HTTPAuth Working Group R. Shekh-Yusef, Ed.
Internet-Draft D. Ahrens, Ed.
Obsoletes: 2617 (if approved) Avaya
Intended Status: Standards Track October 7, 2013
Expires: April 10, 2014
HTTP Digest Access Authentication
draft-ietf-httpauth-digest-00.txt
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
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Copyright and License Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Syntax Convention . . . . . . . . . . . . . . . . . . . . . . . 6
3 Digest Access Authentication Scheme . . . . . . . . . . . . . . 6
3.1 Overall Operation . . . . . . . . . . . . . . . . . . . . . 6
3.2 Representation of Digest Values . . . . . . . . . . . . . . 6
3.3 The WWW-Authenticate Response Header . . . . . . . . . . . . 7
3.4 The Authorization Request Header . . . . . . . . . . . . . . 10
3.4.1 Request-Digest . . . . . . . . . . . . . . . . . . . . . 12
3.4.2 A1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4.3 A2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4.4 Directive Values and Quoted-String . . . . . . . . . . . 13
3.4.5 Various Considerations . . . . . . . . . . . . . . . . . 14
3.6 Digest Operation . . . . . . . . . . . . . . . . . . . . . . 16
3.7 Security Protocol Negotiation . . . . . . . . . . . . . . . 18
3.8 Proxy-Authentication and Proxy-Authorization . . . . . . . . 18
3.9 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4 Internationalization . . . . . . . . . . . . . . . . . . . . . . 21
5 Security Considerations . . . . . . . . . . . . . . . . . . . . 21
5.1 Limitations . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2 Authentication of Clients using Digest Authentication . . . 21
5.3 Limited Use Nonce Values . . . . . . . . . . . . . . . . . . 22
5.4 Replay Attacks . . . . . . . . . . . . . . . . . . . . . . . 22
5.5 Weakness Created by Multiple Authentication Schemes . . . . 23
5.6 Online dictionary attacks . . . . . . . . . . . . . . . . . 24
5.7 Man in the Middle . . . . . . . . . . . . . . . . . . . . . 24
5.8 Chosen plaintext attacks . . . . . . . . . . . . . . . . . . 25
5.9 Precomputed dictionary attacks . . . . . . . . . . . . . . . 25
5.10 Batch brute force attacks . . . . . . . . . . . . . . . . . 25
5.11 Spoofing by Counterfeit Servers . . . . . . . . . . . . . . 26
5.12 Storing passwords . . . . . . . . . . . . . . . . . . . . . 26
5.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 27
7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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7.1 Normative References . . . . . . . . . . . . . . . . . . . . 27
7.2 Informative References . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
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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. It uses an extensible,
case-insensitive token to identify the authentication scheme,
followed by a comma-separated list of attribute-value pairs which
carry the parameters necessary for achieving authentication via that
scheme.
auth-scheme = token
auth-param = token "=" ( token | quoted-string )
The 401 (Unauthorized) response message is used by an origin server
to challenge the authorization of a user agent. This response MUST
include a WWW-Authenticate header field containing at least one
challenge applicable to the requested resource. The 407 (Proxy
Authentication Required) response message is used by a proxy to
challenge the authorization of a client and MUST include a Proxy-
Authenticate header field containing at least one challenge
applicable to the proxy for the requested resource.
challenge = auth-scheme 1*SP 1#auth-param
Note: User agents will need to take special care in parsing the WWW-
Authenticate or Proxy-Authenticate header field value if it contains
more than one challenge, or if more than one WWW-Authenticate header
field is provided, since the contents of a challenge may itself
contain a comma-separated list of authentication parameters.
The authentication parameter realm is defined for all authentication
schemes:
realm = "realm" "=" realm-value
realm-value = quoted-string
The realm directive (case-insensitive) is required for all
authentication schemes that issue a challenge. The realm value (case-
sensitive), in combination with the canonical root URL (the
absoluteURI for the server whose abs_path is empty; see section 5.1.2
of [RFC2616]) of the server being accessed, defines the protection
space. These realms allow the protected resources on a server to be
partitioned into a set of protection spaces, each with its own
authentication scheme and/or authorization database. The realm value
is a string, generally assigned by the origin server, which may have
additional semantics specific to the authentication scheme. Note that
there may be multiple challenges with the same auth-scheme but
different realms.
