One document matched: draft-ietf-http-authentication-00.txt
HTTP Working Group J. Franks, Northwestern University
INTERNET DRAFT P. Hallam-Baker, M.I.T.
<draft-ietf-http-authentication-00> J. Hostetler, Spyglass, Inc.
P. Leach, Microsoft Corporation
A. Luotonen, Netscape Communications Corporation
E. Sink, Spyglass, Inc.
L. Stewart, Open Market, Inc.
Expires: May 21, 1998 November 21, 1997
HTTP Authentication: Basic and Digest Access Authentication
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).
Distribution of this document is unlimited. Please send comments to the
HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions of the
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<URL:http://www.ics.uci.edu/pub/ietf/http/>. General discussions about
HTTP and the applications which use HTTP should take place on the <www-
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Abstract
"HTTP/1.0" includes the specification for a Basic Access Authentication
scheme. This scheme is not considered to be a secure method of user
authentication (unless used in conjunction with some external secure
system such as SSL [5]), as the user name and password are passed over
the network as clear text.
This document also provides the specification for HTTP's authentication
framework, the original Basic authentication scheme and a scheme based
INTERNET-DRAFT HTTP Authentication Friday 21 November 1997
on cryptographic hashes, referred to as "Digest Access Authentication".
It is therefore intended to also serve as a replacement for RFC 2069.[6]
Like Basic, Digest access authentication verifies that both parties to a
communication know a shared secret (a password); unlike Basic, this
verification can be done without sending the password in the clear,
which is Basic's biggest weakness. As with most other authentication
protocols, the greatest sources of risks are usually found not in the
core protocol itself but in policies and procedures surrounding its use.
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Table of Contents
HTTP AUTHENTICATION: BASIC AND DIGEST ACCESS AUTHENTICATION1
Status of this Memo........................................1
Abstract...................................................1
Table of Contents..........................................3
1 Access Authentication .................................5
1.1 Reliance on the HTTP/1.1 Specification ............5
1.2 Access Authentication Framework ...................5
2 Basic Authentication Scheme ...........................6
3 Digest Access Authentication Scheme ...................7
3.1 Introduction ......................................7
3.1.1 Purpose .........................................7
3.1.2 Overall Operation ...............................8
3.1.3 Representation of digest values .................8
3.1.4 Limitations .....................................8
3.2 Specification of Digest Headers ...................9
3.2.1 The WWW-Authenticate Response Header ............9
3.2.2 The Authorization Request Header ...............11
3.2.3 The Authentication-Info Header .................14
3.3 Digest Operation .................................15
3.4 Security Protocol Negotiation ....................16
3.5 Example ..........................................16
3.6 Proxy-Authentication and Proxy-Authorization .....17
4 Security Considerations ..............................18
4.1 Authentication of Clients using Basic Authentication 18
4.2 Authentication of Clients using Digest Authentication 19
4.3 Offering a Choice of Authentication Schemes ......19
4.4 Comparison of Digest with Basic Authentication ...20
4.5 Replay Attacks ...................................20
4.6 Man in the Middle ................................21
4.7 Spoofing by Counterfeit Servers ..................22
4.8 Storing passwords ................................22
4.9 Summary ..........................................23
5 Acknowledgments ......................................23
6 References ...........................................23
7 Authors' Addresses ...................................24
Index.....................................................26
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1 Access Authentication
1.1 Reliance on the HTTP/1.1 Specification
This specification is a companion two the HTTP/1.1 specification [2]. It
uses using the extended BNF section 2.1 of that document, and relies on
both the BNF defined in that document, and other aspects of the HTTP/1.1
specification.
1.2 Access Authentication Framework
HTTP provides a simple challenge-response authentication mechanism
which 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 a challenge applicable to the proxy for the requested
resource.
challenge = auth-scheme 1*SP 1#auth-param
The authentication parameter realm is defined for all authentication
schemes:
realm = "realm" "=" realm-value
realm-value = quoted-string
The realm attribute (case-insensitive) is required for all
authentication schemes which issue a challenge. The realm value (case-
sensitive), in combination with the canonical root URL (see section
5.1.2 of [2]) 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.
