ApacheMD5 is one of the hash algorithms used by Apache HTTP server for basic authentication. It is Apache specific, but e.g. nginx supports this algorithm since version 1.0.3 http://wiki.nginx.org/HttpAuthBasicModule#auth_basic_user_file.

This is a naive implementation that doesn't aim for high speed, but to be reasonably fast it uses MD5() function from OpenSSL library so during compilation you'll nead to have it installed including header files. Many Linux distributions have separate dev packages for this.

Apache comes with utility named htpasswd that allows to create, delete and update flat files normally named .htpasswd that store pairs of usernames and passwords. While both this utility and Apache support more algorithms most of them rely on UNIX crypt() function. ApacheMD5 is not one of them and therefore it is suitable for cross-platform usage. See also htpasswd documentation on http://httpd.apache.org/docs/current/programs/htpasswd.html.

Output of apacheMD5 function is not identical to what htpasswd does. To create htpasswd-like entry one needs to do:

import Data.ByteString (ByteString)
import qualified Data.ByteString.Char8 as C8 (concat, pack, singleton)
import Data.Digest.ApacheMD5 (Salt, apacheMD5, unSalt)

htpasswdEntry :: ByteString -> ByteString -> Salt -> ByteString
htpasswdEntry username password salt = C8.concat
    [ username
    , C8.pack ":$apr1$"
    , unSalt salt
    , C8.singleton '$'
    , apacheMD5 password salt
    ]