Hashcash(3)

NAME
Digest::Hashcash - generate Hashcash stamps (http://www.hashcash.org)

SYNOPSIS
 use Digest::Hashcash;

DESCRIPTION
This module implements the hashcash hash (or digest, although it's not clearly a digest). For all your information needs please visit http://www.hashcash.org.

One thing to note about this module is that it requires ISO C99 support, both in your compiler and your standard library. If you don't have a compiler that supports ISO C, get gcc at http://gcc.gnu.org/ :)

$secs = estimate_time $size
    Estimate the average time necessary to calculate a token of the given size.

    See also "estimate_size".

$size = estimate_size $time[, $min]
    Estimate the size that can be calculated in the given time (which is an upper bound). The function will not return a size less then "min".

    Estimating the time to be used can go wrong by as much as 50% (but is usually quite accurate), and the estimation itself can take as much as a second on slower (<pentium) machines, but faster machines (1Ghz P3 for example) usually handle it within a hundredth of a second or so.

    The estimation will be done only once, so you can call this fucntion as often as you like without incuring the overhead everytime.

$cipher = new Digest::Hashcash [param => value...]
    size => 20
        The number of collisions, in bits. Every bit increases the time to create the token (and thus the cash) by two.

    vers => 1
        Default version 1. Can produce version 0 if required for backwards compatibility.

    uid => ""
        A string used to make the token more unique (e.g. the senders address) and reduce token collisions. The string must only contain characters valid for the trial part of the token, e.g. uuencoded, base64 or e-mail-address-parts are useful here. Deprecated: use extension field if required.

    extrarand => 0
        The extra bytes of randomness to add to the token in addition to the standard amount. Each byte adds a little bit over 6 bit of randomness to the token.

        The standard amount of randomness is 8 (> 51 bits of randomness).

    timestamp => 0
        The timestamp to use. A value of 0 (the default) means to use the current time.

$token = $cipher->hash ($data [, param => value...])
    Creates and returns a new token. This can take some time.

    Any additional parameters are interpreted the same way as arguments to "new".

$prefix = $cipher->verify ($token)
    Version 0: Checks the given token and returns the number of collision bits.

    Version 1: Returns 0 if stated value is more than the computed collision value, otherwise returns the stated stamp value.

    Any additional parameters are interpreted the same way as arguments to "new".

$version = $cipher->version ($token)
    Returns the version of the stamp (currently 0 or 1).

$resource = $cipher->resource ($token)
    Returns the resource part, or "undef".

$tstamp = $cipher->timestamp ($token)
    Returns the timestamp part (in the same format as perls "time"), or "undef".

$extension = $cipher->extension ($token [,$name [,$var]])
    For Version 1 stamps returns the extension part; for Version 0 stamps returns undef. In a scalar context returns the extension string, in array context returns associative array containing extensions as keys and their values as the corresponding value. If the optional name argument is given, returns info but about that extension (ie decodes the options of that extension if called in array context); finally if the optional var argument also is given reports just the (scalar) value of that variable in the named extension.

    Note it is valid for an extension to exist (the key exists in the associative array) but to have undefined value, this corresponds to a boolean option without a value. Like "noreply;foo=bar" has two extensions, noreply and foo; noreply has no associated value, so the method extension would parse such a stamp into an associative array containing key "noreply" with an undefined value; and key "foo" containing associated value "bar".

BUGS
 * There is a y2k+100 problem, as I always assume the same as
   Time::Local.  This is a problem with the hashcash specification,
   which specifies years as 2 digits :( Though it hardly matters --
   after 100 years of Moore's law a 20 bit stamp will be tiny and not
   worth storing.

 * extension method could be more efficient (it does not cache its
   parsed results so if used in a loop reparses on each call)

AUTHOR
 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de

 Adam Back <adam@cypherspace.org> added version 1 support
 http://www.cypherspace.org/adam/