HTTP Working Group

INTERNET-DRAFT

<draft-ietf-http-digest-aa-05.txt>

 

Paul Leach

John Franks

Philip Hallam-Baker

Jeffery L. Hostetler

Ari Luotonen

Eric W. Sink

Lawrence C. Stewart

Expires SIX MONTHS FROM---> Sept. 10, 1996

 

An Extension to HTTP : 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.

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Distribution of this document is unlimited. Please send comments to  the HTTP working group at <http-wg@cuckoo.hpl.hp.com>.  Discussions of the working group are archived at  <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-talk@www10.w3.org> mailing list.

 


 Abstract

The protocol referred to as "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, as the user name and  password are passed over the network as clear text. A specification  for a different authentication scheme is needed to address this severe  limitation. This document provides specification for such a scheme,  referred to as "Digest Access Authentication". 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.

 


Table of Contents

1. 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

 


1. Introduction

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. A  specification for a new authentication scheme is needed for future  versions of the HTTP protocol. This document 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.

It is proposed that this access authentication scheme be included in  the proposed HTTP/1.1 specification.

 

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.

 

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'.

 

 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  scheme. Nevertheless it is better than nothing, better than what is  commonly used with telnet and ftp, and better than Basic  authentication.

 


2. Digest Access Authentication Scheme

2.1 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,  using the extended BNF defined in the HTTP/1.1 specification, section  2.1. In addition, a new header, Authentication-info, is specified.

2.1.1 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 |

[ digest-opaque ] |[ stale ] | [ algorithm ] )

 

realm = "realm" "=" realm-value

realm-value = quoted-string

domain = "domain" "=" <"> 1#URI <">

nonce = "nonce" "=" nonce-value

nonce-value = quoted-string

opaque = "opaque" "=" quoted-string

stale = "stale" "=" ( "true" | "false" )

algorithm = "algorithm" "=" ( "MD5" | token )

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". 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  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 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 3.2 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 3.  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. This field is a "quoted-string" as specified  in section 2.2 of the HTTP/1.1 specification [2].

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  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.

 

 

2.1.2 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 <">

LHEX = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "a" | "b" | "c" | "d" | "e" | "f"

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" as specified in section  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 13.5.1  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 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.

 

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 "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."   

 

2.2 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 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  2.5 below..

 

 2.3 Security Protocol Negotiation

It is useful for a server to be able to know which security schemes a  client is capable of handling.

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  identifies itself as HTTP/1.1 compliant.

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.

 

2.4 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",

realm="testrealm@host.com",

nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",

uri="/dir/index.html",

response="e966c932a9242554e42c8ee200cec7f6",

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

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.

 

3.1 Comparison 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.

 

3.2 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.

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.

 

3.3 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. 

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

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.

 

 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  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 while 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.

 


4. 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.


5. References

[1] T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen.

"Hypertext Transfer Protocol -- HTTP/1.0"

Internet-Draft (work in progress), UC Irvine,

<URL:http://ds.internic.net/internet-drafts/

draft-ietf-http-v10-spec-00.txt>, March 1995.

 

[2] T. Berners-Lee, R. T. Fielding, H. Frystyk Nielsen...

"Hypertext Transfer Protocol -- HTTP/1.1"

<URL:http://ds.internic.net/internet-drafts/

draft-ietf-http-v11-spec-07.txt>

 

[3] RFC 1321. R.Rivest, "The MD5 Message-Digest Algorithm",

<URL:http://ds.internic.net/rfc/rfc1321.txt>,

April 1992.

 

 


6. Authors Addresses

John Franks

john@math.nwu.edu

Professor of Mathematics

Department of Mathematics

Northwestern University

Evanston, IL 60208-2730, USA

 

Phillip M. Hallam-Baker

hallam@w3.org

European Union Fellow

CERN

Geneva

Switzerland

 

Jeffery L. Hostetler

jeff@spyglass.com

Senior Software Engineer

Spyglass, Inc.

3200 Farber Drive

Champaign, IL 61821, USA

 

Paul J. Leach

paulle@microsoft.com

Microsoft Corporation

1 Microsoft Way

Redmond, WA 98052, USA

 

Ari Luotonen

luotonen@netscape.com

Member of Technical Staff

Netscape Communications Corporation

501 East Middlefield Road

Mountain View, CA 94043, USA

 

Eric W. Sink

eric@spyglass.com

Senior Software Engineer

Spyglass, Inc.

3200 Farber Drive

Champaign, IL 61821, USA

 

Lawrence C. Stewart

stewart@OpenMarket.com

Open Market, Inc.

215 First Street

Cambridge, MA 02142, USA

 


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