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A user agent that wishes to authenticate itself with an origin
server--usually, but not necessarily, after receiving a 401
(Unauthorized)--MAY do so by including an Authorization header field
with the request. A client that wishes to authenticate itself with a
proxy--usually, but not necessarily, after receiving a 407 (Proxy
Authentication Required)--MAY do so by including a Proxy-
Authorization header field with the request. Both the Authorization
field value and the Proxy-Authorization field value consist of
credentials containing the authentication information of the client
for the realm of the resource being requested. The user agent MUST
choose to use one of the challenges with the strongest auth-scheme it
understands and request credentials from the user based upon that
challenge.
credentials = auth-scheme #auth-param
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.
The protection space determines the domain over which credentials can
be automatically applied. If a prior request has been authorized, the
same credentials MAY be reused for all other requests within that
protection space for a period of time determined by the
authentication scheme, parameters, and/or user preference. Unless
otherwise defined by the authentication scheme, a single protection
space cannot extend outside the scope of its server.
If the origin server does not wish to accept the credentials sent
with a request, it SHOULD return a 401 (Unauthorized) response. The
response MUST include a WWW-Authenticate header field containing at
least one (possibly new) challenge applicable to the requested
resource. If a proxy does not accept the credentials sent with a
request, it SHOULD return a 407 (Proxy Authentication Required). The
response MUST include a Proxy-Authenticate header field containing a
(possibly new) challenge applicable to the proxy for the requested
resource.
The HTTP protocol does not restrict applications to this simple
challenge-response mechanism for access authentication. Additional
mechanisms MAY be used, such as encryption at the transport level or
via message encapsulation, and with additional header fields
specifying authentication information. However, these additional
mechanisms are not defined by this specification.
Proxies MUST be completely transparent regarding user agent
authentication by origin servers. That is, they must forward the WWW-
Authenticate and Authorization headers untouched, and follow the
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rules found in section 14.8 of [RFC2616]. Both the Proxy-Authenticate
and the Proxy-Authorization header fields are hop-by-hop headers (see
section 13.5.1 of [RFC2616]).
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
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
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. 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.
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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 as
per the framework defined above, which for the digest scheme is
utilized as follows:
challenge = "Digest" digest-challenge
digest-challenge = 1#( realm | [ domain ] | nonce |
[ opaque ] |[ stale ] | [ algorithm ] |
[ qop-options ] | [charset] | [auth-param])
domain = "domain" "=" <"> URI ( 1*SP URI ) <">
URI = absoluteURI | abs_path
nonce = "nonce" "=" nonce-value
nonce-value = quoted-string
opaque = "opaque" "=" quoted-string
stale = "stale" "=" ( "true" | "false" )
algorithm = "algorithm" "=" (
"MD5" | "MD5-sess" |
"SHA2-256" | "SHA2-256-sess" |
"SHA2-512-256" | "SHA2-512-256-sess" |
token)
qop-options = "qop" "=" <"> 1#qop-value <">
qop-value = "auth" | "auth-int" | token
charset = "charset" "=" ("UTF-8" | token)
The meanings of the values of the directives used above are as
follows:
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".
domain
A quoted, space-separated list of URIs, as specified in RFC XURI
[7], 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 absoluteURI 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
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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 directive is omitted or its
value is empty, the client should assume that the protection space
consists of all URIs on the responding server.
This directive 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
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.
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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 flag, indicating that the previous request from the client was
rejected because the nonce value was stale. If stale is TRUE
(case-insensitive), the client may wish to simply retry the
request with a new encrypted response, without reprompting the
user for a new username and password. The server should only set
stale to TRUE if it receives a request for which the nonce is
invalid but with a valid digest for that nonce (indicating that
the client knows the correct username/password). If stale is
FALSE, or anything other than TRUE, or the stale directive is not
present, the username and/or password are invalid, and new values
must be obtained.
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 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)
and
KD(secret, data) = H(concat(secret, ":", data))
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i.e., the digest is the MD5 of the secret concatenated with a
colon concatenated with the data. The "MD5-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-options
This directive is optional, but is made so only for backward
compatibility with RFC 2069 [RFC2069]; it SHOULD be used by all
implementations compliant with this version of the Digest scheme.
