NAME
   Crypt::Curve25519 - Generate shared secret using elliptic-curve
   Diffie-Hellman function

VERSION
   version 0.03

SYNOPSIS
       use Crypt::Curve25519;

       # Alice:
       my $alice_secret_key = curve25519_secret_key(random_32_bytes());
       my $alice_public_key = curve25519_public_key( $alice_secret_key );

       # Bob:
       my $bob_secret_key = curve25519_secret_key(random_32_bytes());
       my $bob_public_key = curve25519_public_key( $bob_secret_key );

       # Alice and Bob exchange their public keys
       my $alice_public_key_hex = unpack('H64', $alice_public_key);
       my $bob_public_key_hex   = unpack('H64', $bob_public_key);

       # Alice calculates shared secret to communicate with Bob
       my $shared_secret_with_bob = curve25519_shared_secret(
           $alice_secret_key,
           pack('H64', $bob_public_key_hex)
       );

       # Bob calculates shared secret to communicate with Alice
       my $shared_secret_with_alice = curve25519_shared_secret(
           $bob_secret_key,
           pack('H64', $alice_public_key_hex)
       );

       # Shared secrets are equal
       die "Something horrible has happend!"
         unless $shared_secret_with_bob eq $shared_secret_with_alice;

   This package provides also simplified OO interface:

       use Crypt::Curve25519 ();

       my $c = Crypt::Curve25519->new();

       # Alice:
       my $alice_secret_key_hex = $c->secret_key(random_hexencoded_32_bytes());
       my $alice_public_key_hex = $c->public_key( $alice_secret_key_hex );

       # Bob:
       my $bob_secret_key_hex = $c->secret_key(random_hexencoded_32_bytes());
       my $bob_public_key_hex = $c->public_key( $bob_secret_key_hex );

       # Alice and Bob exchange their public keys

       # Alice calculates shared secret to communicate with Bob
       my $shared_secret_with_bob_hex = $c->shared_secret(
                                       $alice_secret_key_hex,
                                       $bob_public_key_hex);

       # Bob calculates shared secret to communicate with Alice
       my $shared_secret_with_alice_hex = $c->shared_secret(
                                       $bob_secret_key_hex,
                                       $alice_public_key_hex);

       # Shared secrets are equal
       die "Something horrible has happend!"
         unless $shared_secret_with_bob_hex eq $shared_secret_with_alice_hex;

   Example functions to generate pseudo-random private secret key:

       sub random_32_bytes {
           return join('', map { chr(int(rand(255))) } 1 .. 32);
       }

       sub random_hexencoded_32_bytes {
          return unpack('H64', random_32_bytes());
       }

DESCRIPTION
   Curve25519 is a state-of-the-art Diffie-Hellman function suitable for a
   wide variety of applications.

   Given a user's 32-byte secret key, Curve25519 computes the user's
   32-byte public key. Given the user's 32-byte secret key and another
   user's 32-byte public key, Curve25519 computes a 32-byte secret shared
   by the two users. This secret can then be used to authenticate and
   encrypt messages between the two users.

METHODS
 new
       my $c = Crypt::Curve25519->new();

   Create a new object

 secret_key
       my $my_secret_key_hex = $c->secret_key( $my_random_32byte_string_hex );

   Using hex encoded 32-byte random string from cryptographically safe
   source create masked secret key.

 public_key
       my $public_key_hex = $c->public_key( $my_secret_key_hex );

   Using hex encoded masked secret key generate corresponding hex encoded
   32-byte Curve25519 public key.

 shared_secret
       my $shared_secret_hex = $c->shared_secret(
           $my_secret_key_hex, $his_public_key_hex
       );

   Using provided hex encoded keys generate 32-byte hex encoded shared
   secret, that both parties can use without disclosing their private
   secret keys.

 generate
   Access to primitive method is also provided.

       my $key_hex = $c->generate($my_secret_key_hex, $basepoint_hex);

       # public key
       if ( $basepoint_hex eq unpack("H64", pack("H64", "09")) ) {
           print "\$key_hex is a public key\n";
       }
       elsif ( $basepoint_hex eq $his_public_key_hex ) {
           print "\$key_hex is a shared secret\n";
       }

   Using provided hex encoded secret key and depending on the 32-byte hex
   encoded basepoint generate 32-byte hex encoded public key or shared
   secret.

FUNCTIONS
 curve25519_secret_key
       my $my_secret_key = curve25519_secret_key($my_random_32byte_string);

   Using provided 32-byte random string from cryptographically safe source
   create masked secret key.

 curve25519_public_key
       my $public_key = curve25519_public_key($my_secret_key);

   Using masked secret key generate corresponding 32-byte Curve25519 public
   key.

 curve25519_shared_secret
       my $shared_secret = curve25519_shared_secret(
           $my_secret_key, $his_public_key
       );

   Using provided keys generate 32-byte shared secret, that both parties
   can use without disclosing their private secret keys.

 curve25519
   Access to primitive function is also provided.

       use Crypt::Curve25519 'curve25519';

       my $key = curve25519($my_secret_key, $basepoint);

       # public key
       if ( $basepoint eq pack('H64', '09') ) {
           print "\$key is a public key\n";
       }
       elsif ( $basepoint eq $his_public_key ) {
           print "\$key is a shared secret\n";
       }

   Using provided secret key and depending on the 32-byte basepoint
   generate 32-byte public key or shared secret.

SEE ALSO
   *   <http://cr.yp.to/ecdh.html>

AUTHOR
   Alex J. G. Burzyński <[email protected]>

COPYRIGHT AND LICENSE
   This software is copyright (c) 2014 by Alex J. G. Burzyński
   <[email protected]>.

   This is free software; you can redistribute it and/or modify it under
   the same terms as the Perl 5 programming language system itself.

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