One document matched: draft-ietf-httpauth-digest-08.txt
Differences from draft-ietf-httpauth-digest-07.txt
HTTPAuth Working Group R. Shekh-Yusef, Ed.
Internet-Draft Avaya
Obsoletes: 2617 (if approved) D. Ahrens
Intended Status: Standards Track Independent
Expires: February 24, 2015 S. Bremer
Netzkonform
August 23, 2014
HTTP Digest Access Authentication
draft-ietf-httpauth-digest-08
Abstract
HTTP provides a simple challenge-response authentication mechanism
that may be used by a server to challenge a client request and by a
client to provide authentication information. This document defines
the HTTP Digest Authentication scheme that may be used with the
authentication mechanism.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Copyright and License Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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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
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Syntax Convention . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Algorithm Variants . . . . . . . . . . . . . . . . . . . . . 4
2.3 ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Digest Access Authentication Scheme . . . . . . . . . . . . . . 5
3.1 Overall Operation . . . . . . . . . . . . . . . . . . . . . 5
3.2 Representation of Digest Values . . . . . . . . . . . . . . 5
3.3 The WWW-Authenticate Response Header . . . . . . . . . . . . 5
3.4 The Authorization Request Header . . . . . . . . . . . . . . 8
3.4.1 Response . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4.2 A1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4.3 A2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4.4 Username Hashing . . . . . . . . . . . . . . . . . . . . 11
3.4.5 Parameter Values and Quoted-String . . . . . . . . . . . 11
3.4.6 Various Considerations . . . . . . . . . . . . . . . . . 12
3.5 The Authentication-Info Header . . . . . . . . . . . . . . . 13
3.6 Digest Operation . . . . . . . . . . . . . . . . . . . . . . 15
3.7 Security Protocol Negotiation . . . . . . . . . . . . . . . 16
3.8 Proxy-Authenticate and Proxy-Authorization . . . . . . . . . 16
3.9 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.9.1 Example with SHA-256 and MD5 . . . . . . . . . . . . . . 17
3.9.2 Example with SHA-512-256, Charset, and Userhash . . . . 18
4 Internationalization . . . . . . . . . . . . . . . . . . . . . . 20
5 Security Considerations . . . . . . . . . . . . . . . . . . . . 20
5.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2 Authentication of Clients using Digest Authentication . . . 20
5.3 Limited Use Nonce Values . . . . . . . . . . . . . . . . . . 21
5.4 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . . 21
5.5 Weakness Created by Multiple Authentication Schemes . . . . 22
5.6 Online dictionary attacks . . . . . . . . . . . . . . . . . 23
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5.7 Man in the Middle . . . . . . . . . . . . . . . . . . . . . 23
5.8 Chosen plaintext attacks . . . . . . . . . . . . . . . . . . 24
5.9 Precomputed dictionary attacks . . . . . . . . . . . . . . . 24
5.10 Batch brute force attacks . . . . . . . . . . . . . . . . . 24
5.11 Spoofing by Counterfeit Servers . . . . . . . . . . . . . . 25
5.12 Storing passwords . . . . . . . . . . . . . . . . . . . . . 25
5.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 26
6.1 HTTP Digest Hash Algorithms Registry . . . . . . . . . . . 26
6.2 Digest Scheme Registration . . . . . . . . . . . . . . . . 27
6.3 Authentication-Info Header Registration . . . . . . . . . . 27
7 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 28
8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 29
8.2 Informative References . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
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1 Introduction
HTTP provides a simple challenge-response authentication mechanism
that may be used by a server to challenge a client request and by a
client to provide authentication information. This document defines
the HTTP Digest Authentication scheme that may be used with the
authentication mechanism.
The details of the challenge-response authentication mechanism are
specified in the [HTTP-P7] document.
The combination of this document with Basic [BASIC] and [HTTP-P7]
obsolete RFC2617.
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 RFC 2119 [RFC2119].
2 Syntax Convention
2.1 Examples
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.
2.2 Algorithm Variants
When used with the Digest mechanism, each one of the algorithms has
two variants: Session variant and non-Session variant.
The non-Session variant is denoted by "<algorithm>", e.g. "SHA-256",
and the Session variant is denoted by "<algorithm>-sess", e.g. "SHA-
256-sess".
2.3 ABNF
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
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3 Digest Access Authentication Scheme
3.1 Overall Operation
The Digest 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, the HTTP method, and the requested URI. In this way, the
password is never sent in the clear. The username and password must
be prearranged in some fashion not addressed by this document.
3.2 Representation of Digest Values
An optional header allows the server to specify the algorithm used to
create the checksum or digest. This documents adds SHA-256 and SHA-
512/256 algorithms. 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, SHA-256 and SHA-512/256 by 64 hexadecimal
characters.
3.3 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 with
Digest scheme as per the framework defined above, and include some or
all of the following parameters:
realm
A string to be displayed to users so they know which username and
password to use. This string should contain at least the name of
the host performing the authentication and might additionally
indicate the collection of users who might have access. An example
might be "registered_users@gotham.news.com". (See section 2.2 of
[HTTP-P7] for more details).