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
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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 consists of credentials containing the authentication information
of the client for the realm of the resource being requested.
credentials = basic-credentials | auth-scheme #auth-param
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 rules
found in section 14.8 of [2]. Both the Proxy-Authenticate and the Proxy-
Authorization header fields are hop-by-hop headers (see section 13.5.1
of [2]).
2 Basic Authentication Scheme
The "basic" authentication scheme is based on the model that the client
must authenticate itself with a user-ID and a password for each realm.
The realm value should be considered an opaque string which can only be
compared for equality with other realms on that server. The server will
service the request only if it can validate the user-ID and password for
the protection space of the Request-URI. There are no optional
authentication parameters.
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Upon receipt of an unauthorized request for a URI within the protection
space, the origin server MAY respond with a challenge like the
following:
WWW-Authenticate: Basic realm="WallyWorld"
where "WallyWorld" is the string assigned by the server to identify the
protection space of the Request-URI. A proxy may respond with the same
challenge using the Proxy-Authenticate header field.
To receive authorization, the client sends the userid and password,
separated by a single colon (":") character, within a base64 [7] encoded
string in the credentials.
basic-credentials = "Basic" SP base64-user-pass
base64-user-pass = <base64 [4] encoding of user-pass,
except not limited to 76 char/line>
user-pass = userid ":" password
userid = *<TEXT excluding ":">
password = *TEXT
Userids might be case sensitive.
If the user agent wishes to send the userid "Aladdin" and password "open
sesame", it would use the following header field:
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
A client SHOULD assume that all paths at or deeper than the depth of the
last symbolic element in the path field of the Request-URI also are
within the protection space specified by the Basic realm value of the
current challenge. A client MAY send the corresponding Authorization
header with requests for resources in that space without receipt of
another challenge from the server.
If a client wishes to send the same userid and password to a proxy, it
would use the Proxy-Authorization header field. See section 4 for
security considerations associated with Basic authentication.
3 Digest Access Authentication Scheme
3.1 Introduction
3.1.1 Purpose
The protocol referred to as "HTTP/1.0" includes specification for a
Basic Access Authentication scheme[1]. This scheme is not considered to
be a secure method of user authentication, as the user name and password
are passed over the network in an unencrypted form. This document
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provides specification for such a scheme, referred to as "Digest Access
Authentication".
The Digest Access Authentication scheme is not intended to be a complete
answer to the need for security in the World Wide Web. This scheme
provides no encryption of object content. The intent is simply to create
a weak access authentication method, which avoids the most serious flaws
of Basic authentication.
3.1.2 Overall Operation
Like Basic Access Authentication, 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 (by default the MD5
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. Just as with the Basic scheme, the username and password
must be prearranged in some fashion which is not addressed by this
document.
3.1.3 Representation of digest values
An optional header allows the server to specify the algorithm used to
create the checksum or digest. By default the MD5 algorithm is used and
that is the only algorithm described in this document.
For the purposes of this document, an MD5 digest of 128 bits is
represented as 32 ASCII printable characters. The bits in the 128 bit
digest are converted from most significant to least significant bit,
four bits at a time to their ASCII presentation as follows. Each four
bits is represented by its familiar hexadecimal notation from the
characters 0123456789abcdef. That is, binary 0000 gets represented by
the character '0', 0001, by '1', and so on up to the representation of
1111 as 'f'.
3.1.4 Limitations
The digest authentication scheme described in this document suffers from
many known limitations. It is intended as a replacement for basic
authentication and nothing more. It is a password-based system and (on
the server side) suffers from all the same problems of any password
system. In particular, no provision is made in this protocol for the
initial secure arrangement between user and server to establish the
user's password.
Users and implementors should be aware that this protocol is not as
secure as kerberos, and not as secure as any client-side private-key
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scheme. Nevertheless it is better than nothing, better than what is
commonly used with telnet and ftp, and better than Basic authentication.
3.2 Specification of Digest Headers
The Digest Access Authentication scheme is conceptually similar to the
Basic scheme. The formats of the modified WWW-Authenticate header line
and the Authorization header line are specified below. In addition, a
new header, Authentication-Info, is specified.
