draft-ietf-httpauth-scram-auth-08.txt   draft-ietf-httpauth-scram-auth-09.txt 
HTTPAUTH A. Melnikov HTTPAUTH A. Melnikov
Internet-Draft Isode Ltd Internet-Draft Isode Ltd
Intended status: Standards Track October 18, 2015 Intended status: Standards Track November 13, 2015
Expires: April 20, 2016 Expires: May 16, 2016
Salted Challenge Response (SCRAM) HTTP Authentication Mechanism Salted Challenge Response (SCRAM) HTTP Authentication Mechanism
draft-ietf-httpauth-scram-auth-08.txt draft-ietf-httpauth-scram-auth-09.txt
Abstract Abstract
The secure authentication mechanism most widely deployed and used by The secure 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
skipping to change at page 1, line 45 skipping to change at page 1, line 45
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2016. This Internet-Draft will expire on May 16, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Conventions Used in This Document . . . . . . . . . . . . . . 2 1. Conventions Used in This Document . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . . . . 6 3. SCRAM Algorithm Overview . . . . . . . . . . . . . . . . . . 5
4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . . 7 4. SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . 11 6. Use of Authentication-Info header field with SCRAM . . . . . 11
7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
11. Design Motivations . . . . . . . . . . . . . . . . . . . . . 14 11. Design Motivations . . . . . . . . . . . . . . . . . . . . . 14
12. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 15 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 12.1. Normative References . . . . . . . . . . . . . . . . . . . 15
13.1. Normative References . . . . . . . . . . . . . . . . . . . 15 12.2. Informative References . . . . . . . . . . . . . . . . . . 16
13.2. Informative References . . . . . . . . . . . . . . . . . . 17 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Conventions Used in This Document 1. Conventions Used in This Document
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].
skipping to change at page 7, line 20 skipping to change at page 7, line 8
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) . http://www.iana.org) .
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 [I-D.hansen-scram-sha256]. 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 a 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
; 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 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 "authentication scheme" 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.
skipping to change at page 8, line 15 skipping to change at page 8, line 13
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: [...]
S: HTTP/1.1 401 Unauthorized S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: Digest realm="realm1@host.com", S: WWW-Authenticate: Digest realm="realm1@host.com",
Digest realm="realm2@host.com", Digest realm="realm2@host.com",
Digest realm="realm3@host.com", Digest realm="realm3@host.com",
SCRAM-SHA-1 realm="realm3@host.com", SCRAM-SHA-256 realm="realm3@host.com",
SCRAM-SHA-1 realm="testrealm@host.com" SCRAM-SHA-256 realm="testrealm@host.com"
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-1 realm="testrealm@host.com", C: Authorization: SCRAM-SHA-256 realm="testrealm@host.com",
data=biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8K data=biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8K
C: [...] C: [...]
S: HTTP/1.1 401 Unauthorized S: HTTP/1.1 401 Unauthorized
S: WWW-Authenticate: SCRAM-SHA-1 S: WWW-Authenticate: SCRAM-SHA-256
sid=AAAABBBBCCCCDDDD, sid=AAAABBBBCCCCDDDD,
data=cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJbGo data=cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJbGo
paE5sRixzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTYK paE5sRixzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTYK
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-1 sid=AAAABBBBCCCCDDDD, C: Authorization: SCRAM-SHA-256 sid=AAAABBBBCCCCDDDD,
data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ0FmdXhG data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ0FmdXhG
SWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empmTUhnc3FtbWl6 SWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empmTUhnc3FtbWl6
N0FuZFZRPQo= N0FuZFZRPQo=
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=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5NUc0PQo=
S: [...Other header fields and resource body...] S: [...Other header fields and resource body...]
skipping to change at page 9, line 29 skipping to change at page 9, line 27
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 base64
encoded "client-first-message" [RFC5802] containing: 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 the header always starts with "n", "y" or "p", Note that version of SCRAM doesn't support HTTP channel
otherwise the message is invalid and authentication MUST fail. 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. * SCRAM username and a random, unique nonce attributes.
In HTTP response, the server sends WWW-Authenticate header field In HTTP response, the server sends WWW-Authenticate header field
containing: a unique session identifier (the "sid" attribute) plus containing: a unique session identifier (the "sid" attribute) plus
the "data" attribute containing base64-encoded "server-first-message" the "data" attribute containing base64-encoded "server-first-message"
[RFC5802]. The "server-first-message" contains the user's iteration [RFC5802]. The "server-first-message" contains the user's iteration
count i, the user's salt, and the nonce with a concatenation of the count i, the user's salt, and the nonce with a concatenation of the
client-specified one with a server nonce. [[CREF1: OPEN ISSUE: client-specified one with a server nonce.
Alternatively, the "sid" attribute can be another header field.]]
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 and a ClientProof computed using the latter has the same nonce and a ClientProof computed using the
selected hash function (e.g. SHA-256) as explained earlier. selected hash function (e.g. SHA-256) 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
skipping to change at page 10, line 24 skipping to change at page 10, line 22
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 [RFC5802] and contains the server part of the "r" attribute (see [RFC5802] and
optional "ttl" attribute (which contains the "sr" value validity in optional "ttl" attribute (which contains the "sr" value validity in
seconds). 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 earlies remembers "i" and "s" attributes for the user from earlies
authentication exchanges with the server), it can respond to that authentication exchanges with the server), it can respond to that
with "client-final-message". [[CREF2: Should some counter be added with "client-final-message". [[CREF1: Should some counter be added
to make "sr" unique for each reauth?]] to make "sr" unique for each reauth?]]
