Digest_access_authentication

'''Digest access authentication''' is one of the agreed methods a web page can use to negotiate credentials with a web user (using the HTTP protocol). This method builds upon (and obsoletes) the Basic_authentication_scheme, allowing user identity to be established without having to send a Password in Plaintext over the network. The HTTP scheme was designed at CERN in 1993 and does not represent subsequent improvements in authentication systems, such as the development of keyed-hash message authentication code (HMAC). Although the cryptographic construction that is used is based on the MD5 hash function which is known to be compromised, the known attacks against MD5 do not affect its use in this particular protocol. One consequence of the scheme's design is that the server must know the password (i.e. store it in plain text) or store the same MD5 hash that is used to calculate the client's response (see example, below). This means that if the password database at a site is compromised the attacker will be able to impersonate any user whose access credentials are stolen. Such a compromise should not affect other sites if the MD5 hash is stored rather than the password, because the realm information is used as a salt. The MD5 calculation is intended to be "one way", meaning that it should be difficult to determine the original input when only the output is known. If the password itself is too simple, however, then it may be possible to test all possible inputs and find a matching output – perhaps aided by a dictionary or suitable look-up list. Ideally users should be forced to use long, non-trivial passwords. Digest access authentication was originally specified by RFC 2069 (''An Extension to HTTP: Digest Access Authentication''), which was later replaced by RFC 2617 (''HTTP Authentication: Basic and Digest Access Authentication''). == Example with explanation == : ''Warning: Please refer to the original specifications for a more comprehensive discussion of security issues.'' The following example was originally given in RFC 2617 and is expanded here to show the full text expected for each request and response. Note that only the "auth" (authentication) quality of protection code is covered – at the time of writing only the Opera Web_browser is known to support "auth-int" (authentication with integrity protection). Although the specification mentions HTTP version 1.1 the scheme can be successfully added to a version 1.0 server, as shown here. This typical transaction consists of the following steps. * The client asks for a page that requires authentication but does not provide a User_name and password. Typically this is because the user simply entered the address or followed a link to the page. * The server responds with the "401" response code, providing the authentication realm and a randomly-generated, single-use value called nonce. * At this point, the client will present the authentication realm (typically a description of the computer or system being accessed) to the user and prompt for a user name and password. The user may decide to cancel at this point. * Once a user name and password have been supplied, the client re-sends the same request but adds an authentication header that includes the response code. * In this example, the server accepts the authentication and the page is returned. If the user name is invalid and/or the password is incorrect, the server might return the "401" response code and the client would prompt the user again. Note: A client may pre-emptively send the authentication header in its first request, with no user interaction required. ---- '''Client request (no authentication)''':

GET /dir/index.html HTTP/1.0
Host: localhost

(followed by a new line, in the form of a Carriage_return followed by a Line_feed). '''Server response''':

HTTP/1.0 401 Unauthorised
Server: SokEvo/0.9
Date: Sun, 10 Apr 2005 20:26:47 GMT
WWW-Authenticate: Digest realm="testrealm@host.com",
                         qop="auth,auth-int",
                         nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
                         opaque="5ccc069c403ebaf9f0171e9517f40e41"
Content-Type: text/html
Content-Length: 311



  
    Error
    
  
  401 Unauthorised.

'''Client request (user name "Mufasa", password "Circle Of Life")''':

GET /dir/index.html HTTP/1.0
Host: localhost
Authorization: Digest username="Mufasa",
                      realm="testrealm@host.com",
                      nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
                      uri="/dir/index.html",
                      qop=auth,
                      nc=00000001,
                      cnonce="0a4f113b",
                      response="6629fae49393a05397450978507c4ef1",
                      opaque="5ccc069c403ebaf9f0171e9517f40e41"

(followed by a blank line, as before). '''Server response''':

HTTP/1.0 200 OK
Server: SokEvo/0.9
Date: Sun, 10 Apr 2005 20:27:03 GMT
Content-Type: text/html
Content-Length: 7984

(followed by a blank line and HTML text comprising of the restricted page). ---- RFC 2617 assumes that the scheme is understood and fails to complete the example by explaining where the "response" value comes from. It is calculated in three steps, as follows. Where values are combined, they are delimited by colon symbols. # The MD5 hash of the combined user name, authentication realm and password is calculated. The result is referred to as HA1. # The MD5 hash of the combined method and digest URI is calculated, e.g. of "GET" and "/dir/index.html". The result is referred to as HA2. # The MD5 hash of the combined HA1 result, server nonce (nonce), request counter (nc), client nonce (cnonce), quality of protection code (qop) and HA2 result is calculated. The result is the "response" value provided by the client. Since the server has the same information as the client, the response can be checked by performing the same calculation. In the example given above the result is formed as follows – where MD5() represents a function used to calculate an MD5 hash, backslashes represent a continuation and the quotes shown are not used in the calculation.

    HA1 = MD5( "Mufasa:testrealm@host.com:Circle Of Life" )
        = 939e7578ed9e3c518a452acee763bce9

    HA2 = MD5( "GET:/dir/index.html" )
        = 39aff3a2bab6126f332b942af96d3366

    Response = MD5( "939e7578ed9e3c518a452acee763bce9:\
                     dcd98b7102dd2f0e8b11d0f600bfb0c093:\
                     00000001:0a4f113b:auth:\
                     39aff3a2bab6126f332b942af96d3366" )
             = 6629fae49393a05397450978507c4ef1

At this point the client may make another request, reusing the server nonce value (the server only issues a new nonce for each "401" response) but providing a new client nonce (cnonce). For subsequent requests, the hexadecimal request counter (nc) must be greater than the last value it used – otherwise an attacker could simply "replay" an old request with the same credentials. It is up to the server to ensure that the counter increases for each of the nonce values that it has issued, rejecting any bad requests appropriately. Obviously changing the method, URI and/or counter value will result in a different response value. The server should remember nonce values that it has recently generated. It may also remember when each nonce value was issued, expiring them after a certain amount of time. If an expired value is used, the server should respond with the "401" status code and add stale=TRUE to the authentication header – indicating that the client should re-send with the new nonce provided, without prompting the user for another user name and password. The server does not need to keep any expired nonce values – it can simply assume that any unrecognised values have expired. It is also possible for the server to only allow each nonce value to be returned once, although this forces the client to repeat every request. Note that expiring a server nonce immediately will not work, as the client would never get a chance to use it. ==See also== *AKA_(security) Category:HTTP