3.2.1 The WWW-Authenticate Response Header
If a server receives a request for an access-protected object, and an
acceptable Authorization header is not sent, the server responds with a
"401 Unauthorized" status code, and a WWW-Authenticate header, which is
defined as follows:
WWW-Authenticate = "WWW-Authenticate" ":" "Digest"
digest-challenge
digest-challenge = 1#( realm | [ domain ] | nonce |
[ opaque ] |[ stale ] | [ algorithm ] |
[ digest-required ])
domain = "domain" "=" <"> URI ( 1*SP URI ) <">
nonce = "nonce" "=" nonce-value
nonce-value = quoted-string
opaque = "opaque" "=" quoted-string
stale = "stale" "=" ( "true" | "false" )
algorithm = "algorithm" "=" ( "MD5" | token )
digest-required = "digest-required" "=" ( "true" | "false" )
The meanings of the values of the parameters 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 space-separated list of URIs, as specified in RFC XURI [7]. The
intent is that the client could use this information to know the set
of URIs for which the same authentication information should be sent.
The URIs in this list may exist on different servers. If this keyword
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is omitted or empty, the client should assume that the domain
consists of all URIs on the responding server.
nonce
A server-specified data string which may 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 recommended nonce
would include
H(client-IP ":" time-stamp ":" private-key)
Where client-IP is the dotted quad IP address of the client making
the request, time-stamp is a server-generated time value, private-key
is data known only to the server. With a nonce of this form a server
would normally recalculate the nonce after receiving the client
authentication header and reject the request if it did not match the
nonce from that header. In this way the server can limit the reuse of
a nonce to the IP address to which it was issued and limit the time
of the nonce's validity. Further discussion of the rationale for
nonce construction is in section 4.5 below.
An implementation might choose not to accept a previously used nonce
or a previously used digest 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 4 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. 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 (in
upper or lower case), 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).
algorithm
A string indicating a pair of algorithms used to produce the digest
and a checksum. If this not present it is assumed to be "MD5". In
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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).
For the "MD5" algorithm
H(data) = MD5(data)
and
KD(secret, data) = H(concat(secret, ":", data))
i.e., the digest is the MD5 of the secret concatenated with a
colon concatenated with the data.
digest-required
If the value of the digest-required parameter is "true", then
any request with an entity-body (such as a PUT or a POST) for
the resource(s) to which this response applies MUST include
the "digest" attribute in its Authorization header. If the
request has no entity-body (such as a GET) then the digest-
required value can be ignored. If the digest-required
parameter is not specified, then its value is "false". If the
value of the digest-required parameter is "false", then the
"digest" attribute is OPTIONAL on requests for the resource(s)
to which the response applies.
3.2.2 The Authorization Request Header
The client is expected to retry the request, passing an
Authorization header line, which is defined as follows.
Authorization = "Authorization" ":" "Digest"
digest-response
Digest-response = 1#( username | realm | nonce | digest-uri |
response | [ digest ] | [ algorithm ] |
opaque )
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
response = "response" "=" response-digest
digest = "digest" "=" entity-digest
response-digest = <"> *LHEX <">
entity-digest = <"> *LHEX <">
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LHEX = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7"
|"8" | "9" | "a" | "b" | "c" | "d" | "e" | "f"
The values of the opaque and algorithm fields must be those
supplied in the WWW-Authenticate response header for the entity
being requested.
If the value of the digest-required parameter is "true", the
response to this request MUST either include the "digest" field
in its Authentication-Info header or the response should be an
error message indicating the server is unable or unwilling to
supply this field. In the latter case the requested entity MUST
not be returned as part of the response. If the digest-required
parameter is not specified in the request, then its value is
"false". If the value of the digest-required parameter is
"false", then the "digest" attribute is OPTIONAL for the response
to this request.
