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22 ” HTTP • Initialize a request counter that we’ll call nc with a value of 1. The value of this counter will need to be incremented and retransmitted with each subsequent request to uniquely identify it. Retransmitting a request counter value used in a previous request will result in the server rejecting it. N ote that the value of this counter will need to be expressed to the server as a hexadecimal number. The dechex PHP function is useful for this. • Generate a random hash using the aforementioned hashing functions that we’ll call the client nonce or cnonce. The time and rand functions may be useful here. This can (and probably should) be regenerated and resent with each request. • Take note of the value of the nonce key pro vided by the server, also kno wn as the server nonce. W e’ll refer to this as simply the nonce. This is randomly generated by the server and will expire after a certain period of time, at which point the server will respond with a 401 status code. It will modify the WWW-Authenticate header it returns in two noteworthy ways: 1) the key - value pair stale=TRUE will be added; 2) the nonce value will be changed. When this happens, simply rederive the response code as sho wn belo w with the new nonce value and resubmit the original request (not forgetting to increment the request counter). • Concatenate HA1, the server nonce (nonce), the current request counter (nc) value, the client nonce you generated (cnonce), an appropriate value (most
HTTP ” 23 likely “auth”) from the comma-separated list contained in the qop (quality of protection) key pro vided by the server, and HA2 together separated by colons and take the MD5 hash of that string. This is the final response code. • Lastly, send everything the server originally sent in the WWW-Authenticate header, plus the response value and its constituents (except the password obviously), back to the server in the usual Authorization header. Authorization: Digest username="USERNAME", realm="testrealm@host.com", nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", uri="/wiki/Main_Page", qop="auth", nc=00000001, cnonce="0a4f113b", response="6629fae49393a05397450978507c4ef1", opaque="5ccc069c403ebaf9f0171e9517f40e41" Some third-party clients implement this, some don’t. Again, it’s not commonly used, but it’s good to be aware of ho w to derive the appropriate header value in cases where you must implement it yourself. F or more information on HTTP authentication, see RFC 2617.
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