draft-ietf-http-digest-aa-03.txt   draft-ietf-http-digest-aa-04.txt 
HTTP Working Group Jeffery L. Hostetler HTTP Working Group John Franks
INTERNET-DRAFT John Franks INTERNET-DRAFT Philip Hallam-Baker
<draft-ietf-http-digest-aa-03.txt> Philip Hallam-Baker <draft-ietf-http-digest-aa-04.txt> Jeffery L. Hostetler
Paul Leach Paul Leach
Ari Luotonen Ari Luotonen
Eric W. Sink Eric W. Sink
Lawrence C. Stewart Lawrence C. Stewart
Expires SIX MONTHS FROM---> March 1, 1996 Expires SIX MONTHS FROM---> June 6, 1996
A Proposed Extension to HTTP : Digest Access Authentication A Proposed Extension to HTTP : Digest Access Authentication
Status of this Memo Status of this Memo
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Abstract Abstract
The protocol referred to as "HTTP/1.0" includes specification The protocol referred to as "HTTP/1.0" includes specification for a
for a Basic Access Authentication scheme. This scheme is not Basic Access Authentication scheme. This scheme is not considered to
considered to be a secure method of user authentication, as the be a secure method of user authentication, as the user name and
user name and password are passed over the network in an password are passed over the network in an unencrypted form. A
unencrypted form. A specification for a new authentication scheme specification for a new authentication scheme is needed for future
is needed for future versions of the HTTP protocol. This document versions of the HTTP protocol. This document provides specification
provides specification for such a scheme, referred to as "Digest for such a scheme, referred to as "Digest Access Authentication".
Access Authentication". The encryption method used by default is the The digesting method used by default is the RSA Data Security, Inc.
RSA Data Security, Inc. MD5 Message-Digest Algorithm [2]. MD5 Message-Digest Algorithm [3].
Table of Contents Table of Contents
1. Introduction STATUS OF THIS MEMO....................................................
1.1 Purpose
1.2 Overall Operation
1.3 Representation of MD5 digest values
2. Basic Access Authentication Scheme
2.1 Specification of Digest Headers
2.2 Digest Operation
2.3 Security protocol negotiation
2.4 Example
3. Security Considerations
4. Acknowledgments
5. References
6. Authors Addresses
1. Introduction ABSTRACT...............................................................
TABLE OF CONTENTS......................................................
INTRODUCTION...........................................................
1.1 PURPOSE .........................................................
1.2 OVERALL OPERATION ...............................................
1.3 REPRESENTATION OF DIGEST VALUES .................................
1.4 LIMITATIONS .....................................................
2. DIGEST ACCESS AUTHENTICATION SCHEME.................................
2.1 SPECIFICATION OF DIGEST HEADERS ..................................
2.1.1 THE WWW-AUTHENTICATE RESPONSE HEADER ..........................
2.1.2 THE AUTHORIZATION REQUEST HEADER ..............................
2.1.3 THE AUTHENTICATION-INFO HEADER ................................
2.2 DIGEST OPERATION .................................................
2.3 SECURITY PROTOCOL NEGOTIATION ....................................
2.4 EXAMPLE ..........................................................
2.5 PROXY-AUTHENTICATION AND PROXY-AUTHORIZATION .....................
3. SECURITY CONSIDERATIONS............................................
3.1 COMPARISON WITH BASIC AUTHENTICATION ............................
3.2 REPLAY ATTACKS ..................................................
3.3 MAN IN THE MIDDLE ...............................................
3.4 SPOOFING BY COUNTERFEIT SERVERS .................................
3.5 STORING PASSWORDS ...............................................
3.6 SUMMARY .........................................................
4. ACKNOWLEDGMENTS...................................................
5. REFERENCES.........................................................
6. AUTHORS ADDRESSES..................................................
Introduction
1.1 Purpose 1.1 Purpose
The protocol referred to as "HTTP/1.0" includes specification The protocol referred to as "HTTP/1.0" includes specification for a
for a Basic Access Authentication scheme[1]. This scheme is not Basic Access Authentication scheme[1]. This scheme is not considered
considered to be a secure method of user authentication, as the to be a secure method of user authentication, as the user name and
user name and password are passed over the network in an password are passed over the network in an unencrypted form. A
unencrypted form. A specification for a new authentication scheme specification for a new authentication scheme is needed for future
is needed for future versions of the HTTP protocol. This document versions of the HTTP protocol. This document provides specification
provides specification for such a scheme, referred to as "Digest for such a scheme, referred to as "Digest Access Authentication".