The definitions of response-digest and entity-digest above
indicate the encoding for their values. The following definitions
show how the value is computed:
response-digest =
<"> < KD ( H(A1), unquoted nonce-value ":" H(A2) ) > <">
A1 = unquoted username-value ":" unquoted realm-value
":" password
password = < user's password >
A2 = Method ":" digest-uri-value
The "username-value" field is a "quoted-string". However, the
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 "CircleOfLife" then H(A1) would
be H(Mufasa:myhost@testrealm.com:CircleOfLife) 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:CircleOfLife with no
white space on either side of the colons. Likewise, the other
strings digested by H() must not have white space on either side
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of the colons which delimit their fields unless that white space
was in the quoted strings or entity body being digested.
"Method" is the HTTP request method as specified in section 5.1
of [2]. The "request-uri" value is the Request-URI from the
request line as specified in section 5.1 of [2]. This may be "*",
an "absoluteURL" or an "abs_path" as specified in section 5.1.2
of [2], but it MUST agree with the Request-URI. In particular, it
MUST be an "absoluteURL" if the Request-URI is an "absoluteURL".
The authenticating server must assure that the document
designated by the "uri" parameter is the same as the document
served. 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. This altered
(but presumably semantically equivalent) request would not result
in the same digest as that calculated by the client.
The optional "digest" field contains a digest of the entity body
and some of the associated entity headers. This digest can be
useful in both request and response transactions. In a request it
can insure the integrity of POST data or data being PUT to the
server. In a response it insures the integrity of the served
document. The value of the "digest" field is an <entity-digest>,
which is defined as follows.
entity-digest<"> KD (H(A1), unquoted nonce-value ":" Method ":"
date ":" entity-info ":" H(entity-body)) <">
; format is <"> *LHEX <">
date = rfc1123-date ; see section 3.3.1 of[2]
entity-info =
H(
digest-uri-value ":"
media-type ":" ; Content-Type, see section 3.7 of [2]
*DIGIT ":" ; Content-Length, see 10.12 of [2]
content-coding ":" ; Content-Encoding, see 3.5 of [2]
last-modified ":" ; last modified date, see 10.25 of [2]
expires ; expiration date; see 10.19 of [2]
)
last-modified = rfc1123-date ; see section 3.3.1 of [2]
expires = rfc1123-date
The entity-info elements incorporate the values of the URI used
to request the entity as well as the associated entity headers
Content-Type, Content-Length, Content-Encoding, Last-Modified,
and Expires. These headers are all end-to-end headers (see
section 13.5.1 of [2]) which must not be modified by proxy
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caches. The "entity-body" is as specified by section 10.13 of [2]
or RFC 1864. The content length MUST always be included. The
HTTP/1.1 spec requires that content length is well defined in all
messages, whether or not there is a Content-Length header.
Note that not all entities will have an associated URI or all of
these headers. For example, an entity which is the data of a POST
request will typically not have a digest-uri-value or Last-
modified or Expires headers. If an entity does not have a digest-
uri-value or a header corresponding to one of the entity-info
fields, then that field is left empty in the computation of
entity-info. All the colons specified above are present, however.
For example the value of the entity-info associated with POST
data which has content-type "text/plain", no content-encoding and
a length of 255 bytes would be H(:text/plain:255:::). Similarly a
request may not have a "Date" header. In this case the date field
of the entity-digest should be empty.
In the entity-info and entity-digest computations, except for the
blank after the comma in "rfc1123-date", there must be no white
space between "words" and "separators", and exactly one blank
between "words" (see section 2.2 of [2]).
Implementers should be aware of how authenticated transactions
interact with proxy caches. The HTTP/1.1 protocol specifies that
when a shared cache (see section 13.10 of [2]) 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 [2]) 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.
3.2.3 The Authentication-Info Header
When authentication succeeds, the server may optionally provide a
Authentication-Info header indicating that the server wants to
communicate some information regarding the successful
authentication (such as an entity digest or a new nonce to be
used for the next transaction). It has two fields, digest and
nextnonce. Both are optional.
AuthenticationInfo = "Authentication-Info" ":"
1#( digest | nextnonce )
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nextnonce = "nextnonce" "=" nonce-value
digest = "digest" "=" entity-digest
The optional digest allows the client to verify that the body of
the response has not been changed en-route. The server would
probably only send this when it has the document and can compute
it. The server would probably not bother generating this header
for CGI output. The value of the "digest" is an <entity-digest>
which is computed as described above.
