Digest access authentication is one of the agreed-upon methods a web server can use to negotiate credentials, such as username or password, with a user's web browser. This can be used to confirm the identity of a user before sending sensitive information, such as online banking transaction history. It applies a hash function to the username and password before sending them over the network. In contrast, basic access authentication uses the easily reversible Base64 encoding instead of hashing, making it non-secure unless used in conjunction with TLS.
Digest access authentication was originally specified by RFC 2069 (An Extension to HTTP: Digest Access Authentication). RFC 2069 specifies roughly a traditional digest authentication scheme with security maintained by a server-generated nonce value. The authentication response is formed as follows (where HA1 and HA2 are names of string variables):
HA1 = MD5(username:realm:password) HA2 = MD5(method:digestURI) response = MD5(HA1:nonce:HA2)
A MD5 hash is a 16-byte value. The HA1 and HA2 values used in the computation of the response are the hexadecimal representation (in lowercase) of the MD5 hashes respectively.
RFC 2069 was later replaced by RFC 2617 (HTTP Authentication: Basic and Digest Access Authentication). RFC 2617 introduced a number of optional security enhancements to digest authentication; "quality of protection" (qop), nonce counter incremented by client, and a client-generated random nonce. These enhancements are designed to protect against, for example, chosen-plaintext attack cryptanalysis.
If the algorithm directive's value is "MD5" or unspecified, then HA1 is
HA1 = MD5(username:realm:password)
If the algorithm directive's value is "MD5-sess", then HA1 is
HA1 = MD5(MD5(username:realm:password):nonce:cnonce)
If the qop directive's value is "auth" or is unspecified, then HA2 is
HA2 = MD5(method:digestURI)
If the qop directive's value is "auth-int", then HA2 is
HA2 = MD5(method:digestURI:MD5(entityBody))
If the qop directive's value is "auth" or "auth-int", then compute the response as follows:
response = MD5(HA1:nonce:nonceCount:cnonce:qop:HA2)
If the qop directive is unspecified, then compute the response as follows:
response = MD5(HA1:nonce:HA2)
The above shows that when qop is not specified, the simpler RFC 2069 standard is followed.
The MD5 calculations used in HTTP digest authentication 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 (a brute-force attack) - perhaps aided by a dictionary or suitable look-up list, which for MD5 is readily available.
The HTTP scheme was designed by Phillip Hallam-Baker at CERN in 1993 and does not incorporate 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, collision attacks were in 2004 generally believed to not affect applications where the plaintext (i.e. password) is not known. However, claims in 2006 cause some doubt over other MD5 applications as well. So far, however, MD5 collision attacks have not been shown to pose a threat to digest authentication, and the RFC 2617 allows servers to implement mechanisms to detect some collision and replay attacks.
Some of the security strengths of HTTP digest authentication are:
There are several drawbacks with digest access authentication:
These weak cleartext protocols used together with HTTPS network encryption resolve many of the threats that digest access authentication is designed to prevent.
However, this use of HTTPS relies upon the end user to accurately validate that they are accessing the correct URL each time to prevent sending their password to an untrusted server, which results in phishing attacks. Users often fail to do this, which is why phishing has become the most common form of security breach.
Some strong authentication protocols for web-based applications that are occasionally used include:
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 - as of April 2005 , only the Opera and Konqueror web browsers are 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:
GET /dir/index.html HTTP/1.0 Host: localhost
HTTP/1.0 401 Unauthorized Server: HTTPd/0.9 Date: Sun, 10 Apr 2014 20:26:47 GMT WWW-Authenticate: Digest realm="email@example.com", qop="auth,auth-int", nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", opaque="5ccc069c403ebaf9f0171e9517f40e41" Content-Type: text/html Content-Length: 153 <!DOCTYPE html> <html> <head> <meta charset="UTF-8" /> <title>Error</title> </head> <body> <h1>401 Unauthorized.</h1> </body> </html>
GET /dir/index.html HTTP/1.0 Host: localhost Authorization: Digest username="Mufasa", realm="firstname.lastname@example.org", nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093", uri="/dir/index.html", qop=auth, nc=00000001, cnonce="0a4f113b", response="6629fae49393a05397450978507c4ef1", opaque="5ccc069c403ebaf9f0171e9517f40e41"
(followed by a blank line, as before).
HTTP/1.0 200 OK Server: HTTPd/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 of the restricted page).
The "response" value is calculated in three steps, as follows. Where values are combined, they are delimited by colons.
"/dir/index.html". The result is referred to as HA2.
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.
Completing the example given in RFC 2617 gives the following results for each step.
HA1 = MD5( "Mufasa:email@example.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 username 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.
.htdigest is a flat-file used to store usernames, realm and passwords for digest authentication of Apache HTTP Server. The name of the file is given in the .htaccess configuration, and can be anything, but ".htdigest" is the canonical name. The file name starts with a dot, because most Unix-like operating systems consider any file that begins with dot to be hidden. This file is often maintained with the shell command "htdigest" which can add, and update users, and will properly encode the password for use.
The syntax of the htdigest command:
htdigest [ -c ] passwdfile realm username
The format of the .htdigest file:
Most browsers have substantially implemented the spec, some barring certain features such as auth-int checking or the MD5-sess algorithm. If the server requires that these optional features be handled, clients may not be able to authenticate (though note mod_auth_digest for Apache does not fully implement RFC 2617 either).