Access Authentication".
The Digest Access Authentication scheme is not intended to be The Digest Access Authentication scheme is not intended to be a
a complete answer to the need for security in the World Wide Web. complete answer to the need for security in the World Wide Web. This
This scheme provides no encryption of object content. The intent scheme provides no encryption of object content. The intent is
is simply to facilitate secure access authentication. simply to facilitate secure access authentication.
It is proposed that this access authentication scheme be included It is proposed that this access authentication scheme be included in
in the proposed HTTP/1.1 specification. the proposed HTTP/1.1 specification.
1.2 Overall Operation 1.2 Overall Operation
Like Basic Access Authentication, the Digest scheme is based on a Like Basic Access Authentication, the Digest scheme is based on a
simple challenge-response paradigm. The Digest scheme challenges simple challenge-response paradigm. The Digest scheme challenges
using a nonce value. A valid response contains a checksum (by default using a nonce value. A valid response contains a checksum (by
the MD5 checksum) of the username, the password, the given nonce default the MD5 checksum) of the username, the password, the given
value, and the requested URI. In this way, the password is never sent
in the clear. Just as with the Basic scheme, the username and nonce value, the HTTP method, and the requested URI. In this way, the
password must be prearranged in some fashion which is not addressed password is never sent in the clear. Just as with the Basic scheme,
by this document. the username and password must be prearranged in some fashion which is
not addressed by this document.
1.3 Representation of digest values 1.3 Representation of digest values
An optional header allows the server to specify the algorithm An optional header allows the server to specify the algorithm used to
used to create the checksum or digest. By default the MD5 create the checksum or digest. By default the MD5 algorithm is used
algorithm is used and that is the only algorithm described in and that is the only algorithm described in this document.
this document.
For the purposes of this document, an MD5 digest of 128 bits For the purposes of this document, an MD5 digest of 128 bits is
is represented as 32 ASCII printable characters. The bits represented as 32 ASCII printable characters. The bits in the 128
in the 128 bit digest are converted from most significant bit digest are converted from most significant to least significant
to least significant bit, four bits at a time to their bit, four bits at a time to their ASCII presentation as follows.
ASCII presentation as follows. Each four bits is Each four bits is represented by its familiar hexadecimal notation
represented by its familiar hexadecimal notation from the from the characters 0123456789abcdef. That is, binary 0000 gets
characters 0123456789abcdef. That is binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the
represented by the character '0', 0001, by '1', and so on representation of 1111 as 'f'.
up to the representation of 1111 as 'f'.
1.4 Limitations 1.4 Limitations
The digest authentication scheme described in this document suffers The digest authentication scheme described in this document suffers
from many known limitations. It is intended as a replacement for from many known limitations. It is intended as a replacement for
basic authentication and nothing more. It is a password-based system basic authentication and nothing more. It is a password-based system
and (on the server side) suffers from all the same problems of any and (on the server side) suffers from all the same problems of any
password system. In particular no provision is made in this protocol password system. In particular, no provision is made in this protocol
for the initial secure arrangement between user and server for the initial secure arrangement between user and server to
establishing the user's password. establish the user's password.
Users and implementors should be aware that this protocol is not as 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 secure as kerberos, and not as secure as any client-side private-key
scheme. Nevertheless it is better than nothing, better than what is scheme. Nevertheless it is better than nothing, better than what is
commonly used with telnet and ftp and better than Basic commonly used with telnet and ftp, and better than Basic
authentication. authentication.
Some keyword-value pairs occurring in headers described below are
required to have values which are of the type "quoted-string" as
defined in section 2.2 of the HTTP/1.1 specification [2]. A
consequence is that these values represent strings in the US-ASCII
character set. An unfortunate side effect of this is that digest
authentication is not capable of handling either user names or realm
names (see 2.1.1 below) which are not expressed in this character set.
