NAME

       Crypt::DH - Diffie-Hellman key exchange system

SYNOPSIS

           use Crypt::DH;
           my $dh = Crypt::DH->new;
           $dh->g($g);
           $dh->p($p);

           ## Generate public and private keys.
           $dh->generate_keys;

           $my_pub_key = $dh->pub_key;

           ## Send $my_pub_key to "other" party, and receive "other"
           ## public key in return.

           ## Now compute shared secret from "other" public key.
           my $shared_secret = $dh->compute_secret( $other_pub_key );

DESCRIPTION

       Crypt::DH is a Perl implementation of the Diffie-Hellman key exchange system. Diffie-
       Hellman is an algorithm by which two parties can agree on a shared secret key, known only
       to them.  The secret is negotiated over an insecure network without the two parties ever
       passing the actual shared secret, or their private keys, between them.

THE ALGORITHM

       The algorithm generally works as follows: Party A and Party B choose a property p and a
       property g; these properties are shared by both parties. Each party then computes a random
       private key integer priv_key, where the length of priv_key is at most (number of bits in
       p) - 1. Each party then computes a public key based on g, priv_key, and p; the exact value
       is

           g ^ priv_key mod p

       The parties exchange these public keys.

       The shared secret key is generated based on the exchanged public key, the private key, and
       p. If the public key of Party B is denoted pub_key_B, then the shared secret is equal to

           pub_key_B ^ priv_key mod p

       The mathematical principles involved insure that both parties will generate the same
       shared secret key.

       More information can be found in PKCS #3 (Diffie-Hellman Key Agreement Standard):

           http://www.rsasecurity.com/rsalabs/pkcs/pkcs-3/

USAGE

       Crypt::DH implements the core routines needed to use Diffie-Hellman key exchange. To
       actually use the algorithm, you'll need to start with values for p and g; p is a large
       prime, and g is a base which must be larger than 0 and less than p.

       Crypt::DH uses Math::BigInt internally for big-integer calculations. All accessor methods
       (p, g, priv_key, and pub_key) thus return Math::BigInt objects, as does the compute_secret
       method.  The accessors, however, allow setting with a scalar decimal string, hex string
       (^0x), Math::BigInt object, or Math::Pari object (for backwards compatibility).

   $dh = Crypt::DH->new([ %param ]).
       Constructs a new Crypt::DH object and returns the object.  %param may include none, some,
       or all of the keys p, g, and priv_key.

   $dh->p([ $p ])
       Given an argument $p, sets the p parameter (large prime) for this Crypt::DH object.

       Returns the current value of p.  (as a Math::BigInt object)

   $dh->g([ $g ])
       Given an argument $g, sets the g parameter (base) for this Crypt::DH object.

       Returns the current value of g.

   $dh->generate_keys
       Generates the public and private key portions of the Crypt::DH object, assuming that
       you've already filled p and g with appropriate values.

       If you've provided a priv_key, it's used, otherwise a random priv_key is created using
       either Crypt::Random (if already loaded), or /dev/urandom, or Perl's rand, in that order.

   $dh->compute_secret( $public_key )
       Given the public key $public_key of Party B (the party with which you're performing key
       negotiation and exchange), computes the shared secret key, based on that public key, your
       own private key, and your own large prime value (p).

       The historical method name "compute_key" is aliased to this for compatibility.

   $dh->priv_key([ $priv_key ])
       Returns the private key.  Given an argument $priv_key, sets the priv_key parameter for
       this Crypt::DH object.

   $dh->pub_key
       Returns the public key.

AUTHOR

       Benjamin Trott (cpan:BTROTT) <ben+cpan@stupidfool.org>

       Brad Fitzpatrick (cpan:BRADFITZ) <brad@danga.com>

CONTRIBUTORS

       BinGOs - Chris Williams (cpan:BINGOS) <chris@bingosnet.co.uk>

       Mithaldu - Christian Walde (cpan:MITHALDU) <walde.christian@googlemail.com>

COPYRIGHT

       Copyright (c) 2012 the Crypt::DH "AUTHOR" and "CONTRIBUTORS" as listed above.

LICENSE

       This library is free software and may be distributed under the same terms as perl itself.