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Manual Reference Pages  -  AUTHEN::DECHPWD (3)

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Authen::DecHpwd - DEC VMS password hashing



        use Authen::DecHpwd qw(

        $hash = lgi_hpwd("JRANDOM", "PASSWORD", UAI_C_PURDY_S, 1234);

        use Authen::DecHpwd qw(vms_username vms_password);

        $username = vms_username($username);
        $password = vms_password($password);


This module implements the SYS$HASH_PASSWORD password hashing function from VMS (also known as LGI$HPWD), and some associated VMS username and password handling functions.

The password hashing function is implemented in XS, with a hideously slow pure Perl backup version for systems that can’t handle XS.


UAI_C_PURDY_S These constants are used to identify the four password hashing algorithms used by VMS. They are the UAI$C_ constants in VMS.

UAI_C_AD_II refers to a 32-bit CRC algorithm. The CRC polynomial used is the IEEE CRC-32 polynomial, as used in Ethernet, and in this context is known as AUTODIN-II. The hash is merely the CRC of the password.

UAI_C_PURDY, UAI_C_PURDY_V, and UAI_C_PURDY_S refer to successive refinements of an algorithm based on Purdy polynomials. All of these algorithms use the salt and username parameters as salt, use the whole password, and return an eight-byte (64-bit) hash. The main part of the algorithm, the Purdy polynomial, is identical in all three. They differ in the pre-hashing, particularly in the treatment of the username parameter.

In UAI_C_PURDY the username is truncated or space-padded to 12 characters before being hashed in. UAI_C_PURDY_V accepts a variable-length username. UAI_C_PURDY_S accepts a variable-length username and also includes the password length in the hash. UAI_C_PURDY_S also does some extra bit rotations when hashing in the username and password strings, in order to avoid aliasing.

lgi_hpwd(USERNAME, PASSWORD, ALGORITHM, SALT) This is the SYS$HASH_PASSWORD function from VMS (also known as LGI$HPWD), but with the parameters in a different order. It hashes the PASSWORD string in a manner determined by the other parameters, and returns the hash as a string of bytes.

ALGORITHM determines which hashing algorithm will be used. It must be the value of one of the algorithm constants supplied by this module (see above).

SALT must be an integer in the range [0, 2^16). It modifies the hashing so that the same password does not always produce the same hash.

USERNAME is a string that is used as more salt. In VMS it is the username of the account to which the password controls access.

VMS usernames and passwords are constrained in character set and length, and are case-insensitive. This function does not enforce these restrictions, nor perform canonicalisation. If restrictions and canonicalisation are desired then they must be applied separately. The functions vms_username and vms_password described below may be useful.

vms_username(USERNAME) Checks whether the USERNAME string matches VMS username syntax, and canonicalises it. VMS username syntax is 1 to 31 characters from case-insensitive alphanumerics, "<B>_B>, and <B>$B>". If the string has correct username syntax then the username is returned in canonical form (uppercase). If the string is not a username then undef is returned.
vms_password(PASSWORD) Checks whether the PASSWORD string is an acceptable VMS password, and canonicalises it. VMS password syntax is 1 to 32 characters from case-insensitive alphanumerics, "<B>_B>, and <B>$B>". If the string is an acceptable password then the password is returned in canonical form (uppercase). If the string is not an acceptable password then undef is returned.




The original C implementation of LGI$HPWD was written by Shawn Clifford. The code has since been developed by Davide Casale, Mario Ambrogetti, Terence Lee, Jean-loup Gailly, Solar Designer, and Andrew Main (Zefram).

Mike McCauley <> created the first version of Authen::DecHpwd, establishing the Perl interface. This was based on Shawn Clifford’s code without the later developments.

Andrew Main (Zefram) <> created a new Authen::DecHpwd based on the more developed C code presently used, and added ancillary functions.


Copyright (C) 2002 Jean-loup Gailly <>

Based in part on code from John the Ripper, Copyright (C) 1996-2002 Solar Designer

Copyright (C) 2006, 2007, 2009, 2010, 2011 Andrew Main (Zefram) <>


This module is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
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perl v5.20.3 AUTHEN::DECHPWD (3) 2016-03-17

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