2. Digest Access Authentication Scheme 2. Digest Access Authentication Scheme
2.1 Specification of Digest Headers 2.1 Specification of Digest Headers
The Digest Access Authentication scheme is conceptually similar to the Basic The Digest Access Authentication scheme is conceptually similar to
scheme. The formats of the modified WWW-Authenticate header line and the the Basic scheme. The formats of the modified WWW-Authenticate
Authorization header line are specified below. In addition, a new header, header line and the Authorization header line are specified below,
Digest-MessageDigest, is specified as well. using the extended BNF defined in the HTTP/1.1 specification, section
2.1. In addition, a new header, Authentication-info, is specified.
Due to formatting constraints, all of the headers are depicted here 2.1.1 The WWW-Authenticate Response Header
on multiple lines. In actual usage, they must follow the syntactic
rules for HTTP/1.0 header lines [1]. Whitespace between the
attribute-value pairs is allowed.
If a server receives a request for an access-protected object, and an If a server receives a request for an access-protected object, and an
acceptable Authorization header is not sent, the server responds with: 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:
HTTP/1.1 401 Unauthorized WWW-Authenticate = "WWW-Authenticate" ":" "Digest"
WWW-Authenticate: Digest realm="<realm>", digest-challenge
domain="<domain>",
nonce="<nonce>",
opaque="<opaque>",
stale="<TRUE | FALSE>",
algorithm="<digest-algorithm>"
The meanings of the identifiers used above are as follows: digest-challenge = 1#( realm | [ domain ] | nonce |
[ digest-opaque ] |[ stale ] | [ algorithm ] )
<realm> realm = "realm" "=" realm-value
A string to be displayed to users so they know which realm-value = quoted-string
username and password to use. This string should contain domain = "domain" "=" <"> 1#URI <">
at least the name of the host performing the authentication nonce = "nonce" "=" nonce-value
and might additionally indicate the collection of users who nonce-value = quoted-string
might have access. An example might be opaque = "opaque" "=" quoted-string
"registered users @ gotham.news.com." stale = "stale" "=" ( "true" | "false" )
algorithm = "algorithm" "=" ( "MD5" | token )
<domain> OPTIONAL The meanings of the values of the parameters used above are as
A comma separated list of URIs, as specified for HTTP/1.0. The 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". The realm is a
"quoted-string" as specified in section 2.2 of the HTTP/1.1
specification [2].
domain
A comma-separated list of URIs, as specified for HTTP/1.0. The
intent is that the client could use this information to know the intent is that the client could use this information to know the
set of URIs for which the same authentication information should be set of URIs for which the same authentication information should be
sent. The URIs in this list may exist on different servers. If sent. The URIs in this list may exist on different servers. If
this keyword is omitted or empty, the client should assume that this keyword is omitted or empty, the client should assume that the
the domain consists of all URIs on the responding server. domain consists of all URIs on the responding server.
<nonce> nonce
A server-specified data string which may be uniquely generated each A server-specified data string which may be uniquely generated each
time a 401 response is made. It is recommended that this string be time a 401 response is made. It is recommended that this string be
base64 or hexadecimal data. Specifically, since the string is passed base64 or hexadecimal data. Specifically, since the string is
in the header lines as a quoted string, the double-quote character passed in the header lines as a quoted string, the double-quote
is not allowed. character is not allowed.
The contents of the nonce is implementation dependent. The The contents of the nonce are implementation dependent. The
quality of the implementation depends on a good choice. A quality of the implementation depends on a good choice. A
recommended nonce would include recommended nonce would include
H(<client IP> + ":" + <timestamp> + ":" + <private key> ) H(client-IP ":" time-stamp ":" private-key )
Where <client IP> is the dotted quad IP address of the client Where client-IP is the dotted quad IP address of the client making
making the request, <timestamp> is a server generated time value, the request, time-stamp is a server-generated time value, private-
<private key> is data known only to the server. With a nonce key is data known only to the server. With a nonce of this form a
of this form a server would normally recalculate the nonce server would normally recalculate the nonce after receiving the
after receiving the client authentication header and reject client authentication header and reject the request if it did not
the request if it did not match the nonce from that header. match the nonce from that header. In this way the server can limit
In this way the server can limit the reuse of a nonce to the reuse of a nonce to the IP address to which it was issued and
the IP address to which it was issued and limit the time of limit the time of the nonce's validity. Further discussion of the
the nonce's validity. A server might also wish to include rationale for nonce construction is in section 3.2 below.
the client request or the contents of the Host: header in
the data digested to create the nonce. Further discussion
of the rationale for nonce construction is in section 3.2
below.
