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Author image Sergei Zhmylev


Math::Revhash - Reverse hash computation library


    use Math::Revhash qw( revhash revunhash );

    # OO style
    my $revhash = Math::Revhash->new( $length, $A, $B, $C );
    my $hash = $revhash->hash( $number );
    my $number = $revhash->unhash( $hash );

    # Procedural style
    my $hash = revhash( $number, $length, $A, $B, $C );
    my $hash = revhash( $number, 5 );
    my $number = revunhash( $hash, $length, $A, $B, $C );

    $Math::Revhash::UNSAFE = 1;


This module is intended for fast and lightweight numbers reversible hashing. Say there are millions of entries inside RDBMS and each entry identified with sequential primary key. Sometimes we want to expose this key to users, i.e. in case it is a session ID. Due to several reasons it could be a good idea to hide from the outer world that those session IDs are just a generic sequence of integers. This module will perform fast, lightweight and reversible translation between simple sequence 1, 2, 3, ... and something like 3287, 8542, 1337, ... without need for hash-table lookups, large memory storage and any other expensive mechanisms.

So far, this module is only capable of translating positive non-zero integers. To use the module you can either choose one of hash lengths: 1..9, for which all other parameters are pre-defined, or specify any positive $length with non-default $A parameter (see below). In any case $number for hashing should not exceed predefined hash length. $B and $C parameters could also be specified to avoid extra modular inverse and power calculation, respectively.



Compute $hash = revhash($number, $length, $A, $B, $C)

$number is the source number to be hashed.
$length is required hash length in digits.
$A (optional for pre-defined lengths) is the first parameter of hash function.

There are some hard-coded $A values for pre-defined lengths. You are free to specify any positive $A to customize the function. It is recommended to choose only primary numbers for $A to avoid possible collisions. $A should not be too short or too huge digit number. It is recommended to start with any primary number close to 10 ** ($length + 1). You are encouraged to play around it on your own.

$B (optional) is the second parameter of hash function.

It is a modular inverse of $A and is being computed as $B = Math::BigInt->bmodinv($A, 10 ** $length) unless explicitly specified.

$C (optional) is the third parameter of hash function.

As our numbers are decimal it is just 10 to the power of $length: $C = 10 ** $length.


Compute $number = revunhash($hash, $length, $A, $B, $C). It takes the same arguments as revhash besides:

$hash is hash value that should be translated back to a number.


Just an object oriented alias for revhash: $hash = $obj->hash($number). All the hash function parameters will be taken from the object itself.


Just an object oriented alias for revunhash: $number = $obj->unhash($hash). All the hash function parameters will be taken from the object itself.


$obj = Math::Revhash->new($length, $A, $B, $C) is an object constructor that will firstly check and vivify all the arguments and store them inside new object.


Arguments parsing and parameters auto-computing takes some time thus sometimes it would be preffered to avoid this phase on every translation operation. There is an UNSAFE mode to speed up the whole process (see SYNOPSIS). In this mode all arguments become mandatory on revhash/revunhash calls. You can either use OO style and still imply and check arguments on object creation, or use procedural style and specify each argument on every call. Use this mode with extra caution.


Sergei Zhmylev, <zhmylove@cpan.org>


Please report any bugs or feature requests to official GitHub page at https://github.com/zhmylove/math-revhash. You also can use official CPAN bugtracker by reporting to bug-math-revhash at rt.cpan.org, or through the web interface at https://rt.cpan.org/NoAuth/ReportBug.html?Queue=Math-Revhash. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.


To install this module, run the following commands:

    $ perl Makefile.PL
    $ make
    $ make test
    $ make install


This software is Copyright (c) 2020 by Sergei Zhmylev.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)