$ man htpasswd
       On the Windows and MPE platforms, passwords encrypted with htpasswd are
       limited to no more than 255 characters in length. Longer passwords will
       be truncated to 255 characters.

       The MD5 algorithm used by htpasswd is specific to the Apache  software;
       passwords encrypted using it will not be usable with other Web servers.

       Usernames are limited to 255 bytes and may not include the character :.

Apache HTTP Server                2003-11-25                       HTPASSWD(1)