An implementation might choose not to accept a previously used An implementation might choose not to accept a previously used
<nonce> or a previously used <digest> to protect against a nonce or a previously used digest to protect against a replay
replay attack. Or, an implementation might choose to use attack. Or, an implementation might choose to use one-time nonces
one-time nonces or digests for POST or PUT requests and a or digests for POST or PUT requests and a time-stamp for GET
timestamp for GET requests. For more details on the issues requests. For more details on the issues involved see section 3.
involved see section 3. of this document. of this document.
The nonce is opaque to the client. The nonce is opaque to the client.
<opaque> OPTIONAL opaque
A string of data, specified by the server, which should returned by A string of data, specified by the server, which should be returned by
the client unchanged. It is recommended that this string be the client unchanged. It is recommended that this string be base64
base64 or hexadecimal data. Specifically, since the string is passed or hexadecimal data. This field is a "quoted-string" as specified
in the header lines as a quoted string, the double-quote character in section 2.2 of the HTTP/1.1 specification [2].
is not allowed.
<stale> OPTIONAL stale
A flag, indicating that the previous request from the client A flag, indicating that the previous request from the client was
was rejected because the nonce value was stale. If stale rejected because the nonce value was stale. If stale is TRUE (in
is TRUE, the client may wish to simply retry the request with upper or lower case), the client may wish to simply retry the
a new encrypted response, without reprompting the user for a request with a new encrypted response, without reprompting the user
new username and password. The server should only set stale 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 to true if it receives a request for which the nonce is invalid but
but with a valid digest for that nonce (indicating the the client with a valid digest for that nonce (indicating that the client knows
knows the correct username/password). the correct username/password).
<algorithm> OPTIONAL algorithm
A string indicating the algorithm used to produce the digest
or checksum. If this not present the MD5 algorithm is assumed.
In this document the string obtained by applying this algorithm
to the data "<data>" will be denoted by H(<data>).
The client is expected to retry the request, passing an Authorization header A string indicating a pair of algorithms used to produce the digest
line as follows: and a checksum. If this not present it is assumed to be "MD5". 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).
Authorization: Digest For the "MD5" algorithm
username="<username>", -- required
realm="<realm>", -- required
nonce="<nonce>", -- required
uri="<requested-uri>", -- required
response="<digest>", -- required
message="<message-digest>", -- OPTIONAL
algorithm="<digest-algorithm>" -- OPTIONAL
opaque="<opaque>", -- required if provided
by server
where <digest> := H( H(A1) + ":" + N + ":" + H(A2) ) H(data) = MD5(data)
and <message-digest> := H( H(A1) + ":" + N + ":" + H(<entity-body>) )
where: and
A1 := U + ':' + R + ':' + P KD(secret, data) = H(concat(secret, ":", data))
A2 := <Method> + ':' + <requested-uri>
with: i.e., the digest is the MD5 of the secret concatenated with a colon
N -- nonce value concatenated with the data.
U -- username
R -- realm
P -- password
<Method> is the HTTP method specified at the beginning of the 2.1.2 The Authorization Request Header
first line of the client request. <requested-uri> is the part
of the requested URL transmitted by the client to the server
in the first line of an HTTP request. In particular it does
not include the "http://host:port" part of the URL but does
include any "query" part which might, for example, include form
data after a '?' in the URL.
The purpose of the <message-digest> is to allow the server to The client is expected to retry the request, passing an Authorization
ensure that the content of the request body has not been tampered header line, which is defined as follows.
with after leaving the client. This would normally be used with a
POST or PUT request and would allow the server to check the validity
of the posted data. The <entity-body> is the "entity body" as
prescribed in the Hypertext Transfer Protocol version 1.1.
When authorization succeeds, the Server may optionally provide the Authorization = "Authorization" ":" "Digest" digest-response
following:
HTTP/1.1 200 OK digest-response = 1#( username | realm | nonce | digest-uri |
Digest-MessageDigest: response | [ digest ] | [ algorithm ] |
message="<message-digest>", opaque )
nextnonce="<nextnonce>"
The Digest-MessageDigest header indicates that the server username = "username" "=" username-value
wants to communicate some information regarding the username-value = quoted-string
successful authentication (such as a message digest or a digest-uri = "uri" "=" digest-uri-value
new nonce to be used for the next transaction). digest-uri-value = request-uri ; As specified by HTTP/1.1
response = "response" "=" response-digest
digest = "digest" "=" entity-digest
<message-digest> is computed by the same algorithm given response-digest = <"> *LHEX <">
above for the body of the client request. This allows the entity-digest = <"> *LHEX <">
client to verify that the body of the response has not been LHEX = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
changed en-route. The server would probably only send this "8" | "9" | "a" | "b" | "c" | "d" | "e" | "f"
when it has the document and can compute it. The server would
probably not bother generating this header for CGI output.
