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

VERSION
    version 0.06

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 <ajgb@cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2014 by Alex J. G. Burzyński
    <ajgb@cpan.org>.

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