare" and "server-
first-message" are reconstructed from data known to the client and
the server.
Reauthentication can look like this:
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C: GET /resource HTTP/1.1
C: Host: server.example.com
C: [...]
S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: Digest realm="realm1@example.com",
Digest realm="realm2@example.com",
Digest 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="testrealm2@example.com", sr=AAABBBCCCDDD, ttl=120
S: [...]
[Client authenticates as usual to realm "testrealm@example.com"]
[Some time later client decides to reauthenticate.
It will use the cached "i" (4096) and "s" (W22ZaJ0SNY7soEsUEjb6gQ==)
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".]
C: GET /resource HTTP/1.1
C: Host: server.example.com
C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",
data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU80MDk2JWh2WURwV1VhMlJhVENB
ZnV4RklsailoTmxGLHA9ZEh6YlphcFdJazRqVWhOK1V0ZTl5dGFnOXpqZk1IZ3Nx
bW1pejdBbmRWUT0K
C: [...]
S: HTTP/1.1 200 Ok
S: Authentication-Info: sid=AAAABBBBCCCCDDDD,
data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5NUc0PQo=
S: [...Other header fields and resource body...]
6. Use of Authentication-Info header field with SCRAM
When used with SCRAM, the Authentication-Info header field is allowed
in the trailer of an HTTP message transferred via chunked transfer-
coding.
7. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234].
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ALPHA = <as defined in RFC 5234 appendix B.1>
DIGIT = <as defined in RFC 5234 appendix B.1>
base64-char = ALPHA / DIGIT / "/" / "+"
base64-4 = 4base64-char
base64-3 = 3base64-char "="
base64-2 = 2base64-char "=="
base64 = *base64-4 [base64-3 / base64-2]
sr = "sr=" s-nonce
;; s-nonce is defined in RFC 5802.
data = "data=" base64
;; The data attribute value is base64 encoded
;; SCRAM challenge or response defined in
;; RFC 5802.
ttl = "ttl" = 1*DIGIT
;; "sr" value validity in seconds.
;; No leading 0s.
reauth-s-nonce = nonce-count s-nonce
nonce-count = posit-number
;; posit-number is defined in RFC 5802.
;; The initial value is taken from the "i"
;; attribute for the user and is incremented
;; by 1 on each successful re-authentication.
sid = "sid=" token
;; See token definition in RFC 7235.
stale = "stale=" ( "true" / "false" )
realm = "realm=" <as defined in RFC 7235>
8. Security Considerations
If the authentication exchange is performed without a strong session
encryption (such as TLS with data confidentiality), then a passive
eavesdropper can gain sufficient information to mount an offline
dictionary or brute-force attack which can be used to recover the
user's password. The amount of time necessary for this attack
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depends on the cryptographic hash function selected, the strength of
the password and the iteration count supplied by the server. SCRAM
allows the server/server administrator to increase the iteration
count over time in order to slow down the above attacks. (Note that
a server that is only in posession of "StoredKey" and "ServerKey"
can't automatic increase the iteration count upon successful
authentication. Such increase would require resetting user's
password.) An external security layer with strong encryption will
prevent these attack.
If the authentication information is stolen from the authentication
database, then an offline dictionary or brute-force attack can be
used to recover the user's password. The use of salt mitigates this
attack somewhat by requiring a separate attack on each password.
Authentication mechanisms which protect against this attack are
available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is
an example of such technology.
If an attacker obtains the authentication information from the
authentication repository and either eavesdrops on one authentication
exchange or impersonates a server, the attacker gains the ability to
impersonate that user to all servers providing SCRAM access using the
same hash function, password, iteration count and salt. For this
reason, it is important to use randomly-generated salt values.
SCRAM does not negotiate a hash function to use. Hash function
negotiation is left to the HTTP authentication mechanism negotiation.
It is important that clients be able to sort a locally available list
of mechanisms by preference so that the client may pick the most
preferred of a server's advertised mechanism list. This preference
order is not specified here as it is a local matter. The preference
order should include objective and subjective notions of mechanism
cryptographic strength (e.g., SCRAM with SHA-256 should be preferred
over SCRAM with SHA-1).
This document recommends use of SCRAM with SHA-256 hash. SCRAM-SHA-1
is registered for database compatibility with implementations of RFC
5802 (such as IMAP or XMPP servers) which want to also expose HTTP
access to a related service, but it is not recommended for new
deployments.
A hostile server can perform a computational denial-of-service attack
on clients by sending a big iteration count value. In order to
defend against that, a client implementation can pick a maximum
iteration count that it is willing to use, and that it rejects any
values that exceed that threshold (in such cases the client, of
course, has to fail the authentication).
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See [RFC4086] for more information about generating randomness.
9. IANA Considerations
New mechanisms in the SCRAM- family are registered according to the
IANA procedure specified in [RFC5802].
Note to future SCRAM- mechanism designers: each new SCRAM- HTTP
authentication mechanism MUST be explicitly registered with IANA and
MUST comply with SCRAM- mechanism naming convention defined in
Section 4 of this document.
IANA is requested to add the following entry to the Authentication
Scheme Registry defined in HTTP/1.1, Part 7 [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
and providing extensive comments on the draft.
Thank you to Russ Housley, Stephen Farrell, Barry Leiba and Tim Chown
for doing detailed reviews of the document.
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11. Design Motivations
The following design goals shaped this document. Note that some of
the goals have changed since the initial version of the document.
o The HTTP authentication mechanism has all modern features: support
for internationalized usernames and passwords.
o The protocol supports mutual authentication.
o The authentication information stored in the authentication
database is not sufficient by itself to impersonate the client.
o The server does not gain the ability to impersonate the client to
other servers (with an exception for server-authorized proxies),
unless such other servers allow SCRAM authentication and use the
same salt and iteration count for the user.
o The mechanism is extensible, but [hopefully] not overengineered in
this respect.
o Easier to implement than HTTP Digest in both clients and servers.
o The protocol supports 1 round trip reauthentication.
12. References
12.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
DOI 10.17487/RFC5802, July 2010,
<http://www.rfc-editor.org/info/rfc5802>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<http://www.rfc-editor.org/info/rfc7235>.
[RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 7613,
DOI 10.17487/RFC7613, August 2015,
<http://www.rfc-editor.org/info/rfc7613>.
[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-
Authentication-Info Response Header Fields", RFC 7615,
DOI 10.17487/RFC7615, September 2015,
<http://www.rfc-editor.org/info/rfc7615>.
[RFC7677] Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
Authentication and Security Layer (SASL) Mechanisms",
RFC 7677, DOI 10.17487/RFC7677, November 2015,
<http://www.rfc-editor.org/info/rfc7677>.
12.2. Informative References
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000,
<http://www.rfc-editor.org/info/rfc2865>.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898,
DOI 10.17487/RFC2898, September 2000,
<http://www.rfc-editor.org/info/rfc2898>.
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[RFC2945] Wu, T., "The SRP Authentication and Key Exchange System",
RFC 2945, DOI 10.17487/RFC2945, September 2000,
<http://www.rfc-editor.org/info/rfc2945>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC4510] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): Technical Specification Road Map", RFC 4510,
DOI 10.17487/RFC4510, June 2006,
<http://www.rfc-editor.org/info/rfc4510>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC6331] Melnikov, A., "Moving DIGEST-MD5 to Historic", RFC 6331,
DOI 10.17487/RFC6331, July 2011,
<http://www.rfc-editor.org/info/rfc6331>.
Author's Address
Alexey Melnikov
Isode Ltd
Email: Alexey.Melnikov@isode.com
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