<nextnonce> is the nonce the server wishes the client to use for The definitions of response-digest and entity-digest above indicate
the next authentication response. Either field is optional. In the encoding for their values. The following definitions show how the value
particular the server may send the Digest-MessageDigest header is computed:
with only the nextnonce=<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 Digest-MessageDigest header has many limitations. Only the response-digest =
entity body is digested, not any headers. This limitation is due <"> < KD ( H(A1), unquoted nonce-value ":" H(A2) > <">
to the fact that proxy caches may (and do) alter the headers of
documents which they relay. Future authentication schemes will A1 = unquoted username-value ":" unquoted realm-value
have to deal with the complexities imposed by the behavior of ":" password
intermediaries handling documents on their way from the origin password = < user's password >
server to the client, but those issues are beyond the scope of A2 = Method ":" digest-uri-value
digest authentication, whose purpose is to replace Basic
Authentication. Despite its limitations the Digest-MessageDigest The "username-value" field is a "quoted-string" as specified in section
can be useful. 2.2 of the HTTP/1.1 specification [2]. 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 in the 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 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 TBS of [2])
which must not be modified by proxy caches. The "entity-body" is as
specified by section 10.13 of [2] or RFC 1864.
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 "tspecials", and exactly one blank between "words"
(see section 2.2 of [2]).
Implementors 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 TBS) 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.
2.1.3 The AuthenticationInfo 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 )
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 "digested-entity" 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."
2.2 Digest Operation 2.2 Digest Operation
Upon receiving the Authorization information, the server may check Upon receiving the Authorization header, the server may check
its validity by looking up its known password which corresponds to its validity by looking up its known password which corresponds to
the submitted <username>. Then, the server must perform the same the submitted username. Then, the server must perform the same MD5
MD5 operation performed by the client, and compare the result to operation performed by the client, and compare the result to the
the given <response>. given response-digest.
Note that the HTTP server does not actually need to know the user's 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 clear text password. As long as H(A1) is available to the server,
validity of an Authorization header may be verified. the validity of an Authorization header may be verified.
All keyword-value pairs must be expressed in characters from the
US-ASCII character set, excluding control characters.
A client may remember the username, password and nonce values, so that A client may remember the username, password and nonce values, so
future requests within the specified <domain> may include the Authorization that future requests within the specified <domain> may include the
line preemptively. The server may choose to accept the old Authorization Authorization header preemptively. The server may choose to accept the
information, even though the nonce value included might not be fresh. old Authorization header information, even though the nonce value
Alternatively, the server could return a 401 response with a new nonce included might not be fresh. Alternatively, the server could return a
value, causing the client to retry the request. By specifying stale=TRUE 401 response with a new nonce value, causing the client to retry the
with this response, the server hints to the client that the request should request. By specifying stale=TRUE with this response, the server
be retried with the new nonce, without reprompting the user for a new hints to the client that the request should be retried with the new
username and password. nonce, without reprompting the user for a new username and password.
The <opaque> data is useful for transporting state information around. The opaque data is useful for transporting state information around.
For example, a server could be responsible for authenticating content For example, a server could be responsible for authenticating content
which actual sits on another server. The first 401 response would include which actually sits on another server. The first 401 response would
a <domain> which includes the URI on the second server, and the <opaque> include a domain field which includes the URI on the second server,
for specifying state information. The client will retry the request, at and the opaque field for specifying state information. The client
which time the server may respond with a 301/302 redirection, pointing will retry the request, at which time the server may respond with a
to the URI on the second server. The client will follow the redirection, 301/302 redirection, pointing to the URI on the second server. The
and pass the same Authorization line, including the <opaque> data which client will follow the redirection, and pass the same Authorization
the second server may require. header, including the <opaque> data which the second server may
require.
