Ed25519(3p)

NAME
Crypt::Ed25519 - bare-bones Ed25519 public key signing/verification system

SYNOPSIS
 use Crypt::Ed25519; # no symbols exported

 ############################################
 # Ed25519 API - public/private keypair

 # generate a public/private key pair once
 ($pubkey, $privkey) = Crypt::Ed25519::generate_keypair;

 # sign a message
 $signature = Crypt::Ed25519::sign $message, $pubkey, $privkey;

 # verify message
 $valid = Crypt::Ed25519::verify $message, $pubkey, $signature;

 # verify, but croak on failure
 Crypt::Ed25519::verify_croak $message, $pubkey, $signature;

 ############################################
 # EdDSA API - secret key and derived public key

 # generate a secret key
 $secret = Crypt::EdDSA::eddsa_secret_key;

 # derive public key as needed
 $pubkey = Crypt::EdDSA::eddsa_public_key $secret;

 # sign a message
 $signature = Crypt::Ed25519::eddsa_sign $message, $pubkey, $secret;

 # verify message
 $valid = Crypt::Ed25519::eddsa_verify $message, $pubkey, $signature;

 # verify, but croak on failure
 Crypt::Ed25519:eddsa_verify_croak $message, $pubkey, $signature;

 ############################################
 # Key exchange

 # side A:
 ($pubkey_a, $privkey_a) = Crypt::Ed25519::generate_keypair;
 # send $pubkey to side B

 # side B:
 ($pubkey_b, $privkey_b) = Crypt::Ed25519::generate_keypair;
 # send $pubkey to side A

 # side A then calculates their shared secret:
 $shared_secret = Crypt::Ed25519::key_exchange $pubkey_b, $privkey_a;

 # and side B does this:
 $shared_secret = Crypt::Ed25519::key_exchange $pubkey_a, $privkey_b;

 # the generated $shared_secret will be the same - you cna now
 # hash it with hkdf or something else to generate symmetric private keys

DESCRIPTION
This module implements Ed25519 public key generation, message signing and verification. It is a pretty bare-bones implementation that implements the standard Ed25519 variant with SHA512 hash, as well as a slower API compatible with the upcoming EdDSA RFC.

The security target for Ed25519 is to be equivalent to 3000 bit RSA or AES-128.

The advantages of Ed25519 over most other signing algorithms are: small public/private key and signature sizes (<= 64 octets), good key generation, signing and verification performance, no reliance on random number generators for signing and by-design immunity against branch or memory access pattern side-channel attacks.

More detailed praise and other info can be found at <http://ed25519.cr.yp.to/index.html>.

CRYPTOGRAPHY IS HARD
A word of caution: don't use this module unless you really know what you are doing - even if this module were completely error-free, that still doesn't mean that every way of using it is correct. When in doubt, it's best not to design your own cryptographic protocol.

CONVENTIONS
Public/private/secret keys, messages and signatures are all opaque and architecture-independent octet strings, and, except for the message, have fixed lengths.

Ed25519 API

($public_key, $private_key) = Crypt::Ed25519::generate_keypair
    Creates and returns a new random public and private key pair. The public key is always 32 octets, the private key is always 64 octets long.

($public_key, $private_key) = Crypt::Ed25519::generate_keypair $secret_key
    Instead of generating a random keypair, generate them from the given $secret_key (e.g. as returned by "Crypt::Ed25519::eddsa_secret_key". The derivation is deterministic, i.e. a specific $secret_key will always result in the same keypair.

    A secret key is simply a random bit string, so if you have a good source of key material, you can simply generate 32 octets from it and use this as your secret key.

$signature = Crypt::Ed25519::sign $message, $public_key, $private_key
    Generates a signature for the given message using the public and private keys. The signature is always 64 octets long and deterministic, i.e. it is always the same for a specific combination of $message, $public_key and $private_key, i.e. no external source of randomness is required for signing.

$valid = Crypt::Ed25519::verify $message, $public_key, $signature
    Checks whether the $signature is valid for the $message and $public_ke.

Crypt::Ed25519::verify_croak $message, $public_key, $signature
    Same as "Crypt::Ed25519::verify", but instead of returning a boolean, simply croaks with an error message when the signature isn't valid, so you don't have to think about what the return value really means.

EdDSA compatible API
The upcoming EdDSA draft RFC uses a slightly different (and slower) API for Ed25519. This API is provided by the following functions:

$secret_key = Crypt::Ed25519::eddsa_secret_key
    Creates and returns a new secret key, which is always 32 octets long. The secret key can be used to generate the public key via "Crypt::Ed25519::eddsa_public_key" and is not the same as the private key used in the Ed25519 API.

    A secret key is simply a random bit string, so if you have a good source of key material, you can simply generate 32 octets from it and use this as your secret key.

$public_key = Crypt::Ed25519::eddsa_public_key $secret_key
    Takes a secret key generated by "Crypt::Ed25519::eddsa_secret_key" and returns the corresponding $public_key. The derivation is deterministic, i.e. the $public_key generated for a specific $secret_key is always the same.

    This public key corresponds to the public key in the Ed25519 API above.

$signature = Crypt::Ed25519::eddsa_sign $message, $public_key, $secret_key
    Generates a signature for the given message using the public and secret keys. Apart from specifying the $secret_key, this function is identical to "Crypt::Ed25519::sign", so everything said about it is true for this function as well.

    Internally, "Crypt::Ed25519::eddsa_sign" derives the corresponding private key first and then calls "Crypt::Ed25519::sign", so it is always slower.

$valid = Crypt::Ed25519::eddsa_verify $message, $public_key, $signature
Crypt::Ed25519::eddsa_verify_croak $message, $public_key, $signature
    Really the same as "Crypt::Ed25519::verify" and "Crypt::Ed25519::verify_croak", i.e. the functions without the "eddsa_" prefix. These aliases are provided so it's clear that you are using EdDSA and not Ed25519 API.

CONVERTING BETWEEN Ed25519 and EdDSA
The Ed25519 and EdDSA compatible APIs handle keys slightly differently: The Ed25519 API gives you a public/private key pair, while EdDSA takes a secret and generates a public key from it.

You can convert an EdDSA secret to an Ed25519 private/public key pair using "Crypt::Ed25519::generate_keypair":

   ($public_key, $private_key) = Crypt::Ed25519::generate_keypair $secret

As such, the EdDSA-style API allows you to store only the secret key and derive the public key as needed. On the other hand, signing using the private key is faster than using the secret key, so converting the secret key to a public/private key pair allows you to sign a small message, or many messages, faster.

Key Exchange
As an extension to Ed25519, this module implements a key exchange similar (But not identical) to Curve25519. For this, both sides generate a keypair and send their public key to the other side. Then both sides can generate the same shared secret using this function:

$shared_secret = Crypt::Ed25519::key_exchange $other_public_key, $own_private_key
    Return the 32 octet shared secret generated from the given public and private key. See SYNOPSIS for an actual example.

SUPPORT FOR THE PERL MULTICORE SPECIFICATION
This module supports the perl multicore specification (<http://perlmulticore.schmorp.de/>) for all operations, although it makes most sense to use it when signing or verifying longer messages.

IMPLEMENTATION
This module currently uses "Nightcracker's Ed25519" implementation, which is unmodified except for some portability fixes and static delcarations, but the interface is kept implementation-agnostic to allow usage of other implementations in the future.

AUTHOR
 Marc Lehmann <schmorp@schmorp.de>
 http://software.schmorp.de/pkg/Crypt-Ed25519.html