Internet DRAFT - draft-ietf-httpauth-digest-update
draft-ietf-httpauth-digest-update
HTTPAuth Working Group R. Shekh-Yusef
Internet-Draft D. Ahrens
Updates: 2617 (if approved) Avaya
Intended Status: Standards Track September 2, 2013
Expires: March 6, 2014
HTTP Digest Update
draft-ietf-httpauth-digest-update-05
Abstract
This documents specifies extensions to the HTTP Digest Authentication
mechanism to add support for new digest algorithms to the HTTP Digest
Access Authentication scheme.
This document also defines an extension to the HTTP Digest
Authentication mechanism to allow the server to indicate its support
for the UTF-8 character encoding scheme.
Status of this Memo
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Copyright and License Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Syntax Convention . . . . . . . . . . . . . . . . . . . . . . . 3
3 Digest Access Authentication Scheme . . . . . . . . . . . . . . 3
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1 Representation of digest values . . . . . . . . . . . . 4
3.1.2 Limitations . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Specification of Digest Headers . . . . . . . . . . . . . . 5
3.2.1 The WWW-Authenticate Response Header . . . . . . . . . . 5
3.2.2 The Authorization Request Header . . . . . . . . . . . . 6
3.3 Digest Operation . . . . . . . . . . . . . . . . . . . . . . 6
3.4 Security Protocol Operation . . . . . . . . . . . . . . . . 6
3.5 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 Internationalization . . . . . . . . . . . . . . . . . . . . . 9
5 Security Considerations . . . . . . . . . . . . . . . . . . . . 9
6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 9
7 References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1 Normative References . . . . . . . . . . . . . . . . . . . 10
7.2 Informative References . . . . . . . . . . . . . . . . . . 10
8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
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1 Introduction
This document specifies extensions to the HTTP Digest Access
Authentication scheme by adding support for SHA2-256 [FIPS 180-3] and
SHA2-512/256 [FIPS 180-3] hash algorithms. RFC2617 specifies the MD5
algorithm as the default hash algorithm used in the digest access
authentication scheme. Since RFC2617 was first proposed, the MD5
algorithm has been broken. In 2008 the US-CERT issued a note that
MD5 "should be considered cryptographically broken and unsuitable for
further use" [CERT-VU].
RFC2617 does not define how to treat Unicode characters [UNICODE]
outside the ASCII range [RFC20] with the "Digest" scheme. This
document also defines an extension to the HTTP Digest Authentication
mechanism to allow the server to indicate its support for the UTF-8
character encoding scheme.
1.1 Terminology
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 [RFC2119].
2 Syntax Convention
In the interest of clarity and readability, the extended parameters
or the headers and parameters in the examples in this document might
be broken into multiple lines. Any line that is indented in this
document is a continuation of the preceding line.
3 Digest Access Authentication Scheme
The Digest Access Authentication scheme is based on a simple
challenge-response paradigm. The Digest scheme challenges using a
nonce value. A valid response contains a checksum of the username,
the password, the given nonce value and the requested URI. In this
way the password is never sent in the clear.
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3.1 Introduction
3.1.1 Representation of digest values
An optional header allows the server to specify the algorithm used to
create the checksum or digest. By default the SHA2-256 algorithm is
used, with SHA2-512/256 being used as a backup algorithm. To
maintain backwards compatibility, the MD5 algorithm is still
supported but not recommended.
The size of the digest depends on the algorithm used. The bits in
the digest are converted from the most significant to the least
significant bit, four bits at a time to the ASCII representation as
follows. Each four bits is represented by its familiar hexadecimal
notation from the characters 0123456789abcdef, that is binary 0000 is
represented by the character '0', 0001 by '1' and so on up to the
representation of 1111 as 'f'. If the MD5 algorithm is used to
calculate the digest, then the digest will be represented as 32
hexadecimal characters, SHA2-256 and SHA2-512/256 by 64 hexadecimal
characters.
3.1.2 Limitations
The Digest authentication scheme suffers from many known limitations
as specified in RFC2617, section 3.1.4. The update in this document
does not address those limitations.