As with the basic scheme, proxies must be completely transparent in As with the basic scheme, proxies must be completely transparent in
the Digest access authentication scheme. That is, they must forward the the Digest access authentication scheme. That is, they must forward
WWW-Authenticate, Digest-MessageDigest and Authorization headers untouched. the WWW-Authenticate, Authentication-info and Authorization headers
If a proxy wants to authenticate a client before a request is forwarded to untouched. If a proxy wants to authenticate a client before a request
the server, it can be done using the Proxy-Authenticate and is forwarded to the server, it can be done using the Proxy-
Proxy-Authorization headers. Authenticate and Proxy-Authorization headers described in section
2.5 below..
2.3 Security Protocol Negotiation 2.3 Security Protocol Negotiation
It is useful for a server to be able to know which security schemes It is useful for a server to be able to know which security schemes a
a client is capable of handling. client is capable of handling.
If this proposal is accepted as a required part of the HTTP/1.1 If this proposal is accepted as a required part of the HTTP/1.1
specification, then a server may assume Digest support when a client specification, then a server may assume Digest support when a client
identifies itself as HTTP/1.1 compliant. identifies itself as HTTP/1.1 compliant.
It is possible that a server may want to require Digest as its 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 authentication method, even if the server does not know that the
supports it. A client is encouraged to fail gracefully if the server client supports it. A client is encouraged to fail gracefully if the
specifies any authentication scheme it cannot handle. server specifies any authentication scheme it cannot handle.
2.4 Example 2.4 Example
The following example assumes that an access-protected document is being The following example assumes that an access-protected document is
requested from the server. The URI of the document is being requested from the server. The URI of the document is
"http://www.nowhere.org/dir/index.html". "http://www.nowhere.org/dir/index.html". Both client and server know
that the username for this document is "Mufasa", and the password is
Both client and server know that the username for this document is "CircleOfLife".
"Mufasa", and the password is "CircleOfLife".
The first time the client requests the document, no Authorization header The first time the client requests the document, no Authorization
is sent, so the server responds with: header is sent, so the server responds with:
HTTP/1.1 401 Unauthorized HTTP/1.1 401 Unauthorized
WWW-Authenticate: Digest realm="testrealm@host.com", WWW-Authenticate: Digest realm="testrealm@host.com",
nonce="72540723369", nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
opaque="5ccc069c403ebaf9f0171e9517f40e41" opaque="5ccc069c403ebaf9f0171e9517f40e41"
The client may prompt the user for the username and password, after which it The client may prompt the user for the username and password, after
will respond with a new request, including the following Authorization which it will respond with a new request, including the following
header: Authorization header:
Authorization: Digest username="Mufasa", Authorization: Digest username="Mufasa",
realm="testrealm@host.com", realm="testrealm@host.com",
nonce="72540723369", nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
uri="/dir/index.html", uri="/dir/index.html",
response="e966c932a9242554e42c8ee200cec7f6", response="e966c932a9242554e42c8ee200cec7f6",
opaque="5ccc069c403ebaf9f0171e9517f40e41" opaque="5ccc069c403ebaf9f0171e9517f40e41"
2.5 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 2.1.
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 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.
3. Security Considerations 3. Security Considerations
Digest Authentication does not provide provide a strong authentication Digest Authentication does not provide a strong authentication
mechanism. That is not its intent. It is intended solely to replace mechanism. That is not its intent. It is intended solely to replace
a much weaker and even dangerous authentication mechanism: Basic a much weaker and even more dangerous authentication mechanism: Basic
Authentication. An important design constraint is that the new Authentication. An important design constraint is that the new
authentication scheme be free of patent and export restrictions. authentication scheme be free of patent and export restrictions.
Most needs for secure HTTP transactions cannot be met by Digest Most needs for secure HTTP transactions cannot be met by Digest
Authentication. For those needs SSL or SHTTP are more appropriate Authentication. For those needs SSL or SHTTP are more appropriate
protocols. In particular digest authentication cannot be used for any protocols. In particular digest authentication cannot be used for
transaction requiring encrypted content. Nevertheless many functions any transaction requiring encrypted content. Nevertheless many
remain for which digest authentication is both useful and appropriate. functions remain for which digest authentication is both useful and
appropriate.
3.1 Comparison with Basic Authentication 3.1 Comparison with Basic Authentication
Both Digest and Basic Authentication are very much on the weak end of Both Digest and Basic Authentication are very much on the weak end of
the security strength spectrum. But a comparison between the security strength spectrum. But a comparison between the two
the two points out the utility, even necessity, of replacing Basic points out the utility, even necessity, of replacing Basic by Digest.
by Digest.
