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#  Signature/DSS.py : DSS.py
#
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# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
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"""
Digital Signature Standard (DSS), as specified in `FIPS PUB 186-3`__.

A sender signs a message in the following way:

        >>> from Crypto.Hash import SHA256
        >>> from Crypto.PublicKey import ECC
        >>> from Crypto.Signature import DSS
        >>>
        >>> message = b'I give my permission to order #4355'
        >>> key = ECC.import_key(open('privkey.der').read())
        >>> h = SHA256.new(message)
        >>> signer = DSS.new(key, 'fips-186-3')
        >>> signature = signer.sign(h)

The receiver can verify authenticity of the message:

        >>> key = ECC.import_key(open('pubkey.der').read())
        >>> h = SHA256.new(received_message)
        >>> verifier = DSS.new(key, 'fips-186-3')
        >>> try:
        >>>     verifier.verify(h, signature):
        >>>     print "The message is authentic."
        >>> except ValueError:
        >>>     print "The message is not authentic."

.. __: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf

"""

__all__ = ['new', 'DssSigScheme']

from Crypto.Util.py3compat import bchr, b


from Crypto.Util.asn1 import DerSequence
from Crypto.Util.number import long_to_bytes
from Crypto.Math.Numbers import Integer

from Crypto.Hash import HMAC
from Crypto.PublicKey.ECC import _curve, EccKey


class DssSigScheme(object):
    """This signature scheme can perform DSS signature or verification.

    :undocumented: __init__
    """

    def __init__(self, key, encoding, order):
        """Create a new Digital Signature Standard (DSS) object.

        Do not instantiate this object directly,
        use `Crypto.Signature.DSS.new` instead.
        """

        self._key = key
        self._encoding = encoding
        self._order = order

        self._order_bits = self._order.size_in_bits()
        self._order_bytes = (self._order_bits - 1) // 8 + 1

    def can_sign(self):
        """Return True if this signature object can be used
        for signing messages."""

        return self._key.has_private()

    def _compute_nonce(self, msg_hash):
        raise NotImplementedError("To be provided by subclasses")

    def _valid_hash(self, msg_hash):
        raise NotImplementedError("To be provided by subclasses")

    def sign(self, msg_hash):
        """Produce the DSS signature of a message.

        :Parameters:
          msg_hash : hash object
            The hash that was carried out over the message.
            The object belongs to the `Crypto.Hash` package.

            Under mode *'fips-186-3'*, the hash must be a FIPS
            approved secure hash (SHA-1 or a member of the SHA-2 family),
            of cryptographic strength appropriate for the DSA key.
            For instance, a 3072/256 DSA key can only be used
            in combination with SHA-512.

        :Return: The signature encoded as a byte string.
        :Raise ValueError:
            If the hash algorithm is incompatible to the DSA key.
        :Raise TypeError:
            If the DSA key has no private half.
        """

        if not self._valid_hash(msg_hash):
            raise ValueError("Hash is not sufficiently strong")

        # Generate the nonce k (critical!)
        nonce = self._compute_nonce(msg_hash)

        # Perform signature using the raw API
        z = Integer.from_bytes(msg_hash.digest()[:self._order_bytes])
        sig_pair = self._key._sign(z, nonce)

        # Encode the signature into a single byte string
        if self._encoding == 'binary':
            output = b("").join([long_to_bytes(x, self._order_bytes)
                                 for x in sig_pair])
        else:
            # Dss-sig  ::=  SEQUENCE  {
            #               r       OCTET STRING,
            #               s       OCTET STRING
            # }
            output = DerSequence(sig_pair).encode()

        return output

    def verify(self, msg_hash, signature):
        """Verify that a certain DSS signature is authentic.

        This function checks if the party holding the private half of the key
        really signed the message.

        :Parameters:
          msg_hash : hash object
            The hash that was carried out over the message.
            This is an object belonging to the `Crypto.Hash` module.

            Under mode *'fips-186-3'*, the hash must be a FIPS
            approved secure hash (SHA-1 or a member of the SHA-2 family),
            of cryptographic strength appropriate for the DSA key.
            For instance, a 3072/256 DSA key can only be used in
            combination with SHA-512.

          signature : byte string
            The signature that needs to be validated.

