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