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domain
A quoted, space-separated list of URIs, as specified in RFC 3986
[RFC3986], that define the protection space. If a URI is an
abs_path, it is relative to the canonical root URL of the server
being accessed. An absolute-URI in this list may refer to a
different server than the one being accessed. The client can use
this list to determine the set of URIs for which the same
authentication information may be sent: any URI that has a URI in
this list as a prefix (after both have been made absolute) may be
assumed to be in the same protection space. If this parameter is
omitted or its value is empty, the client should assume that the
protection space consists of all URIs on the responding server.
This parameter is not meaningful in Proxy-Authenticate headers,
for which the protection space is always the entire proxy; if
present it should be ignored.
nonce
A server-specified data string which should be uniquely generated
each time a 401 response is made. It is recommended that this
string be base64 or hexadecimal data. Specifically, since the
string is passed in the header lines as a quoted string, the
double-quote character is not allowed.
The contents of the nonce are implementation dependent. The
quality of the implementation depends on a good choice. A nonce
might, for example, be constructed as the base 64 encoding of
time-stamp H(time-stamp ":" ETag ":" private-key)
where time-stamp is a server-generated time or other non-repeating
value, ETag is the value of the HTTP ETag header associated with
the requested entity, and private-key is data known only to the
server. With a nonce of this form a server would recalculate the
hash portion after receiving the client authentication header and
reject the request if it did not match the nonce from that header
or if the time-stamp value is not recent enough. In this way the
server can limit the time of the nonce's validity. The inclusion
of the ETag prevents a replay request for an updated version of
the resource. (Note: including the IP address of the client in the
nonce would appear to offer the server the ability to limit the
reuse of the nonce to the same client that originally got it.
However, that would break proxy farms, where requests from a
single user often go through different proxies in the farm. Also,
IP address spoofing is not that hard.)
An implementation might choose not to accept a previously used
nonce or a previously used digest, in order to protect against a
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replay attack. Or, an implementation might choose to use one-time
nonces or digests for POST or PUT requests and a time-stamp for
GET requests. For more details on the issues involved see section
5 of this document.
The nonce is opaque to the client.
opaque
A string of data, specified by the server, which should be
returned by the client unchanged in the Authorization header of
subsequent requests with URIs in the same protection space. It is
recommended that this string be base64 or hexadecimal data.
stale
A case-insensitive flag, indicating that the previous request from
the client was rejected because the nonce value was stale. If
stale is TRUE, the client may wish to simply retry the request
with 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 parameter is not present,
the username and/or password are invalid, and new values must be
obtained.
algorithm
A string indicating a pair of algorithms used to produce the
digest and a checksum. If this is not present it is assumed to be
"MD5". If the algorithm is not understood, the challenge should be
ignored (and a different one used, if there is more than one).
In this document the string obtained by applying the digest
algorithm to the data "data" with secret "secret" will be denoted
by 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 "<algorithm>" and "<algorithm>-sess"
H(data) = <algorithm>(data)
and
KD(secret, data) = H(concat(secret, ":", data))
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For example:
For the "SHA-256" and "SHA-256-sess" algorithms
H(data) = SHA-256(data)
i.e., the digest is the SHA-256 of the secret concatenated with
a colon concatenated with the data. The "SHA-256-sess"
algorithm is intended to allow efficient 3rd party
authentication servers; for the difference in usage, see the
description in section 3.4.2.
qop
This parameter MUST be used by all implementations compliant with
this version of the Digest scheme. It is a quoted string of one or
more tokens indicating the "quality of protection" values
supported by the server. The value "auth" indicates
authentication; the value "auth-int" indicates authentication with
integrity protection; see the descriptions below for calculating
the response parameter value for the application of this choice.
Unrecognized options MUST be ignored.
charset
This is an optional parameter that is used by the server to
indicate the encoding scheme it supports.
userhash
This is an optional parameter that is used by the server to
indicate that it supports username hashing. Valid value are:
"true" or "false".
3.4 The Authorization Request Header
The client is expected to retry the request, passing an
Authorization header line with Digest scheme, which is defined
according to the framework above. The values of the opaque and
algorithm fields must be those supplied in the WWW-Authenticate
response header for the entity being requested.
The request includes some or all of the following parameters:
response
A string of the hex digits computed as defined below, which proves
that the user knows a password.
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username
The user's name in the specified realm.
uri
The URI from request-target of the Request-Line; duplicated here
because proxies are allowed to change the Request-Line in transit.
qop
Indicates what "quality of protection" the client has applied to
the message. Its value MUST be one of the alternatives the server
indicated it supports in the WWW-Authenticate header. These values
affect the computation of the response. Note that this is a single
token, not a quoted list of alternatives as in WWW-Authenticate.
