"""
<Module Name>
  dsa.py

<Author>
  Santiago Torres-Arias <santiago@nyu.edu>

<Started>
  Nov 15, 2017

<Copyright>
  See LICENSE for licensing information.

<Purpose>
  DSA-specific handling routines for signature verification and key parsing
"""
import binascii

CRYPTO = True
NO_CRYPTO_MSG = 'DSA key support for GPG requires the cryptography library'
try:
  import cryptography.hazmat.primitives.asymmetric.dsa as dsa
  import cryptography.hazmat.backends as backends
  import cryptography.hazmat.primitives.asymmetric.utils as dsautils
  import cryptography.exceptions
except ImportError:
  CRYPTO = False

import securesystemslib.gpg.util
import securesystemslib.gpg.exceptions
import securesystemslib.exceptions
import securesystemslib.formats


def create_pubkey(pubkey_info):
  """
  <Purpose>
    Create and return a DSAPublicKey object from the passed pubkey_info
    using pyca/cryptography.

  <Arguments>
    pubkey_info:
            The DSA pubkey info dictionary as specified by
            securesystemslib.formats.GPG_DSA_PUBKEY_SCHEMA

  <Exceptions>
    securesystemslib.exceptions.FormatError if
      pubkey_info does not match securesystemslib.formats.GPG_DSA_PUBKEY_SCHEMA

    securesystemslib.exceptions.UnsupportedLibraryError if
      the cryptography module is not available

  <Returns>
    A cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicKey based on the
    passed pubkey_info.

  """
  if not CRYPTO: # pragma: no cover
    raise securesystemslib.exceptions.UnsupportedLibraryError(NO_CRYPTO_MSG)

  securesystemslib.formats.GPG_DSA_PUBKEY_SCHEMA.check_match(pubkey_info)

  y = int(pubkey_info['keyval']['public']['y'], 16)
  g = int(pubkey_info['keyval']['public']['g'], 16)
  p = int(pubkey_info['keyval']['public']['p'], 16)
  q = int(pubkey_info['keyval']['public']['q'], 16)
  parameter_numbers = dsa.DSAParameterNumbers(p, q, g)
  pubkey = dsa.DSAPublicNumbers(y, parameter_numbers).public_key(
      backends.default_backend())

  return pubkey


def get_pubkey_params(data):
  """
  <Purpose>
    Parse the public-key parameters as multi-precision-integers.

  <Arguments>
    data:
           the RFC4880-encoded public key parameters data buffer as described
           in the fifth paragraph of section 5.5.2.

  <Exceptions>
    securesystemslib.gpg.exceptions.PacketParsingError:
           if the public key parameters are malformed

  <Side Effects>
    None.

  <Returns>
    The parsed DSA public key in the format
    securesystemslib.formats.GPG_DSA_PUBKEY_SCHEMA.

  """
  ptr = 0

  prime_p_length = securesystemslib.gpg.util.get_mpi_length(data[ptr: ptr + 2])
  ptr += 2
  prime_p = data[ptr:ptr + prime_p_length]
  if len(prime_p) != prime_p_length: # pragma: no cover
    raise securesystemslib.gpg.exceptions.PacketParsingError(
        "This MPI was truncated!")
  ptr += prime_p_length

  group_order_q_length = securesystemslib.gpg.util.get_mpi_length(
      data[ptr: ptr + 2])
  ptr += 2
  group_order_q = data[ptr:ptr + group_order_q_length]
  if len(group_order_q) != group_order_q_length: # pragma: no cover
    raise securesystemslib.gpg.exceptions.PacketParsingError(
        "This MPI has been truncated!")
  ptr += group_order_q_length

  generator_length = securesystemslib.gpg.util.get_mpi_length(
      data[ptr: ptr + 2])
  ptr += 2
  generator = data[ptr:ptr + generator_length]
  if len(generator) != generator_length: # pragma: no cover
    raise securesystemslib.gpg.exceptions.PacketParsingError(
        "This MPI has been truncated!")
  ptr += generator_length

  value_y_length = securesystemslib.gpg.util.get_mpi_length(data[ptr: ptr + 2])
  ptr += 2
  value_y = data[ptr:ptr + value_y_length]
  if len(value_y) != value_y_length: # pragma: no cover
    raise securesystemslib.gpg.exceptions.PacketParsingError(
        "This MPI has been truncated!")

  return {
    "y": binascii.hexlify(value_y).decode('ascii'),
    "p": binascii.hexlify(prime_p).decode("ascii"),
    "g": binascii.hexlify(generator).decode("ascii"),
    "q": binascii.hexlify(group_order_q).decode("ascii"),
  }


def get_signature_params(data):
  """
  <Purpose>
    Parse the signature parameters as multi-precision-integers.