The greatest threat to the type of transactions for which these The greatest threat to the type of transactions for which these
protocols are used is network snooping. This kind of transaction protocols are used is network snooping. This kind of transaction
might involve, for example, online access to a database whose use is might involve, for example, online access to a database whose use is
restricted to paying subscribers. With Basic authentication an restricted to paying subscribers. With Basic authentication an
eavesdropper can obtain the password of the user. This not only eavesdropper can obtain the password of the user. This not only
permits him to access anything in the data base, but often worse, will permits him to access anything in the database, but, often worse,
permit access to anything else the user protects with the same will permit access to anything else the user protects with the same
password. password.
By contrast, with Digest Authentication the eavesdropper only gets By contrast, with Digest Authentication the eavesdropper only gets
access to the transaction in question and not to the user's password. access to the transaction in question and not to the user's password.
The information gained by the eavesdropper would permit a replay The information gained by the eavesdropper would permit a replay
attack, but only with a request for the same document and even attack, but only with a request for the same document, and even that
that might be difficult. might be difficult.
3.2 Replay Attacks 3.2 Replay Attacks
A replay attack against digest authentication would usually be A replay attack against digest authentication would usually be
pointless for a simple GET request since an eavesdropper would pointless for a simple GET request since an eavesdropper would
already have seen the only document he could obtain with a replay. already have seen the only document he could obtain with a replay.
This is because the URI of the requested document is digested in This is because the URI of the requested document is digested in the
the client response and the server will only deliver that document. client response and the server will only deliver that document. By
By contrast under Basic Authentication once the eavesdropper has contrast under Basic Authentication once the eavesdropper has the
the user's password any document protected by that password is open user's password, any document protected by that password is open to
to him. A GET request containing form data could only be "replayed" him. A GET request containing form data could only be "replayed"
with the identical data. However, this could be problematic if it with the identical data. However, this could be problematic if it
caused a CGI script to take some action on the server. caused a CGI script to take some action on the server.
Thus, for some purposes, it is necessary to protect against replay Thus, for some purposes, it is necessary to protect against replay
attacks. A good digest implementation can do this in various ways. attacks. A good digest implementation can do this in various ways.
The server created "nonce" value is implementation dependent, but if The server created "nonce" value is implementation dependent, but if
it contains a digest of the client IP, a timestamp, and a private 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 server key (as recommended above) then a replay attack is not simple.
simple. An attacker must convince the server that the request is An attacker must convince the server that the request is coming from
coming from a false IP address and must cause the server to deliver a false IP address and must cause the server to deliver the document
the document to an IP address different from the address to which it to an IP address different from the address to which it believes it
believes it is sending the document. An attack can only succeed in is sending the document. An attack can only succeed in the period
the period before the timestamp expires. Digesting the client before the time-stamp expires. Digesting the client IP and time-stamp
IP and timestamp in the nonce permits an implementation which does in the nonce permits an implementation which does not maintain state
not maintain state between transactions. between transactions.
For applications where no possibility of replay attack can be For applications where no possibility of replay attack can be
tolerated the server can use one-time response digests which will tolerated the server can use one-time response digests which will not
not be honored for a second use. This requires the overhead of be honored for a second use. This requires the overhead of the
the server remembering which digests have been used until the server remembering which digests have been used until the nonce
nonce timestamp (and hence the digest built with it) has expired, time-stamp (and hence the digest built with it) has expired, but it
but it effectively protects against replay attacks. Instead of effectively protects against replay attacks. Instead of maintaining a
maintaining a list of the values of used digests, a server would list of the values of used digests, a server would hash these values
hash these values and require re-authentication whenever a hash and require re-authentication whenever a hash collision occurs.
collision occurs.
An implementation must give special attention to the possibility of An implementation must give special attention to the possibility of
replay attacks with POST and PUT requests. A successful replay replay attacks with POST and PUT requests. A successful replay attack
attack could result in counterfeit form data or a counterfeit could result in counterfeit form data or a counterfeit version of a
version of a PUT file. The use of one-time digests or one-time PUT file. The use of one-time digests or one-time nonces is
nonces is recommended. It is also recommended that the optional recommended. It is also recommended that the optional <digest> be
<message-digest> be implemented for use with POST or PUT requests implemented for use with POST or PUT requests to assure the integrity
to assure the integrity of the posted data. Alternatively, a server of the posted data. Alternatively, a server may choose to allow
may choose to allow digest authentication only with GET requests. digest authentication only with GET requests. Responsible server
Responsible server implementors will document the risks described implementors will document the risks described here as they pertain to
here as they pertain to a given implementation. a given implementation.