.in 3
cnonce
This MUST be specified if a qop parameter is sent (see above), and
MUST NOT be specified if the server did not send a qop parameter
in the WWW-Authenticate header field. The cnonce value is an
opaque quoted string value provided by the client and used by both
client and server to avoid chosen plaintext attacks, to provide
mutual authentication, and to provide some message integrity
protection. See the descriptions below of the calculation of the
rspauth and response values.
nc
The "nc" parameter stands for "nonce count". This MUST be
specified if a qop parameter is sent (see above), and MUST NOT be
specified if the server did not send a qop parameter in the WWW-
Authenticate header field. The nc value is the hexadecimal count
of the number of requests (including the current request) that the
client has sent with the nonce value in this request. For
example, in the first request sent in response to a given nonce
value, the client sends "nc=00000001". The purpose of this
parameter is to allow the server to detect request replays by
maintaining its own copy of this count - if the same nc value is
seen twice, then the request is a replay. See the description
below of the construction of the response value.
userhash
This optional parameter is used by the client to indicate that the
username has been hashed. Valid value are: "true" or "false".
If a parameter or its value is improper, or required parameters are
missing, the proper response is 400 Bad Request. If the request-
digest is invalid, then a login failure should be logged, since
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repeated login failures from a single client may indicate an attacker
attempting to guess passwords.
The definition of response above indicates the encoding for its
value. The following definitions show how the value is computed.
3.4.1 Response
If the "qop" value is "auth" or "auth-int":
response = <"> < KD ( H(A1), unq(nonce)
":" nc
":" unq(cnonce)
":" unq(qop)
":" H(A2)
) <">
See below for the definitions for A1 and A2.
3.4.2 A1
If the "algorithm" parameter's value is "<algorithm>", e.g. "SHA-
256", then A1 is:
A1 = unq(username) ":" unq(realm) ":" passwd
where
passwd = < user's password >
If the "algorithm" parameter's value is "<algorithm>-sess", e.g.
"SHA-256-sess", then A1 is calculated using the initial nonce and
cnounce value from the first request by the client following receipt
of a WWW-Authenticate challenge from the server. It uses the server
nonce from that challenge, herin called nonce-prime, and the first
client nonce value to construct A1, herin called cnonce-prime as
follows:
A1 = H( unq(username) ":" unq(realm)
":" passwd )
":" unq(nonce-prime) ":" unq(cnonce-prime)
This creates a 'session key' for the authentication of subsequent
requests and responses which is different for each "authentication
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session", thus limiting the amount of material hashed with any one
key. (Note: see further discussion of the authentication session in
section 3.6.) Because the server need only use the hash of the user
credentials in order to create the A1 value, this construction could
be used in conjunction with a third party authentication service so
that the web server would not need the actual password value. The
specification of such a protocol is beyond the scope of this
specification.
3.4.3 A2
If the "qop" parameter's value is "auth" or is unspecified, then A2
is:
A2 = Method ":" request-uri
If the "qop" value is "auth-int", then A2 is:
A2 = Method ":" request-uri ":" H(entity-body)
3.4.4 Username Hashing
To protect the transport of the username from the client to the
server, the server SHOULD set the "userhash" parameter with the value
of "true" in the WWW-Authentication header.
If the client supports the "userhash" parameter, and the "userhash"
parameter value in the WWW-Authentication header is set to "true",
then the client MUST calculate a hash of the username after any other
hash calculation and include the "userhash" parameter with the value
of "true" in the Authorization Request Header. If the client does not
provide the "username" as a hash value or the "userhash" parameter
with the value of "true", the server MAY reject the request.
The following is the operation that the client will take to hash the
username:
username = H( unq(username) ":" unq(realm) )
3.4.5 Parameter Values and Quoted-String
Note that the value of many of the parameters, such as "username"
value, are defined as a "quoted-string". However, the "unq" notation
indicates that surrounding quotation marks are removed in forming the
string A1. Thus if the Authorization header includes the fields
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username="Mufasa", realm=myhost@testrealm.com
and the user Mufasa has password "Circle Of Life" then H(A1) would be
H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks
in the digested string.
No white space is allowed in any of the strings to which the digest
function H() is applied unless that white space exists in the quoted
strings or entity body whose contents make up the string to be
digested. For example, the string A1 illustrated above must be
Mufasa:myhost@testrealm.com:Circle Of Life
with no white space on either side of the colons, but with the white
space between the words used in the password value. Likewise, the
other strings digested by H() must not have white space on either
side of the colons which delimit their fields unless that white space
was in the quoted strings or entity body being digested.
Also note that if integrity protection is applied (qop=auth-int), the
H(entity-body) is the hash of the entity body, not the message body -
it is computed before any transfer encoding is applied by the sender
and after it has been removed by the recipient. Note that this
includes multipart boundaries and embedded headers in each part of
any multipart content-type.
3.4.6 Various Considerations
The "Method" value is the HTTP request method as specified in section
3.1.1 of [HTTP-P1]. The "request-target" value is the request-target
from the request line as specified in section 3.1.1 of [HTTP-P1].