HTTP Digest authentication, when used with human-memorable passwords,
is vulnerable to dictionary attacks. Such attacks are much easier
than cryptographic attacks on any widely used algorithm, including
those that are no longer considered secure. In other words, algorithm
agility does not make this usage any more secure.
As a result, Digest authentication SHOULD be used only with passwords
that have a reasonable amount of entropy, e.g. 128-bit or more. Such
passwords typically cannot be memorized by humans but can be used for
automated web services.
It is recommended that Digest authentication be used over a secure
channel like HTTPS.
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3.2 Specification of Digest Headers
The modifications to the formats of the WWW-Authenticate Header line
and the Authorization header line are specified below.
3.2.1 The WWW-Authenticate Response Header
If a server receives a request for an access protected object, and an
acceptable Authorization header is not sent, the server responds with
a "401 Unauthorized" status code, and a WWW-Authenticate header. The
server MAY include multiple WWW-Authenticate headers to allow the
server to utilize the best available algorithm supported by the
client.
The algorithm directive is extended as follows:
algorithm = "algorithm" "=" (
"MD5" | "MD5-sess" |
"SHA2-256" | "SHA2-256-sess" |
"SHA2-512-256" | "SHA2-512-256-sess" |
token)
Algorithm
A string indicating a pair of algorithms used to produce the
digest and a checksum. If the algorithm parameter is not present
it is assumed to be "MD5" to maintain backwards compatibility
with existing implementations. If the algorithm is not
understood, the challenge should be ignored and a different
challenge used if there is more than one.
The string obtained by applying the digest algorithm to the data
"data" with "secret" will be denoted KD(secret, data) and the
string obtained by applying the checksum algorithm to the data
"data" will be denoted H(data). The notation unq(x) means the
value of the quoted string X without the surrounding quotes.
For the "MD5 and "MD5-sess" algorithms
H(data) = MD5(data)
For the "SHA2-256" and "SHA2-256-sess" algorithms
H(data) = SHA2-256(data)
For the "SHA2-512-256" and "SHA2-512-256-sess" algorithms
H(data) = SHA2-512-256(data)
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and
KD(secret, data) = H(concat(secret, ":", data))
i.e the digest is the hash of the secret concatenated with a
colon concatenated with the data. The " -sess" algorithm is
intended to allow efficient 3rd party authentication servers;
for the difference in usage see the description in section
RFC2617, Section 3.2.2.2.
3.2.2 The Authorization Request Header
The client is expected to retry the request, passing an Authorization
Request Header line. The Authorization Request Header line is
modified as follows:
request-digest = <"> digest-size LHEX <">
digest-size = "32" | "64"
The values of the opaque and algorithm fields must match those
supplied in the WWW-Authenticate response header for the entity being
requested.
response
A string of hex digits as defined in RFC2617 which proves
that the user knows a password.
3.3 Digest Operation
The modifications specified in this document does not introduce any
change to the digest operation specified in RFC2617.
3.4 Security Protocol Operation
When a server receives a request to access a resource, the server
might challenge the client by responding with "401 Unauthorized"
status code, and include one or more WWW-Authenticate headers. If the
server challenges with multiple Digest headers, then each one of
these headers MUST use a different digest algorithm. The server MUST
add these Digest headers to the response in order of preference,
starting with the most preferred header, followed by the less
preferred headers.
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This specification defines the following preference list, starting
with the most preferred algorithm:
* SHA2-256 as the default algorithm.
* SHA2-512/256 as a backup algorithm.
* MD5 for backward compatibility.
A future version of this document might add SHA3 [SHA3] as a backup
algorithm, once its definition has been finalized and published.
When the client receives the response it SHOULD use the topmost
header that it supports, unless a local policy dictates otherwise.
The client should ignore any challenge it does not understand.
NOTE: There is some concern around the support for the SHA2-512/256
algorithm in the common implementation of SHA2.
3.5 Example
The following example is borrowed from RFC2617 and assumes that an
access protected document is being requested from the server via a
GET request. The URI of the document is
http://www.nowhere.org/dir/index.html". Both client and server know
that the username for this document is "Mufasa" and the password is
"Circle of Life" ( with one space between each of the three words).
