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.