This may be "*", an "absolute-URI" or an "absolute-path" as specified
in section 2.7 of [HTTP-P1], but it MUST agree with the request-
target. In particular, it MUST be an "absolute-URI" if the request-
target is an "absolute-URI". The "cnonce" value is an optional
client-chosen value whose purpose is to foil chosen plaintext
attacks.
The authenticating server must assure that the resource designated by
the "uri" parameter is the same as the resource specified in the
Request-Line; if they are not, the server SHOULD return a 400 Bad
Request error. (Since this may be a symptom of an attack, server
implementers may want to consider logging such errors.) The purpose
of duplicating information from the request URL in this field is to
deal with the possibility that an intermediate proxy may alter the
client's Request-Line. This altered (but presumably semantically
equivalent) request would not result in the same digest as that
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calculated by the client.
Implementers should be aware of how authenticated transactions
interact with shared caches. The HTTP/1.1 protocol specifies that
when a shared cache (see [HTTP-P6]) has received a request containing
an Authorization header and a response from relaying that request, it
MUST NOT return that response as a reply to any other request, unless
one of two Cache-Control (see section 3.2 of [HTTP-P6]) directive was
present in the response. If the original response included the "must-
revalidate" Cache-Control directive, the cache MAY use the entity of
that response in replying to a subsequent request, but MUST first
revalidate it with the origin server, using the request headers from
the new request to allow the origin server to authenticate the new
request. Alternatively, if the original response included the
"public" Cache-Control directive, the response entity MAY be returned
in reply to any subsequent request.
3.5 The Authentication-Info Header
The Authentication-Info header is used by the server to communicate
some information regarding the successful authentication in the
response.
Authentication-Info = auth-info
auth-info = *auth-param
The auth-param is defined in [RFC7235].
The request includes some or all of the following parameters:
nextnonce
The value of the nextnonce parameter is the nonce the server
wishes the client to use for a future authentication response.
The server may send the Authentication-Info header with a
nextnonce field as a means of implementing one-time or otherwise
changing nonces. If the nextnonce field is present the client
SHOULD use it when constructing the Authorization header for its
next request. Failure of the client to do so may result in a
request to re-authenticate from the server with the "stale=TRUE".
Server implementations should carefully consider the
performance implications of the use of this mechanism;
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pipelined requests will not be possible if every response
includes a nextnonce parameter that must be used on the next
request received by the server. Consideration should be given
to the performance vs. security tradeoffs of allowing an old
nonce value to be used for a limited time to permit request
pipelining. Use of the "nc" parameter can retain most of the
security advantages of a new server nonce without the
deleterious affects on pipelining.
qop
Indicates the "quality of protection" options applied to the
response by the server. The value "auth" indicates
authentication; the value "auth-int" indicates authentication with
integrity protection. The server SHOULD use the same value for the
qop parameter in the response as was sent by the client in the
corresponding request.
rspauth
The optional response digest in the "rspauth" parameter supports
mutual authentication -- the server proves that it knows the
user's secret, and with qop=auth-int also provides limited
integrity protection of the response. The "rspauth" value is
calculated as for the response in the Authorization header, except
that if "qop=auth" or is not specified in the Authorization header
for the request, A2 is
A2 = ":" request-uri
and if "qop=auth-int", then A2 is
A2 = ":" request-uri ":" H(entity-body)
cnonce and nc
The "cnonce" value and "nc" value MUST be the ones for the client
request to which this message is the response. The "rspauth",
"cnonce", and "nc" parameters MUST be present if "qop=auth" or
"qop=auth-int" is specified.
The Authentication-Info header is allowed in the trailer of an HTTP
message transferred via chunked transfer-coding.
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3.6 Digest Operation
Upon receiving the Authorization header, the server may check its
validity by looking up the password that corresponds to the submitted
username. Then, the server must perform the same digest operation
(e.g., MD5) performed by the client, and compare the result to the
given response value.
Note that the HTTP server does not actually need to know the user's
cleartext password. As long as H(A1) is available to the server, the
validity of an Authorization header may be verified.
The client response to a WWW-Authenticate challenge for a protection
space starts an authentication session with that protection space.
The authentication session lasts until the client receives another
WWW-Authenticate challenge from any server in the protection space. A
client should remember the username, password, nonce, nonce count and
opaque values associated with an authentication session to use to
construct the Authorization header in future requests within that
protection space. The Authorization header may be included
preemptively; doing so improves server efficiency and avoids extra
round trips for authentication challenges. The server may choose to
accept the old Authorization header information, even though the
nonce value included might not be fresh. Alternatively, the server
may return a 401 response with a new nonce value, causing the client
to retry the request; by specifying stale=TRUE with this response,
the server tells the client to retry with the new nonce, but without
prompting for a new username and password.
Because the client is required to return the value of the opaque
parameter given to it by the server for the duration of a session,
the opaque data may be used to transport authentication session state
information. (Note that any such use can also be accomplished more
easily and safely by including the state in the nonce.) For example,
a server could be responsible for authenticating content that
actually sits on another server. It would achieve this by having the
first 401 response include a domain parameter whose value includes a
URI on the second server, and an opaque parameter whose value
contains the state information. The client will retry the request, at
which time the server might respond with a 301/302 redirection,
pointing to the URI on the second server. The client will follow the
redirection, and pass an Authorization header , including the
<opaque> data.
