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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.
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