The value of the nextnonce parameter is the nonce the server
wishes the client to use for the next authentication response.
Note that either field is optional. In particular the server may
send the Authentication-Info header with only the nextnonce field
as a means of implementing one-time nonces. If the nextnonce
field is present the client is strongly encouraged to use it for
the next WWW- Authenticate header. Failure of the client to do so
may result in a request to re-authenticate from the server with
the "stale=TRUE ".
The Authentication-Info header is allowed in the trailer of an
HTTP message transferred via chunked transfer-coding.
3.3 Digest Operation
Upon receiving the Authorization header, the server may check its
validity by looking up its known password which corresponds to
the submitted username. Then, the server must perform the same
MD5 operation performed by the client, and compare the result to
the given response-digest.
Note that the HTTP server does not actually need to know the
user's clear text password. As long as H(A1) is available to the
server, the validity of an Authorization header may be verified.
A client may remember the username, password and nonce values, so
that future requests within the specified <domain> may include
the Authorization header preemptively. The server may choose to
accept the old Authorization header information, even though the
nonce value included might not be fresh. Alternatively, the
server could 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 hints to the client that the
request should be retried with the new nonce, without reprompting
the user for a new username and password.
The opaque data is useful for transporting state information
around. For example, a server could be responsible for
authenticating content which actually sits on another server. The
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first 401 response would include a domain field which includes
the URI on the second server, and the opaque field for specifying
state information. The client will retry the request, at which
time the server may respond with a 301/302 redirection, pointing
to the URI on the second server. The client will follow the
redirection, and pass the same Authorization header, including
the <opaque> data which the second server may require.
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.
3.4 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 any authentication scheme it cannot handle.
3.5 Example
The following example assumes that an access-protected document
is being requested from the server. 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 "CircleOfLife".
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",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
opaque="5ccc069c403ebaf9f0171e9517f40e41"
The client may prompt the user for the username and password,
after which it will respond with a new request, including the
following Authorization header:
Authorization: Digest username="Mufasa",
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realm="testrealm@host.com",
nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
uri="/dir/index.html",
response="1949323746fe6a43ef61f9606e7febea",
opaque="5ccc069c403ebaf9f0171e9517f40e41"
3.6 Proxy-Authentication and Proxy-Authorization
The digest authentication scheme may also be used for
authenticating users to proxies, proxies to proxies, or proxies
to end servers by use of the Proxy-Authenticate and Proxy-
Authorization headers. These headers are instances of the general
Proxy-Authenticate and Proxy-Authorization headers specified in
sections 10.30 and 10.31 of the HTTP/1.1 specification [2] 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 "HTTP/1.1 401
Unauthorized" header followed by a "Proxy-Authenticate" header of
the form
Proxy-Authentication = "Proxy-Authentication" ":"
"Digest"
digest-challenge
where digest-challenge is as defined above in section 2.1. The
client/proxy must then re-issue the request with a Proxy-
Authenticate header of the form
Proxy-Authorization = "Proxy-Authorization" ":"
digest-response
where digest-response is as defined above in section 2.1. When
authentication succeeds, the server may optionally provide a
Proxy-Authentication-info header of the form
Proxy-Authentication-Info = "Proxy-Authentication-Info" ":"
nextnonce
where nextnonce has the same semantics as the nextnonce field in
the Authentication-Info header described above in section 3.2.3.
Note that in principle a client could be asked to authenticate
itself to both a proxy and an end-server. It might receive an
"HTTP/1.1 401 Unauthorized" header followed by both a WWW-
Authenticate and a Proxy-Authenticate header. However, it can
never receive more than one Proxy-Authenticate header since such
headers are only for immediate connections and must not be passed
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on by proxies. If the client receives both headers, it must
respond with both the Authorization and Proxy-Authorization
headers as described above, which will likely involve different
combinations of username, password, nonce, etc.
4 Security Considerations
4.1 Authentication of Clients using Basic Authentication
The Basic authentication scheme is not a secure method of user
authentication, nor does it in any way protect the entity, which is
transmitted in clear text across the physical network used as the
carrier. HTTP does not prevent additional authentication schemes and
encryption mechanisms from being employed to increase security or the
addition of enhancements (such as schemes to use one-time passwords) to
Basic authentication.