If present, 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 directive value
for the application of this choice. Unrecognized options MUST be
ignored.
charset
This is an optional parameter that could be used by the server to
indicate the encoding scheme it supports.
auth-param
This directive allows for future extensions. Any unrecognized
directive MUST be ignored.
3.4 The Authorization Request Header
The client is expected to retry the request, passing an Authorization
header line, which is defined according to the framework above,
utilized as follows.
credentials = "Digest" digest-response
digest-response = 1#( username | realm | nonce | digest-uri |
response | [ algorithm ] | [cnonce] |
[opaque] | [message-qop] |
[nonce-count] | [charset] | [auth-param] )
username = "username" "=" username-value
username-value = quoted-string
digest-uri = "uri" "=" digest-uri-value
digest-uri-value = request-uri ; As specified by HTTP/1.1
message-qop = "qop" "=" qop-value
cnonce = "cnonce" "=" cnonce-value
cnonce-value = nonce-value
nonce-count = "nc" "=" nc-value
nc-value = 8LHEX
response = "response" "=" request-digest
request-digest = <"> digest-size LHEX <">
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digest-size = "32" | "64"
LHEX = "0" | "1" | "2" | "3" |
"4" | "5" | "6" | "7" |
"8" | "9" | "a" | "b" |
"c" | "d" | "e" | "f"
charset = "charset" "=" ("UTF-8" | token)
The values of the opaque and algorithm fields must be those supplied
in the WWW-Authenticate response header for the entity being
requested.
response
A string of digest-size hex digits computed as defined below,
which proves that the user knows a password
username
The user's name in the specified realm.
digest-uri
The URI from Request-URI 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. If present, 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 request-digest. Note
that this is a single token, not a quoted list of alternatives as
in WWW- Authenticate. This directive is optional in order to
preserve backward compatibility with a minimal implementation of
RFC 2069 [RFC2069], but SHOULD be used if the server indicated
that qop is supported by providing a qop directive in the WWW-
Authenticate header field.
cnonce
This MUST be specified if a qop directive is sent (see above), and
MUST NOT be specified if the server did not send a qop directive
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
response- digest and request-digest values.
nonce-count
This MUST be specified if a qop directive is sent (see above), and
MUST NOT be specified if the server did not send a qop directive
in the WWW-Authenticate header field. The nc-value is the
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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 directive 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 request-digest value.
charset
This is an optional parameter that could be used by the client to
indicate the encoding scheme it supports.
auth-param
This directive allows for future extensions. Any unrecognized
directive MUST be ignored.
If a directive or its value is improper, or required directives are
missing, the proper response is 400 Bad Request. If the request-
digest is invalid, then a login failure should be logged, since
repeated login failures from a single client may indicate an attacker
attempting to guess passwords.
The definition of request-digest above indicates the encoding for its
value. The following definitions show how the value is computed.
3.4.1 Request-Digest
If the "qop" value is "auth" or "auth-int":
request-digest = <"> < KD ( H(A1), unq(nonce-value)
":" nc-value
":" unq(cnonce-value)
":" unq(qop-value)
":" H(A2)
) <">
If the "qop" directive is not present (this construction is for
compatibility with RFC 2069):
request-digest =
<"> < KD ( H(A1), unq(nonce-value) ":" H(A2) ) >
<">
See below for the definitions for A1 and A2.
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3.4.2 A1
If the "algorithm" directive's value is "MD5", "SHA2-256", or "SHA2-
512-256", then A1 is:
A1 = unq(username-value) ":" unq(realm-value) ":" passwd
where
passwd = < user's password >
If the "algorithm" directive's value is "MD5-sess", "SHA2-256", or
"SHA2-512-256", then A1 is calculated only once - on the first
request by the client following receipt of a WWW-Authenticate
challenge from the server. It uses the server nonce from that
challenge, and the first client nonce value to construct A1 as
follows:
A1 = H( unq(username-value) ":" unq(realm-value)
":" passwd )
":" unq(nonce-value) ":" unq(cnonce-value)
This creates a 'session key' for the authentication of subsequent
requests and responses which is different for each "authentication
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" directive's value is "auth" or is unspecified, then A2
is:
A2 = Method ":" digest-uri-value
If the "qop" value is "auth-int", then A2 is:
A2 = Method ":" digest-uri-value ":" H(entity-body)
3.4.4 Directive Values and Quoted-String
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Note that the value of many of the directives, 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
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.