        :Raise ValueError:
            If the signature is not authentic.
        """

        if not self._valid_hash(msg_hash):
            raise ValueError("Hash does not belong to SHS")

        if self._encoding == 'binary':
            if len(signature) != (2 * self._order_bytes):
                raise ValueError("The signature is not authentic (length)")
            r_prime, s_prime = [Integer.from_bytes(x)
                                for x in (signature[:self._order_bytes],
                                          signature[self._order_bytes:])]
        else:
            try:
                der_seq = DerSequence().decode(signature)
            except (ValueError, IndexError):
                raise ValueError("The signature is not authentic (DER)")
            if len(der_seq) != 2 or not der_seq.hasOnlyInts():
                raise ValueError("The signature is not authentic (DER content)")
            r_prime, s_prime = der_seq[0], der_seq[1]

        if not (0 < r_prime < self._order) or not (0 < s_prime < self._order):
            raise ValueError("The signature is not authentic (d)")

        z = Integer.from_bytes(msg_hash.digest()[:self._order_bytes])
        result = self._key._verify(z, (r_prime, s_prime))
        if not result:
            raise ValueError("The signature is not authentic")
        # Make PyCrypto code to fail
        return False


class DeterministicDsaSigScheme(DssSigScheme):
    # Also applicable to ECDSA

    def __init__(self, key, encoding, order, private_key):
        super(DeterministicDsaSigScheme, self).__init__(key, encoding, order)
        self._private_key = private_key

    def _bits2int(self, bstr):
        """See 2.3.2 in RFC6979"""

        result = Integer.from_bytes(bstr)
        q_len = self._order.size_in_bits()
        b_len = len(bstr) * 8
        if b_len > q_len:
            result >>= (b_len - q_len)
        return result

    def _int2octets(self, int_mod_q):
        """See 2.3.3 in RFC6979"""

        assert 0 < int_mod_q < self._order
        return long_to_bytes(int_mod_q, self._order_bytes)

    def _bits2octets(self, bstr):
        """See 2.3.4 in RFC6979"""

        z1 = self._bits2int(bstr)
        if z1 < self._order:
            z2 = z1
        else:
            z2 = z1 - self._order
        return self._int2octets(z2)

    def _compute_nonce(self, mhash):
        """Generate k in a deterministic way"""

        # See section 3.2 in RFC6979.txt
        # Step a
        h1 = mhash.digest()
        # Step b
        mask_v = bchr(1) * mhash.digest_size
        # Step c
        nonce_k = bchr(0) * mhash.digest_size

        for int_oct in 0, 1:
            # Step d/f
            nonce_k = HMAC.new(nonce_k,
                               mask_v + bchr(int_oct) +
                               self._int2octets(self._private_key) +
                               self._bits2octets(h1), mhash).digest()
            # Step e/g
            mask_v = HMAC.new(nonce_k, mask_v, mhash).digest()

        nonce = -1
        while not (0 < nonce < self._order):
            # Step h.C (second part)
            if nonce != -1:
                nonce_k = HMAC.new(nonce_k, mask_v + bchr(0),
                                   mhash).digest()
                mask_v = HMAC.new(nonce_k, mask_v, mhash).digest()

            # Step h.A
            mask_t = b("")

            # Step h.B
            while len(mask_t) < self._order_bytes:
                mask_v = HMAC.new(nonce_k, mask_v, mhash).digest()
                mask_t += mask_v

            # Step h.C (first part)
            nonce = self._bits2int(mask_t)
        return nonce

    def _valid_hash(self, msg_hash):
        return True


class FipsDsaSigScheme(DssSigScheme):

    #: List of L (bit length of p) and N (bit length of q) combinations
    #: that are allowed by FIPS 186-3. The security level is provided in
    #: Table 2 of FIPS 800-57 (rev3).
    _fips_186_3_L_N = (
                        (1024, 160),    # 80 bits  (SHA-1 or stronger)
                        (2048, 224),    # 112 bits (SHA-224 or stronger)
                        (2048, 256),    # 128 bits (SHA-256 or stronger)
                        (3072, 256)     # 256 bits (SHA-512)
                      )

    def __init__(self, key, encoding, order, randfunc):
        super(FipsDsaSigScheme, self).__init__(key, encoding, order)
        self._randfunc = randfunc

