NAME

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

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

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.

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.

AUTHOR

Alex J. G. Burzyński <ajgb@cpan.org>

MAINTAINER

Dmitry Karasik <dmitry@karasik.eu.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.