The most serious flaw in Basic authentication is that it results in the
essentially clear text transmission of the user's password over the
physical network. It is this problem which Digest Authentication
attempts to address.
Because Basic authentication involves the clear text transmission of
passwords it SHOULD never be used (without enhancements) to protect
sensitive or valuable information.
A common use of Basic authentication is for identification purposes --
requiring the user to provide a user name and password as a means of
identification, for example, for purposes of gathering accurate usage
statistics on a server. When used in this way it is tempting to think
that there is no danger in its use if illicit access to the protected
documents is not a major concern. This is only correct if the server
issues both user name and password to the users and in particular does
not allow the user to choose his or her own password. The danger arises
because naive users frequently reuse a single password to avoid the task
of maintaining multiple passwords.
If a server permits users to select their own passwords, then the threat
is not only illicit access to documents on the server but also illicit
access to the accounts of all users who have chosen to use their account
password. If users are allowed to choose their own password that also
means the server must maintain files containing the (presumably
encrypted) passwords. Many of these may be the account passwords of
users perhaps at distant sites. The owner or administrator of such a
system could conceivably incur liability if this information is not
maintained in a secure fashion.
Basic Authentication is also vulnerable to spoofing by counterfeit
servers. If a user can be led to believe that he is connecting to a host
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containing information protected by basic authentication when in fact he
is connecting to a hostile server or gateway then the attacker can
request a password, store it for later use, and feign an error. This
type of attack is not possible with Digest Authentication. Server
implementers SHOULD guard against the possibility of this sort of
counterfeiting by gateways or CGI scripts. In particular it is very
dangerous for a server to simply turn over a connection to a gateway.
That gateway can then use the persistent connection mechanism to engage
in multiple transactions with the client while impersonating the
original server in a way that is not detectable by the client.
4.2 Authentication of Clients using Digest Authentication
Digest Authentication does not provide a strong authentication
mechanism. That is not its intent. It is intended solely to
replace a much weaker and even more dangerous authentication
mechanism: Basic Authentication. An important design constraint
is that the new authentication scheme be free of patent and
export restrictions.
Most needs for secure HTTP transactions cannot be met by Digest
Authentication. For those needs SSL or SHTTP are more appropriate
protocols. In particular digest authentication cannot be used for
any transaction requiring encrypted content. Nevertheless many
functions remain for which digest authentication is both useful
and appropriate.
4.3 Offering a Choice of Authentication Schemes
An HTTP/1.1 server may return multiple challenges with a 401
(Authenticate) response, and each challenge may use a different scheme.
The order of the challenges returned to the user agent is in the order
that the server would prefer they be chosen. The server should order its
challenges with the "most secure" authentication scheme first. A user
agent should choose as the challenge to be made to the user the first
one that the user agent understands.
When the server offers choices of authentication schemes using the WWW-
Authenticate header, the "security" of the authentication is only as
good as the security of the weakest of the authentication schemes. A
malicious user could capture the set of challenges and try to
authenticate him/herself using the weakest of the authentication
schemes. Thus, the ordering serves more to protect the user's
credentials than the server's information.
A possible man-in-the-middle (MITM) 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
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reason, the client should always use the strongest scheme that it
understands from the choices accepted.
An even better MITM attack would be to remove all offered choices, and
to insert a challenge that requests Basic authentication. For this
reason, user agents that are concerned about this kind of attack could
remember the strongest authentication scheme ever requested by a server
and produce a warning message that requires user confirmation before
using a weaker one. A particularly insidious way to mount such a MITM
attack would be to offer a "free" proxy caching service to gullible
users.
4.4 Comparison of Digest with Basic Authentication
Both Digest and Basic Authentication are very much on the weak
end of the security strength spectrum. But a comparison between
the two points out the utility, even necessity, of replacing
Basic by Digest.
The greatest threat to the type of transactions for which these
protocols are used is network snooping. This kind of transaction
might involve, for example, online access to a database whose use
is restricted to paying subscribers. With Basic authentication an
eavesdropper can obtain the password of the user. This not only
permits him to access anything in the database, but, often worse,
will permit access to anything else the user protects with the
same password.