        L = Integer(key.p).size_in_bits()
        if (L, self._order_bits) not in self._fips_186_3_L_N:
            error = ("L/N (%d, %d) is not compliant to FIPS 186-3"
                     % (L, self._order_bits))
            raise ValueError(error)

    def _compute_nonce(self, msg_hash):
        # hash is not used
        return Integer.random_range(min_inclusive=1,
                                    max_exclusive=self._order,
                                    randfunc=self._randfunc)

    def _valid_hash(self, msg_hash):
        """Verify that SHA-1, SHA-2 or SHA-3 are used"""
        return (msg_hash.oid == "1.3.14.3.2.26" or
                msg_hash.oid.startswith("2.16.840.1.101.3.4.2."))


class FipsEcDsaSigScheme(DssSigScheme):

    def __init__(self, key, encoding, order, randfunc):
        super(FipsEcDsaSigScheme, self).__init__(key, encoding, order)
        self._randfunc = randfunc

    def _compute_nonce(self, msg_hash):
        return Integer.random_range(min_inclusive=1,
                                    max_exclusive=_curve.order,
                                    randfunc=self._randfunc)

    def _valid_hash(self, msg_hash):
        """Verify that SHA-[23] (256|384|512) bits are used to
        match the 128-bit security of P-256"""

        approved = ("2.16.840.1.101.3.4.2.1",
                    "2.16.840.1.101.3.4.2.2",
                    "2.16.840.1.101.3.4.2.3",
                    "2.16.840.1.101.3.4.2.8",
                    "2.16.840.1.101.3.4.2.9",
                    "2.16.840.1.101.3.4.2.10")

        return msg_hash.oid in approved


def new(key, mode, encoding='binary', randfunc=None):
    """Return a signature scheme object `DSS_SigScheme` that
    can be used to perform DSS signature or verification.

    :Parameters:
      key : a `Crypto.PublicKey.DSA` or `Crypto.PublicKey.ECC` key object
        If the key has got its private half, both signature and
        verification are possible.

        If it only has the public half, verification is possible
        but not signature generation.

        For DSA keys, let *L* and *N* be the bit lengths of the modules *p*
        and *q*: the combination *(L,N)* must appear in the following list,
        in compliance to section 4.2 of `FIPS-186`__:

        - (1024, 160)
        - (2048, 224)
        - (2048, 256)
        - (3072, 256)

      mode : string
        The parameter can take these values:

        - *'fips-186-3'*. The signature generation is carried out
          according to `FIPS-186`__: the nonce *k* is taken from the RNG.
        - *'deterministic-rfc6979'*. The signature generation
          process does not rely on a random generator.
          See RFC6979_.

      encoding : string
        How the signature is encoded. This value determines the output of
        ``sign`` and the input of ``verify``.

        The following values are accepted:

        - *'binary'* (default), the signature is the raw concatenation
          of *r* and *s*. The size in bytes of the signature is always
          two times the size of *q*.

        - *'der'*, the signature is a DER encoded SEQUENCE with two
          INTEGERs, *r* and *s*. The size of the signature is variable.

      randfunc : callable
        The source of randomness. If ``None``, the internal RNG is used.
        Only used for the *'fips-186-3'* mode.

    .. __: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf
    .. __: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf
    .. _RFC6979: http://tools.ietf.org/html/rfc6979
    """

    # The goal of the 'mode' parameter is to avoid to
    # have the current version of the standard as default.
    #
    # Over time, such version will be superseded by (for instance)
    # FIPS 186-4 and it will be odd to have -3 as default.

    if encoding not in ('binary', 'der'):
        raise ValueError("Unknown encoding '%s'" % encoding)

    if isinstance(key, EccKey):
        order = _curve.order
        private_key_attr = 'd'
    else:
        order = Integer(key.q)
        private_key_attr = 'x'

    if key.has_private():
        private_key = getattr(key, private_key_attr)
    else:
        private_key = None

    if mode == 'deterministic-rfc6979':
        return DeterministicDsaSigScheme(key, encoding, order, private_key)
    elif mode == 'fips-186-3':
        if isinstance(key, EccKey):
            return FipsEcDsaSigScheme(key, encoding, order, randfunc)
        else:
            return FipsDsaSigScheme(key, encoding, order, randfunc)
    else:
        raise ValueError("Unknown DSS mode '%s'" % mode)