As with the basic scheme, proxies must be completely transparent in
the Digest access authentication scheme. That is, they must forward
the WWW-Authenticate, Authentication-Info and Authorization headers
untouched. If a proxy wants to authenticate a client before a request
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is forwarded to the server, it can be done using the Proxy-
Authenticate and Proxy-Authorization headers described in section 3.6
below.
3.7 Security Protocol Negotiation
It is useful for a server to be able to know which security schemes a
client is capable of handling.
It is possible that a server may want to require Digest as its
authentication method, even if the server does not know that the
client supports it. A client is encouraged to fail gracefully if the
server specifies only authentication schemes it cannot handle.
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.
This specification defines the following algorithms:
* SHA2-256 (mandatory to implement)
* SHA2-512/256 (as a backup algorithm)
* MD5 (for backward compatibility).
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.
3.8 Proxy-Authenticate and Proxy-Authorization
The digest authentication scheme may also be used for authenticating
users to proxies, proxies to proxies, or proxies to origin servers by
use of the Proxy-Authenticate and Proxy-Authorization headers. These
headers are instances of the Proxy-Authenticate and Proxy-
Authorization headers specified in sections 4.2 and 4.3 of the
HTTP/1.1 specification [HTTP-P7] and their behavior is subject to
restrictions described there. The transactions for proxy
authentication are very similar to those already described. Upon
receiving a request which requires authentication, the proxy/server
must issue the "407 Proxy Authentication Required" response with a
"Proxy-Authenticate" header. The digest-challenge used in the Proxy-
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Authenticate header is the same as that for the WWW- Authenticate
header as defined above in section 3.2.1.
The client/proxy must then re-issue the request with a Proxy-
Authorization header, with parameters as specified for the
Authorization header in section 3.4 above.
On subsequent responses, the server sends Proxy-Authenticate-Info
with parameters the same as those for the Authentication-Info header
field.
Note that in principle a client could be asked to authenticate itself
to both a proxy and an end-server, but never in the same response.
3.9 Examples
3.9.1 Example with SHA-256 and MD5
The following example 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="SHA-256",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
opaque="5ccc069c403ebaf9f0171e9517f40e41"
WWW-Authenticate: Digest
realm="testrealm@host.com",
qop="auth, auth-int",
algorithm="MD5",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
opaque="5ccc069c403ebaf9f0171e9517f40ef41"
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:
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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 SHA-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="SHA-256",
nc=00000001,
cnonce="0a4f113b",
response="5abdd07184ba512a22c53f41470e5eea7dcaa3a93
a59b630c13dfe0a5dc6e38b",
opaque="5ccc069c403ebaf9f0171e9517f40e41"
3.9.2 Example with SHA-512-256, Charset, and Userhash
The following example assumes that an access protected document is
being requested from the server via a GET request. The URI for the
request is "http://api.example.org/doe.json". Both client and server
know the userhash of the username, support the UTF-8 charset, and use
the SHA-512-256 algorithm. The username for the request is "Jason
Doe" and the password is "Secret, or not?".
The first time the client requests the document, no Authorization
header is sent, so the server responds with:
HTTP/2.0 401 Unauthorized
WWW-Authenticate: Digest
realm="api@example.org",
qop=auth,
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algorithm=SHA-512-256,
nonce="e145a96d70d40739596e60c6340f13be03290bd73c676d
3f25c01271af522eb2",
opaque="192cbcf2a2576846522c1a367c1dfdf0359a87719c5cc1
839e4f3d2ffeb82517",
charset=UTF-8,
userhash=true
The client may prompt the user for the required credentials and send
a new request with following Authorization header:
Authorization: Digest
username="298bc3decec198ec5e7ecc1d69f059ca33044dd15baf45
a1f87bbd7adb3784fd",
realm="api@example.org",
uri="/doe.json",
algorithm=SHA-512-256,
nonce="e145a96d70d40739596e60c6340f13be03290bd73c676d
3f25c01271af522eb2",
nc=00000001,
cnonce="cde966df34a49d5d842a263604159141c81db8d468e1bf
657230429424fc337a",
qop=auth,
response="ec180fc03b7a0dcd43c414f66f2335399bbe5f4d4ad469
f8233106ba453213c8",
opaque="192cbcf2a2576846522c1a367c1dfdf0359a87719c5cc1
839e4f3d2ffeb82517",
userhash=true
If the client can not provide a hashed username for any reason, the
client may try a request with this Authorization header:
Authorization: Digest
username="Jason Doe",
realm="api@example.org",
uri="/doe.json",
algorithm=SHA-512-256,
nonce="e145a96d70d40739596e60c6340f13be03290bd73c676d
3f25c01271af522eb2",
nc=00000001,
cnonce="cde966df34a49d5d842a263604159141c81db8d468e1bf
657230429424fc337a",
qop=auth,
response="ec180fc03b7a0dcd43c414f66f2335399bbe5f4d4ad469
f8233106ba453213c8",
opaque="192cbcf2a2576846522c1a367c1dfdf0359a87719c5cc1
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839e4f3d2ffeb82517",
userhash=false
4 Internationalization
In challenges, servers SHOULD use the "charset" authentication
parameter (case-insensitive) to express the character encoding they
expect the user agent to use when generating A1 (see section 3.4.2)
and username hashing (see section 3.4.4).