The first time the client requests the document, no Authorization
header is sent, so the server responds with:
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest
realm = "testrealm@host.com",
qop="auth, auth-int",
algorithm="SHA2-256",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
opaque="5ccc069c403ebaf9f0171e9517f40e41"
WWW-Authenticate: Digest
realm="testrealm@host.com",
qop="auth, auth-int",
algorithm="MD5",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
opaque="5ccc069c403ebaf9f0171e9517f40ef41"
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The client may prompt the user for their username and password, after
which it will respond with a new request, including the following
Authorization header if the client chooses MD5 digest:
Authorization:Digest username="Mufasa",
realm="testrealm@host.com",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
uri="/dir/index.html",
qop="auth",
algorithm="MD5",
nc=00000001,
cnonce="0a4f113b",
response="6629fae49393a05397450978507c4ef1",
opaque="5ccc069c403ebaf9f0171e9517f40e41"
If the client chooses to use the SHA2-256 algorithm for calculating
the response, the client responds with a new request including the
following Authorization header:
Authorization:Digest username="Mufasa",
realm="testrealm@host.com",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
uri="/dir/index.html",
qop="auth",
algorithm="SHA2-256",
nc=00000001,
cnonce="0a4f113b",
response="5abdd07184ba512a22c53f41470e5eea7dcaa3a93
a59b630c13dfe0a5dc6e38b",
opaque="5ccc069c403ebaf9f0171e9517f40e41"
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4 Internationalization
The "Digest" mechanism allows for new parameters to be defined and
used with Authenticate and Authorization requests. This document
defines a new optional "charset" auth-param that could be used by the
server to indicate the encoding scheme it supports.
In challenges, servers MAY use the "charset" authentication parameter
(case-insensitive) to express the character encoding they expect the
user agent to use.
The only allowed value is "UTF-8", to be matched case-insensitively,
indicating that the server expects the UTF-8 character encoding to be
used ([RFC3629]).
5 Security Considerations
This specification updates the Digest Access Authentication scheme
specified in RFC2617 to add support for the SHA2-256 and SHA2-512/256
hash algorithms. Support for these additional hash algorithms does
not alter the security properties of the Digest Access Authentication
scheme.
6 Acknowledgments
The authors would like to thank Geoff Baskwill and Eric Cooper for
their careful review and comments on the pre published version of
this document.
The authors would also like to thank Stephen Farrell, Yoav Nir,
Phillip Hallam-Baker, Manu Sporny, Paul Hoffman, Julian Reschke, and
Sean Turner for their careful review and comments on and off the
mailing list.
Special thanks to Yaron Sheffer for his thorough review, comments on
and off the list, and for the text he provided for the limitation
section.
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7 References
7.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC6365] Hoffman, P., Klensin, J., "Terminology Used in
Internationalization in the IETF", BCP: 166, RFC 6365,
September 2011.
[UNICODE] The Unicode Consortium, "The Unicode Standard,
Version 6.0".
<http://www.unicode.org/versions/Unicode6.0.0/>.
[RFC20] Cerf, V., "ASCII format for Network Interchange", RFC 20,
October 1969.
7.2 Informative References
[FIPS180-3] National Institute of Standards and Technology
(NIST), FIPS Publication 180-3: Digital Signature
Standard, June 2009.
[CERT-VU] Vulnerability Note VU#836068, "MD5 vulnerable to
collision attacks", December 2008.
[SHA3] National Institute of Standards and Technology (NIST),
"CRYPTOGRAPHIC HASH AND SHA-3 STANDARD DEVELOPMENT".
http://csrc.nist.gov/groups/ST/hash/index.html
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8 Authors' Addresses
Rifaat Shekh-Yusef
Avaya
250 Sydney Street
Belleville, Ontario
Canada
Phone: +1-613-967-5267
Email: rifaat.ietf@gmail.com
David Ahrens
Avaya
EMail: ahrensdc@gmail.com
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