By contrast, with Digest Authentication the eavesdropper only
gets access to the transaction in question and not to the user's
password. The information gained by the eavesdropper would permit
a replay attack, but only with a request for the same document,
and even that might be difficult.
4.5 Replay Attacks
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 response 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. A GET request containing form data
could only be "replayed" with the identical data. However, this
could be problematic if it caused a CGI script to take some
action on the server.
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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, 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 response digests which will
not be honored for a second use. This requires the overhead of
the server remembering which digests have been used until the
nonce time-stamp (and hence the digest built with it) has
expired, but it effectively protects against replay attacks.
Instead of maintaining a list of the values of used digests, a
server would hash these values and require re-authentication
whenever a hash collision occurs.
An implementation must give special attention to the possibility
of replay attacks with POST and PUT requests. A successful replay
attack could result in counterfeit form data or a counterfeit
version of a PUT file. The use of one-time digests or one-time
nonces is recommended. It is also recommended that the optional
<digest> be implemented for use with POST or PUT requests to
assure the integrity of the posted data. Alternatively, a server
may choose to allow digest authentication only with GET requests.
Responsible server implementors will document the risks described
here as they pertain to a given implementation.
4.6 Man in the Middle
Both Basic and Digest authentication are vulnerable to "man in the
middle" attacks, for example, from a hostile or compromised proxy.
Clearly, this would present all the problems of eavesdropping. But it
could also offer some additional threats.
A simple but effective attack would be to replace the Digest challenge
with a Basic challenge, to spoof the client into revealing their
password. To protect against this attack, clients should remember if a
site has used Digest authentication in the past, and warn the user if
the site stops using it. It might also be a good idea for the browser to
be configured to demand Digest authentication in general, or from
specific sites.
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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.
There are several attacks on the "digest" field in the Authentication-
Info header. A simple but effective attack is just to remove the field,
so that the client will not be able to use it to detect modifications to
the response entity. Sensitive applications may wish to allow
configuration to require that the digest field be present when
appropriate. More subtly, the attacker can alter any of the entity-
headers not incorporated in the computation of the digest. The attacker
can alter most of the request headers in the client's request, and can
alter any response header in the origin-server's reply, except those
headers whose values are incorporated into the "digest" field.
Alteration of Accept* or User-Agent request headers can only result in a
denial of service attack that returns content in an unacceptable media
type or language. Alteration of cache control headers also can only
result in denial of service. Alteration of Host will be detected, if the
full URL is in the response-digest. Alteration of Referer or From is not
important, as these are only hints.
4.7 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.
4.8 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
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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.
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.
4.9 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 many, 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 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.
5 Acknowledgments
In addition to the authors, valuable discussion instrumental in creating
this document has come from Peter J. Churchyard, Ned Freed, and David M.
Kristol.
Jim Gettys edited this document for its update.
6 References
[1] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
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INTERNET-DRAFT HTTP Authentication Friday 21 November 1997
[2] Fielding, R., Gettys, J., Mogul, J. C., Frysyk, H, Berners-Lee,
T., " Hypertext Transfer Protocol -- HTTP/1.1", Work In Progress of
the HTTP working group, November 1997.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[4] Freed, N., and N. Borenstein. "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies." RFC
2045, Innosoft, First Virtual, November 1996.
[5] Dierks, T. and C. Allen "The TLS Protocol, Version 1.0," Work In
Progress of the TLS working group, November, 1997.
[6] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
Luotonen, A., Sink, E., Stewart, L.," An Extension to HTTP : Digest
Access Authentication." RFC 2069, January, 1997.
[7] Berners Lee, T, Fielding, R., Masinter, L., "Uniform Resource
Identifiers (URI): Generic Syntax and Semantics ," Work in Progress,
November, 1997.
7 Authors' Addresses
John Franks
Professor of Mathematics
Department of Mathematics
Northwestern University
Evanston, IL 60208-2730, USA
EMail: john@math.nwu.edu
Phillip M. Hallam-Baker
Principal Consultant
Verisign Inc.