The only allowed value is "UTF-8", to be matched case-insensitively
(see [RFC2978], Section 2.3). It indicates that the server expects
user name and password to be converted to Unicode Normalization Form
C ("NFC", see Section 3 of [RFC5198]) and to be encoded into octets
using the UTF-8 character encoding scheme ([RFC3629]).
If the user agent does not support the encoding indicated by the
server, it MUST fail the request.
5 Security Considerations
5.1 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.
Digest authentication SHOULD be used over a secure channel like HTTPS
[RFC2818].
5.2 Authentication of Clients using Digest Authentication
Digest Authentication does not provide a strong authentication
mechanism, when compared to public key based mechanisms, for example.
However, it is significantly stronger than (e.g.) CRAM-MD5, which has
been proposed for use with LDAP [RFC4513], POP and IMAP (see
[RFC2195]). It is intended to replace the much weaker and even more
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dangerous Basic mechanism.
Digest Authentication offers no confidentiality protection beyond
protecting the actual username and password. All of the rest of the
request and response are available to an eavesdropper.
Digest Authentication offers only limited integrity protection for
the messages in either direction. If qop=auth-int mechanism is used,
those parts of the message used in the calculation of the WWW-
Authenticate and Authorization header field response parameter values
(see section 3.2 above) are protected. Most header fields and their
values could be modified as a part of a man-in-the-middle attack.
Many needs for secure HTTP transactions cannot be met by Digest
Authentication. For those needs TLS or SHTTP are more appropriate
protocols. In particular Digest authentication cannot be used for any
transaction requiring confidentiality protection. Nevertheless many
functions remain for which Digest authentication is both useful and
appropriate.
5.3 Limited Use Nonce Values
The Digest scheme uses a server-specified nonce to seed the
generation of the response value (as specified in section 3.4.1
above). As shown in the example nonce in section 3.2.1, the server
is free to construct the nonce such that it may only be used from a
particular client, for a particular resource, for a limited period of
time or number of uses, or any other restrictions. Doing so
strengthens the protection provided against, for example, replay
attacks (see 4.5). However, it should be noted that the method
chosen for generating and checking the nonce also has performance and
resource implications. For example, a server may choose to allow
each nonce value to be used only once by maintaining a record of
whether or not each recently issued nonce has been returned and
sending a next-nonce parameter in the Authentication-Info header
field of every response. This protects against even an immediate
replay attack, but has a high cost checking nonce values, and perhaps
more important will cause authentication failures for any pipelined
requests (presumably returning a stale nonce indication). Similarly,
incorporating a request-specific element such as the Etag value for a
resource limits the use of the nonce to that version of the resource
and also defeats pipelining. Thus it may be useful to do so for
methods with side effects but have unacceptable performance for those
that do not.
5.4 Replay Attacks
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A replay attack against Digest authentication would usually be
pointless for a simple GET request since an eavesdropper would
already have seen the only document he could obtain with a replay.
This is because the URI of the requested document is digested in the
client request and the server will only deliver that document. By
contrast under Basic Authentication once the eavesdropper has the
user's password, any document protected by that password is open to
him.
Thus, for some purposes, it is necessary to protect against replay
attacks. A good Digest implementation can do this in various ways.
The server created "nonce" value is implementation dependent, but if
it contains a digest of the client IP, a time-stamp, the resource
ETag, and a private server key (as recommended above) then a replay
attack is not simple. An attacker must convince the server that the
request is coming from a false IP address and must cause the server
to deliver the document to an IP address different from the address
to which it believes it is sending the document. An attack can only
succeed in the period before the time-stamp expires. Digesting the
client IP and time-stamp in the nonce permits an implementation which
does not maintain state between transactions.
For applications where no possibility of replay attack can be
tolerated the server can use one-time nonce values which will not be
honored for a second use. This requires the overhead of the server
remembering which nonce values have been used until the nonce time-
stamp (and hence the digest built with it) has expired, but it
effectively protects against replay attacks.
An implementation must give special attention to the possibility of
replay attacks with POST and PUT requests. Unless the server employs
one-time or otherwise limited-use nonces and/or insists on the use of
the integrity protection of qop=auth-int, an attacker could replay
valid credentials from a successful request with counterfeit form
data or other message body. Even with the use of integrity protection
most metadata in header fields is not protected. Proper nonce
generation and checking provides some protection against replay of
previously used valid credentials, but see 4.8.
