Authen::Passphrase::MD5Crypt - passphrases using the MD5-based Unix crypt()


        use Authen::Passphrase::MD5Crypt;

        $ppr = Authen::Passphrase::MD5Crypt->new(
                        salt => "Vd3f8aG6",
                        hash_base64 => "GcsdF4YCXb0PM2UmXjIoI1");

        $ppr = Authen::Passphrase::MD5Crypt->new(
                        salt_random => 1,
                        passphrase => "passphrase");

        $ppr = Authen::Passphrase::MD5Crypt->from_crypt(

        $ppr = Authen::Passphrase::MD5Crypt->from_rfc2307(

        $salt = $ppr->salt;
        $hash_base64 = $ppr->hash_base64;

        if($ppr->match($passphrase)) { ...

        $passwd = $ppr->as_crypt;
        $userPassword = $ppr->as_rfc2307;


An object of this class encapsulates a passphrase hashed using the MD5-based Unix crypt() hash function. This is a subclass of Authen::Passphrase, and this document assumes that the reader is familiar with the documentation for that class.

The crypt() function in a modern Unix actually supports several different passphrase schemes. This class is concerned only with one particular scheme, an MD5-based algorithm designed by Poul-Henning Kamp and originally implemented in FreeBSD. To handle the whole range of passphrase schemes supported by the modern crypt(), see the from_crypt constructor and the as_crypt method in Authen::Passphrase.

The MD5-based crypt() scheme uses the whole passphrase, a salt which can in principle be an arbitrary byte string, and the MD5 message digest algorithm. First the passphrase and salt are hashed together, yielding an MD5 message digest. Then a new digest is constructed, hashing together the passphrase, the salt, and the first digest, all in a rather complex form. Then this digest is passed through a thousand iterations of a function which rehashes it together with the passphrase and salt in a manner that varies between rounds. The output of the last of these rounds is the resulting passphrase hash.

In the crypt() function the raw hash output is then represented in ASCII as a 22-character string using a base 64 encoding. The base 64 digits are ".", "/", "0" to "9", "A" to "Z", "a" to "z" (in ASCII order). Because the base 64 encoding can represent 132 bits in 22 digits, more than the 128 required, the last digit can only take four of the base 64 digit values. An additional complication is that the bytes of the raw algorithm output are permuted in a bizarre order before being represented in base 64.

There is no tradition of handling these passphrase hashes in raw binary form. The textual encoding described above, including the final permutation, is used universally, so this class does not support any binary format.

The complex algorithm was designed to be slow to compute, in order to resist brute force attacks. However, the complexity is fixed, and the operation of Moore's Law has rendered it far less expensive than intended. If efficiency of a brute force attack is a concern, see Authen::Passphrase::BlowfishCrypt.


Authen::Passphrase::MD5Crypt->new(ATTR => VALUE, ...)

Generates a new passphrase recogniser object using the MD5-based crypt() algorithm. The following attributes may be given:


The salt, as a raw string. It may be any byte string, but in crypt() usage it is conventionally limited to zero to eight base 64 digits.


Causes salt to be generated randomly. The value given for this attribute is ignored. The salt will be a string of eight base 64 digits. The source of randomness may be controlled by the facility described in Data::Entropy.


The hash, as a string of 22 base 64 digits. This is the final part of what crypt() outputs.


A passphrase that will be accepted.

The salt must be given, and either the hash or the passphrase.


Generates a new passphrase recogniser object using the MD5-based crypt() algorithm, from a crypt string. The crypt string must consist of "$1$", the salt, "$", then 22 base 64 digits giving the hash. The salt may be up to 8 characters long, and cannot contain "$" or any character that cannot appear in a crypt string.


Generates a new passphrase recogniser object using the MD5-based crypt() algorithm, from an RFC 2307 string. The string must consist of "{CRYPT}" (case insensitive) followed by an acceptable crypt string.



Returns the salt, in raw form.


Returns the hash value, as a string of 22 base 64 digits.


These methods are part of the standard Authen::Passphrase interface. Not every passphrase recogniser of this type can be represented as a crypt string: the crypt format only allows the salt to be up to eight bytes, and it cannot contain any NUL or "$" characters.


Authen::Passphrase, Crypt::PasswdMD5


Andrew Main (Zefram) <>


Copyright (C) 2006, 2007, 2009, 2010, 2012 Andrew Main (Zefram) <>


This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.