  <Arguments>
    data:
           the RFC4880-encoded signature data buffer as described
           in the fourth paragraph of section 5.2.2

  <Exceptions>
    securesystemslib.gpg.exceptions.PacketParsingError:
           if the public key parameters are malformed

    securesystemslib.exceptions.UnsupportedLibraryError:
           if the cryptography module is not available

  <Side Effects>
    None.

  <Returns>
    The decoded signature buffer
  """
  if not CRYPTO: # pragma: no cover
    return securesystemslib.exceptions.UnsupportedLibraryError(NO_CRYPTO_MSG)

  ptr = 0
  r_length = securesystemslib.gpg.util.get_mpi_length(data[ptr:ptr+2])
  ptr += 2
  r = data[ptr:ptr + r_length]
  if len(r) != r_length: # pragma: no cover
    raise securesystemslib.gpg.exceptions.PacketParsingError(
        "r-value truncated in signature")
  ptr += r_length

  s_length = securesystemslib.gpg.util.get_mpi_length(data[ptr: ptr+2])
  ptr += 2
  s = data[ptr: ptr + s_length]
  if len(s) != s_length: # pragma: no cover
    raise securesystemslib.gpg.exceptions.PacketParsingError(
        "s-value truncated in signature")

  s = int(binascii.hexlify(s), 16)
  r = int(binascii.hexlify(r), 16)

  signature = dsautils.encode_dss_signature(r, s)

  return signature


def verify_signature(signature_object, pubkey_info, content,
    hash_algorithm_id):
  """
  <Purpose>
    Verify the passed signature against the passed content with the passed
    DSA public key using pyca/cryptography.

  <Arguments>
    signature_object:
            A signature dictionary as specified by
            securesystemslib.formats.GPG_SIGNATURE_SCHEMA

    pubkey_info:
            The DSA public key info dictionary as specified by
            securesystemslib.formats.GPG_DSA_PUBKEY_SCHEMA

    hash_algorithm_id:
            one of SHA1, SHA256, SHA512 (see securesystemslib.gpg.constants)
            used to verify the signature
            NOTE: Overrides any hash algorithm specification in "pubkey_info"'s
            "hashes" or "method" fields.

    content:
            The signed bytes against which the signature is verified

  <Exceptions>
    securesystemslib.exceptions.FormatError if:
      signature_object does not match securesystemslib.formats.GPG_SIGNATURE_SCHEMA
      pubkey_info does not match securesystemslib.formats.GPG_DSA_PUBKEY_SCHEMA

    securesystemslib.exceptions.UnsupportedLibraryError if:
      the cryptography module is not available

    ValueError:
      if the passed hash_algorithm_id is not supported (see
      securesystemslib.gpg.util.get_hashing_class)

  <Returns>
    True if signature verification passes and False otherwise

  """
  if not CRYPTO: # pragma: no cover
    raise securesystemslib.exceptions.UnsupportedLibraryError(NO_CRYPTO_MSG)

  securesystemslib.formats.GPG_SIGNATURE_SCHEMA.check_match(signature_object)
  securesystemslib.formats.GPG_DSA_PUBKEY_SCHEMA.check_match(pubkey_info)

  hasher = securesystemslib.gpg.util.get_hashing_class(hash_algorithm_id)

  pubkey_object = create_pubkey(pubkey_info)

  digest = securesystemslib.gpg.util.hash_object(
      binascii.unhexlify(signature_object['other_headers']),
      hasher(), content)

  try:
    pubkey_object.verify(
      binascii.unhexlify(signature_object['signature']),
      digest,
      dsautils.Prehashed(hasher())
    )
    return True
  except cryptography.exceptions.InvalidSignature:
    return False