3.4.5 Various Considerations
The "Method" value is the HTTP request method as specified in section
5.1.1 of [RFC2616]. The "request-uri" value is the Request-URI from
the request line as specified in section 5.1.2 of [RFC2616]. This may
be "*", an "absoluteURL" or an "abs_path" as specified in section
5.1.2 of [RFC2616], but it MUST agree with the Request-URI. In
particular, it MUST be an "absoluteURL" if the Request-URI is an
"absoluteURL". 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" directive 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
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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
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 section 13.7 of [RFC2616]) 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 14.9
of [RFC2616]) directives 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.
AuthenticationInfo = "Authentication-Info" ":" auth-info
auth-info = 1#(nextnonce | [ message-qop ]
| [ response-auth ] | [ cnonce ]
| [nonce-count] )
nextnonce = "nextnonce" "=" nonce-value
response-auth = "rspauth" "=" response-digest
response-digest = <"> *LHEX <">
The value of the nextnonce directive 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; pipelined requests will
not be possible if every response includes a nextnonce directive
that must be used on the next request received by the server.
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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 nonce-count can
retain most of the security advantages of a new server nonce
without the deleterious affects on pipelining.
message-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
message- qop directive in the response as was sent by the client
in the corresponding request.
The optional response digest in the "response-auth" directive
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 "response-digest" value is calculated
as for the "request-digest" in the Authorization header, except that
if "qop=auth" or is not specified in the Authorization header for the
request, A2 is
A2 = ":" digest-uri-value
and if "qop=auth-int", then A2 is
A2 = ":" digest-uri-value ":" H(entity-body)
where "digest-uri-value" is the value of the "uri" directive on the
Authorization header in the request. The "cnonce-value" and "nc-
value" MUST be the ones for the client request to which this message
is the response. The "response-auth", "cnonce", and "nonce-count"
directives 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.
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 request-digest 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
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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
directive 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 directive whose value includes a
URI on the second server, and an opaque directive 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
is forwarded to the server, it can be done using the Proxy-
Authenticate and Proxy-Authorization headers described in section 3.6
below.
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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 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.
3.8 Proxy-Authentication 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 10.33 and 10.34 of the
HTTP/1.1 specification [RFC2616] 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-
Authenticate header is the same as that for the WWW- Authenticate
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header as defined above in section 3.2.1.
The client/proxy must then re-issue the request with a Proxy-
Authorization header, with directives as specified for the
Authorization header in section 3.4 above.
On subsequent responses, the server sends Proxy-Authentication-Info
with directives 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 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
In challenges, servers SHOULD 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]).
If the user agent supports the encoding indicated by the server, it
MAY add the "charset" parameter, with the value it received from the
server, to the Proxy-Authenticate or WWW-Authenticate header fields
it sends back to the server.
If the user agent does not support the encoding indicated by the
server, it MAY add the "charset" parameter to the Proxy-Authenticate
or WWW-Authenticate header fields it sends back to the server, but
the value in the parameter should be preceded by an exclamation point
(!).
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.
It is recommended that Digest authentication be used over a secure
channel like HTTPS.
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 [10], POP and IMAP (see RFC 2195
[9]). 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 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 directive 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. Any service in present use that uses Basic should be
switched to Digest as soon as practical.
5.3 Limited Use Nonce Values
The Digest scheme uses a server-specified nonce to seed the
generation of the request-digest value (as specified in section
3.2.2.1 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 directive 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 4.8 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
warning message before using a weaker one. It might also be a good
idea for the user agent to be configured to demand Digest
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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 [8].
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" directive;
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" directive.
5.10 Batch brute force attacks
With Digest authentication, a MITM can execute a chosen plaintext
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
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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" directive.
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,
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.
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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
<IANA considerations text>
7 References
7.1 Normative References
7.2 Informative References
Authors' Addresses
Rifaat Shekh-Yusef
Avaya
250 Sydney Street
Belleville, Ontario
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Canada
Phone: +1-613-967-5267
Email: rifaat.ietf@gmail.com
David Ahrens
Avaya
California
USA
EMail: ahrensdc@gmail.com
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