3.3 Man in the Middle 3.3 Man in the Middle
Both Basic and Digest authentication are vulnerable to "man in the Both Basic and Digest authentication are vulnerable to "man in the
middle" attacks, for example, from a hostile or compromised proxy. middle" attacks, for example, from a hostile or compromised proxy.
Clearly, this would present all the problems of eavesdropping. But it Clearly, this would present all the problems of eavesdropping. But
could also offer some additional threats. In particular, even with it could also offer some additional threats.
digest authentication, a hostile proxy might spoof the client into
making a request the attacker wanted rather than one the client A simple but effective attack would be to replace the Digest challenge
wanted. Of course, this is still much harder than a comparable with a Basic challenge, to spoof the client into revealing their
attack against Basic Authentication. 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.
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.
3.4 Spoofing by Counterfeit Servers 3.4 Spoofing by Counterfeit Servers
Basic Authentication is vulnerable to spoofing by counterfeit servers. Basic Authentication is vulnerable to spoofing by counterfeit
If a user can be led to believe that she is connecting to a host servers. If a user can be led to believe that she is connecting to a
containing information protected by a password she knows when host containing information protected by a password she knows, when in
in fact she is connecting to a hostile server then the hostile server 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 can request a password, store it away for later use, and feign an
error. This type of attack is not possible with Digest Authentication. 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.
3.5 Summary 3.5 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.
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.
3.6 Summary
By modern cryptographic standards Digest Authentication is weak. But By modern cryptographic standards Digest Authentication is weak. But
for a large range of purposes it is valuable as a replacement for 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. It remedies many, but not all, weaknesses of
Basic Authentication. Its strength may vary depending on the Basic Authentication. Its strength may vary depending on the
implementation. In particular the structure of the nonce (which is implementation. In particular the structure of the nonce (which is
dependent on the server implementation) may affect the ease of dependent on the server implementation) may affect the ease of
mounting a replay attack. A range of server options is appropriate mounting a replay attack. A range of server options is appropriate
since, for example, some implementations may be willing to accept the since, for example, some implementations may be willing to accept the
server overhead of one-time nonces or digests to eliminate the server overhead of one-time nonces or digests to eliminate the
skipping to change at line 528 skipping to change at line 762
the one recommended above restricted to a single IP address and with the one recommended above restricted to a single IP address and with
a limited lifetime. a limited lifetime.
The bottom line is that *any* compliant implementation will be The bottom line is that *any* compliant implementation will be
relatively weak by cryptographic standards, but *any* compliant relatively weak by cryptographic standards, but *any* compliant
implementation will be far superior to Basic Authentication. implementation will be far superior to Basic Authentication.
4. Acknowledgments 4. Acknowledgments
In addition to the authors, valuable discussion instrumental in In addition to the authors, valuable discussion instrumental in
creating this document have come from Peter J Churchyard, Ned Freed, creating this document has come from Peter J. Churchyard, Ned Freed,
and David Kristol. and David M. Kristol.
5. References 5. References
[1] T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen. [1] T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen.
"Hypertext Transfer Protocol -- HTTP/1.0" "Hypertext Transfer Protocol -- HTTP/1.0"
Internet-Draft (work in progress), UC Irvine, Internet-Draft (work in progress), UC Irvine,
<URL:http://ds.internic.net/internet-drafts/ <URL:http://ds.internic.net/internet-drafts/
draft-ietf-http-v10-spec-00.txt>, March 1995. draft-ietf-http-v10-spec-00.txt>, March 1995.
[2] RFC 1321. R.Rivest, "The MD5 Message-Digest Algorithm", [2] T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen...
"Hypertext Transfer Protocol -- HTTP/1.1"
TBS
[3] RFC 1321. R.Rivest, "The MD5 Message-Digest Algorithm",
<URL:http://ds.internic.net/rfc/rfc1321.txt>, <URL:http://ds.internic.net/rfc/rfc1321.txt>,
April 1992. April 1992.
6. Authors Addresses 6. Authors Addresses
John Franks John Franks
john@math.nwu.edu john@math.nwu.edu
Professor of Mathematics Professor of Mathematics
Department of Mathematics Department of Mathematics
Northwestern University Northwestern University
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