signing.py

from __future__ import division

from . import six

from . import nacl
from .exceptions import CryptoError
from .random import random


class BadSignatureError(CryptoError):
    """
    Raised when the signature was forged or otherwise corrupt.
    """


class SignedMessage(six.binary_type):

    @property
    def signature(self):
        return self[:nacl.lib.crypto_sign_BYTES]

    @property
    def message(self):
        return self[nacl.lib.crypto_sign_BYTES:]


class VerifyKey(object):

    def __init__(self, key):
        if len(key) != nacl.lib.crypto_sign_PUBLICKEYBYTES:
            raise ValueError("The key must be exactly %s bytes long" %
                                nacl.lib.crypto_sign_PUBLICKEYBYTES)

        self._key = key

    def verify(self, smessage, signature=None):
        if signature is not None:
            # If we were given the message and signature separately, combine
            #   them.
            smessage = signature + smessage

        message = nacl.ffi.new("unsigned char[]", len(smessage))
        message_len = nacl.ffi.new("unsigned long long *")

        if not nacl.lib.crypto_sign_open(message, message_len, smessage, len(smessage), self._key):
            raise BadSignatureError("Signature was forged or corrupt")

        return nacl.ffi.buffer(message, message_len[0])[:]


class SigningKey(object):

    def __init__(self, seed):
        # Verify that our seed is the proper size
        seed_size = nacl.lib.crypto_sign_SECRETKEYBYTES // 2
        if len(seed) != seed_size:
            raise ValueError(
                'The seed must be exactly %d bytes long' % (seed_size,))

        pk = nacl.ffi.new("unsigned char[]", nacl.lib.crypto_sign_PUBLICKEYBYTES)
        sk = nacl.ffi.new("unsigned char[]", nacl.lib.crypto_sign_SECRETKEYBYTES)

        if not nacl.lib.crypto_sign_seed_keypair(pk, sk, seed):
            raise CryptoError("Failed to generate a key pair")

        # Secret values
        self._seed = seed
        self._signing_key = nacl.ffi.buffer(sk, nacl.lib.crypto_sign_SECRETKEYBYTES)[:]

        # Public values
        self.verify_key = VerifyKey(nacl.ffi.buffer(pk, nacl.lib.crypto_sign_PUBLICKEYBYTES)[:])

    @classmethod
    def generate(cls):
        return cls(random(nacl.lib.crypto_sign_SECRETKEYBYTES // 2))

    def sign(self, message):
        sm = nacl.ffi.new("unsigned char[]", len(message) + nacl.lib.crypto_sign_BYTES)
        smlen = nacl.ffi.new("unsigned long long *")

        if not nacl.lib.crypto_sign(sm, smlen, message, len(message), self._signing_key):
            raise CryptoError("Failed to sign the message")

        return SignedMessage(nacl.ffi.buffer(sm, smlen[0])[:])