If the server considers the server part of the nonce (the "r" If the server considers the server part of the nonce (the "r"
attribute) to be still valid, it will provide access to the requested attribute) to be still valid, it will provide access to the requested
resource (assuming the client hash verifies correctly, of course). resource (assuming the client hash verifies correctly, of course).
However if the server considers that the server part of the nonce is 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" stale (for example if the "sr" value is used after the "ttl"
seconds), the server returns "401 Unauthorized" containing the SCRAM seconds), the server returns "401 Unauthorized" containing the SCRAM
mechanism name with the following attributes: a new "sr", mechanism name with the following attributes: a new "sr",
"stale=true" and an optional "ttl". The "stale" attribute signals to "stale=true" and an optional "ttl". The "stale" attribute signals to
skipping to change at page 11, line 16 skipping to change at page 11, line 13
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@host.com", S: WWW-Authenticate: Digest realm="realm1@host.com",
Digest realm="realm2@host.com", Digest realm="realm2@host.com",
Digest realm="realm3@host.com", Digest realm="realm3@host.com",
SCRAM-SHA-1 realm="realm3@host.com", SCRAM-SHA-256 realm="realm3@host.com",
SCRAM-SHA-1 realm="testrealm@host.com", sr=pWUa2RaTCAfuxFIlj)hNlF$k0 SCRAM-SHA-256 realm="testrealm@host.com", sr=pWUa2RaTCAfuxFIlj)hNlF$k0
SCRAM-SHA-1 realm="testrealm2@host.com", sr=AAABBBCCCDDD, ttl=120 SCRAM-SHA-256 realm="testrealm2@host.com", sr=AAABBBCCCDDD, ttl=120
S: [...] S: [...]
[Client authenticates as usual to realm "testrealm@host.com"] [Client authenticates as usual to realm "testrealm@host.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==) from earlies exchanges. It will use the cached "i" (4096) and "s" (W22ZaJ0SNY7soEsUEjb6gQ==) from earlies exchanges.
It will use the server advertised "sr" value as the server part of the "r".] It will use 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-1 realm="testrealm@host.com", C: Authorization: SCRAM-SHA-256 realm="testrealm@host.com",
data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ0FmdXhG data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ0FmdXhG
SWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empmTUhnc3FtbWl6 SWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empmTUhnc3FtbWl6
N0FuZFZRPQo= N0FuZFZRPQo=
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=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5NUc0PQo=
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 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]. "UTF8-2", "UTF8-3" Form (ABNF) notation as specified in [RFC5234]. The "UTF8-2",
and "UTF8-4" non-terminal are defined in [RFC3629]. "UTF8-3" and "UTF8-4" non-terminals are defined in [RFC3629].
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 "="
skipping to change at page 13, line 4 skipping to change at page 12, line 45
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 security If the authentication exchange is performed without a strong security
layer (such as TLS with data confidentiality), then a passive layer (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 which 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. An the password and the iteration count supplied by the server. SCRAM
external security layer with strong encryption will prevent this allows to increase the iteration count over time in order to slow
attack. down the above attacks. (Note that a server that is only in
posession of "StoredKey" and "ServerKey" can't automatic increase the
If the external security layer used to protect the SCRAM exchange iteration count upon successful authentication. Such increase would
uses an anonymous key exchange, then the SCRAM channel binding require resetting user's password.) An external security layer with
mechanism can be used to detect a man-in-the-middle attack on the strong encryption will prevent these attack.
security layer and cause the authentication to fail as a result.
However, the man-in-the-middle attacker will have gained sufficient
information to mount an offline dictionary or brute-force attack.
For this reason, SCRAM allows to increase the iteration count over
time. (Note that a server that is only in posession of "StoredKey"
and "ServerKey" can't automatic increase the iteration count upon
successful authentication. Such increase would require resetting
user's password.)
If the authentication information is stolen from the authentication 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 which protect against this attack are
available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is
an example of such technology. an example of such technology.
If an attacker obtains the authentication information from the If an attacker obtains the authentication information from the
skipping to change at page 15, line 21 skipping to change at page 15, line 5
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 overengineered in
this respect. this respect.
o Easier to implement than HTTP Digest in both clients and servers. o Easier to implement than HTTP Digest in both clients and servers.
12. Open Issues 12. References
Should "sid" directive be an attribute or a new HTTP header field
shared with other HTTP authentication mechanisms?
Username/password normalization algorithm needs to be picked, once
Precis WG concludes its work.
13. References
13.1. Normative References
[I-D.hansen-scram-sha256] 12.1. Normative References
Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS SASL
Mechanisms", draft-hansen-scram-sha256-02 (work in
progress), October 2014.
[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 10 skipping to change at page 16, line 31
Enforcement, and Comparison of Internationalized Strings Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 7613, Representing Usernames and Passwords", RFC 7613,
DOI 10.17487/RFC7613, August 2015, DOI 10.17487/RFC7613, August 2015,
<http://www.rfc-editor.org/info/rfc7613>. <http://www.rfc-editor.org/info/rfc7613>.
[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>.
13.2. Informative References [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, [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>.
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