5.5 Weakness Created by Multiple Authentication Schemes
An HTTP/1.1 server may return multiple challenges with a 401
(Authenticate) response, and each challenge may use a different auth-
scheme. A user agent MUST choose to use the strongest auth- scheme it
understands and request credentials from the user based upon that
challenge.
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Note that many browsers will only recognize Basic and will require
that it be the first auth-scheme presented. Servers should only
include Basic if it is minimally acceptable.
When the server offers choices of authentication schemes using the
WWW-Authenticate header, the strength of the resulting authentication
is only as good as that of the of the weakest of the authentication
schemes. See section 5.7 below for discussion of particular attack
scenarios that exploit multiple authentication schemes.
5.6 Online dictionary attacks
If the attacker can eavesdrop, then it can test any overheard
nonce/response pairs against a list of common words. Such a list is
usually much smaller than the total number of possible passwords. The
cost of computing the response for each password on the list is paid
once for each challenge.
The server can mitigate this attack by not allowing users to select
passwords that are in a dictionary.
5.7 Man in the Middle
Both Basic and Digest authentication are vulnerable to "man in the
middle" (MITM) attacks, for example, from a hostile or compromised
proxy. Clearly, this would present all the problems of eavesdropping.
But it also offers some additional opportunities to the attacker.
A possible man-in-the-middle attack would be to add a weak
authentication scheme to the set of choices, hoping that the client
will use one that exposes the user's credentials (e.g. password). For
this reason, the client should always use the strongest scheme that
it understands from the choices offered.
An even better MITM attack would be to remove all offered choices,
replacing them with a challenge that requests only Basic
authentication, then uses the cleartext credentials from the Basic
authentication to authenticate to the origin server using the
stronger scheme it requested. A particularly insidious way to mount
such a MITM attack would be to offer a "free" proxy caching service
to gullible users.
User agents should consider measures such as presenting a visual
indication at the time of the credentials request of what
authentication scheme is to be used, or remembering the strongest
authentication scheme ever requested by a server and produce a
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warning message before using a weaker one. It might also be a good
idea for the user agent to be configured to demand Digest
authentication in general, or from specific sites.
Or, a hostile proxy might spoof the client into making a request the
attacker wanted rather than one the client wanted. Of course, this is
still much harder than a comparable attack against Basic
Authentication.
5.8 Chosen plaintext attacks
With Digest authentication, a MITM or a malicious server can
arbitrarily choose the nonce that the client will use to compute the
response. This is called a "chosen plaintext" attack. The ability to
choose the nonce is known to make cryptanalysis much easier.
However, no way to analyze the MD5 one-way function used by Digest
using chosen plaintext is currently known.
The countermeasure against this attack is for clients to be
configured to require the use of the optional "cnonce" parameter;
this allows the client to vary the input to the hash in a way not
chosen by the attacker.
5.9 Precomputed dictionary attacks
With Digest authentication, if the attacker can execute a chosen
plaintext attack, the attacker can precompute the response for many
common words to a nonce of its choice, and store a dictionary of
(response, password) pairs. Such precomputation can often be done in
parallel on many machines. It can then use the chosen plaintext
attack to acquire a response corresponding to that challenge, and
just look up the password in the dictionary. Even if most passwords
are not in the dictionary, some might be. Since the attacker gets to
pick the challenge, the cost of computing the response for each
password on the list can be amortized over finding many passwords. A
dictionary with 100 million password/response pairs would take about
3.2 gigabytes of disk storage.
The countermeasure against this attack is to for clients to be
configured to require the use of the optional "cnonce" parameter.
5.10 Batch brute force attacks
With Digest authentication, a MITM can execute a chosen plaintext
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attack, and can gather responses from many users to the same nonce.
It can then find all the passwords within any subset of password
space that would generate one of the nonce/response pairs in a single
pass over that space. It also reduces the time to find the first
password by a factor equal to the number of nonce/response pairs
gathered. This search of the password space can often be done in
parallel on many machines, and even a single machine can search large
subsets of the password space very quickly -- reports exist of
searching all passwords with six or fewer letters in a few hours.
The countermeasure against this attack is to for clients to be
configured to require the use of the optional "cnonce" parameter.
5.11 Spoofing by Counterfeit Servers
Basic Authentication is vulnerable to spoofing by counterfeit
servers. If a user can be led to believe that she is connecting to a
host containing information protected by a password she knows, when
in fact she is connecting to a hostile server, then the hostile
server can request a password, store it away for later use, and feign
an error. This type of attack is more difficult with Digest
Authentication -- but the client must know to demand that Digest
authentication be used, perhaps using some of the techniques
described above to counter "man-in-the-middle" attacks. Again, the
user can be helped in detecting this attack by a visual indication of
the authentication mechanism in use with appropriate guidance in
interpreting the implications of each scheme.
5.12 Storing passwords
Digest authentication requires that the authenticating agent (usually
the server) store some data derived from the user's name and password
in a "password file" associated with a given realm. Normally this
might contain pairs consisting of username and H(A1), where H(A1) is
the digested value of the username, realm, and password as described
above.