One Alewife Center
Cambridge, MA 02138, USA
EMail: pbaker@verisign.com
Jeffery L. Hostetler
Senior Software Engineer
Spyglass, Inc.
3200 Farber Drive
Champaign, IL 61821, USA
EMail: jeff@spyglass.com
Franks, et al. [Page 24]
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Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052, USA
EMail: paulle@microsoft.com
Ari Luotonen
Member of Technical Staff
Netscape Communications Corporation
501 East Middlefield Road
Mountain View, CA 94043, USA
EMail: luotonen@netscape.com
Eric W. Sink
Senior Software Engineer
Spyglass, Inc.
3200 Farber Drive
Champaign, IL 61821, USA
EMail: eric@spyglass.com
Lawrence C. Stewart
Open Market, Inc.
215 First Street
Cambridge, MA 02142, USA
EMail: stewart@OpenMarket.com
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Index
While some care was taken producing this index, there is no guarantee
that all occurrences of an index term have been entered into the index.
Italics indicate the definition of a term; bold face is used for the
definition of a header.
credentials, 6
301, 16
13
digest, 11, 12, 13, 14, 15, 21,
22
Digest Access Authentication, 2,
401, 5, 6, 9, 10, 15, 16, 17, 19 8, 9
407, 5, 6 Digest Authentication, 18, 19
411, 6 digest-challenge, 9, 17
digest-required, 9, 11, 12
digest-response, 11, 17
digest-uri, 11
absoluteURL, 13 digest-uri-value, 11, 12, 13, 14
Accept*, 22 domain, 9, 10, 15, 16
Access Authentication, 5
algorithm, 8, 9, 10, 11, 12
AuthenticationInfo, 302, 16 date, 14
Authentication-Info, 9, 12, 14, entity-body, 13, 14
15, 16, 17, 22 entity-digest, 11, 12, 13, 14, 15
Authorization, 5, 6, 7, 9, 11, entity-info, 13, 14
12, 14, 15, 16, 17, 18 expires, 13
auth-param, 5 Expires, 13, 14
auth-scheme, 5
From, 22
base64-user-pass, 7
Basic Access Authentication, 1,
7, 8
Basic authentication, 7, 18, 20 GET, 10, 11, 20, 21
Basic Authentication Scheme, 6
basic-credentials, 7
last-modified, 13
Last-Modified, 13
Cache-Control, 14 LHEX, 11, 12, 13
challenge, 5
content-coding, 13
Content-Encoding, 13
Content-Length, 13 MD5, 8, 9, 10, 11, 15, 24
Content-Type, 13 media-type, 13
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Method, 12, 13 response, 11, 17
MIME, 24 response-digest, 11, 12, 15, 22
must-revalidate, 14 rfc1123-date, 13, 14
, 17 Security Considerations
nonce, 8, 9, 10, 11, 14, 15, 16, basic scheme is insecure, 18
17, 18, 21, 23 comparison of digest with basic,
nonce-value, 9, 12, 13, 15 20
man in the middle attacks, 21
offering multiple authentication
schemes, 19
opaque, 9, 10, 11, 12, 15, 16, 17 replay attacks against digest nextnonce, 14, 15
authentication, 20
spoofing by counterfeit servers,
22
password, 1, 7, 8, 9, 10, 12, 15, digest weak, 23
16, 18, 20, 21, 22, 23 separators, 14
POST, 10, 11, 13, 14, 21 stale, 9, 10, 15
Proxy-Authenticate, 5, 6, 7, 16,
17
Proxy-Authentication, 17
Proxy-Authentication-Info, 17 token, 5
Proxy-Authorization, 6 true, 12
public, 14
PUT, 10, 11, 13, 21
User-Agent, 22
userid, 7
quoted-string, 5, 9, 11, 12 username, 8, 9, 10, 11, 12, 15,
16, 18, 22, 23
username-value, 11, 12
user-pass, 7
realm, 5, 9, 11, 12, 16, 17, 22,
23
realm-value, 5, 12
Referer, 22 words, 14
request-uri, 11, 13 WWW-Authenticate, 5, 6, 7, 9, 12,
Request-URI, 6, 7, 13 16, 17, 19
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