The security implications of this are that if this password file is
compromised, then an attacker gains immediate access to documents on
the server using this realm. Unlike, say a standard UNIX password
file, this information need not be decrypted in order to access
documents in the server realm associated with this file. On the other
hand, decryption, or more likely a brute force attack, would be
necessary to obtain the user's password. This is the reason that the
realm is part of the digested data stored in the password file. It
means that if one Digest authentication password file is compromised,
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it does not automatically compromise others with the same username
and password (though it does expose them to brute force attack).
There are two important security consequences of this. First the
password file must be protected as if it contained unencrypted
passwords, because for the purpose of accessing documents in its
realm, it effectively does.
A second consequence of this is that the realm string should be
unique among all realms which any single user is likely to use. In
particular a realm string should include the name of the host doing
the authentication. The inability of the client to authenticate the
server is a weakness of Digest Authentication.
5.13 Summary
By modern cryptographic standards Digest Authentication is weak. But
for a large range of purposes it is valuable as a replacement for
Basic Authentication. It remedies some, but not all, weaknesses of
Basic Authentication. Its strength may vary depending on the
implementation. In particular the structure of the nonce (which is
dependent on the server implementation) may affect the ease of
mounting a replay attack. A range of server options is appropriate
since, for example, some implementations may be willing to accept the
server overhead of one-time nonces or digests to eliminate the
possibility of replay. Others may satisfied with a nonce like the one
recommended above restricted to a single IP address and a single ETag
or with a limited lifetime.
The bottom line is that *any* compliant implementation will be
relatively weak by cryptographic standards, but *any* compliant
implementation will be far superior to Basic Authentication.
6 IANA Considerations
6.1 HTTP Digest Hash Algorithms Registry
This specification creates a new IANA registry named "HTTP Digest
Hash Algorithms". When registering a new hash algorithm, the
following information MUST be provided:
o Hash Algorithm
The textual name of the hash algorithm.
o Digest Size
The size of the algorithm's output in bits.
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o Reference
A reference to the specification that describes the new algorithm.
The update policy for this registry shall be Specification Required.
The initial registry will contain the following entries:
Hash Algorithm Digest Size Reference
-------------- ----------- ---------
"MD5" 128 RFC XXXX
"SHA-512-256" 256 RFC XXXX
"SHA-256" 256 RFC XXXX
Each one of the algorithms defined in the registry might have a -sess
variant, e.g. MD5-sess, SHA-256-sess, etc.
6.2 Digest Scheme Registration
This specification registers the Digest scheme with the
Authentication Scheme Registry.
Authentication Scheme Name: Digest
Pointer to specification text: RFCXXX
6.3 Authentication-Info Header Registration
This specification registers the Authentication-Info Header with the
Message Header Field Registry.
Header Field Name: Authentication-Info
Protocol: http
Status: standard
Reference: RFCXXXX, Section 3.5
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7 Acknowledgments
The authors of this document would like to thank the authors of
RFC2617, as this document heavily borrows text from their document to
provide a complete description of the digest scheme and its
operations.
The authors would like to thank Stephen Farrell, Yoav Nir, Phillip
Hallam-Baker, Manu Sporny, Paul Hoffman, Julian Reschke, Yaron
Sheffer, Sean Turner, Geoff Baskwill, Eric Cooper, Bjoern Hoehrmann,
Martin Durst, Peter Saint-Andre, Michael Sweet, Daniel Stenberg,
Brett Tate, Paul Leach, Ilari Liusvaara, and Gary Mort for their
careful review and comments.
The authors would like to thank Jonathan Stoke, Nico Williams, Harry
Halpin, and Phil Hunt for their comments on the mailing list when
discussing various aspects of this document.
The authors would like to thank Paul Kyzivat and Dale Worley for
their careful review and feedback on some aspects of this document.
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8 References
8.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[RFC7230] Fielding, R., Reschke, J., "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
[RFC7234] Fielding, R., Nottingham, M., Reschke, J., "Hypertext
Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
2014.
[RFC7235] Fielding, R., Reschke, J., "Hypertext Transfer Protocol
(HTTP/1.1): Authentication", RFC 7235, June 2014.
[BASIC] Reschke, J., "The 'Basic' HTTP Authentication Scheme",
draft-ietf-httpauth-basicauth-update (Work in Progress),
September 2013.
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8.2 Informative References
[RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
AUTHorize Extension for Simple Challenge/Response",
RFC 2195, September 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
Authors' Addresses
Rifaat Shekh-Yusef (Editor)
Avaya
250 Sydney Street
Belleville, Ontario
Canada
Phone: +1-613-967-5267
Email: rifaat.ietf@gmail.com
David Ahrens
Independent
California
USA
EMail: ahrensdc@gmail.com
Sophie Bremer
Netzkonform
Germany
Email: sophie.bremer@netzkonform.de
Shekh-Yusef, et. al. Expires February 24, 2015 [Page 30]
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