python/tink/daead/_deterministic_aead_wrapper.py

# Copyright 2019 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Deterministic AEAD wrapper."""

from typing import Type
from absl import logging

from tink import core
from tink.daead import _deterministic_aead


class _WrappedDeterministicAead(_deterministic_aead.DeterministicAead):
  """Implements DeterministicAead for a set of DeterministicAead primitives."""

  def __init__(self, pset: core.PrimitiveSet):
    self._primitive_set = pset

  def encrypt_deterministically(self, plaintext: bytes,
                                associated_data: bytes) -> bytes:
    primary = self._primitive_set.primary()
    return primary.identifier + primary.primitive.encrypt_deterministically(
        plaintext, associated_data)

  def decrypt_deterministically(self, ciphertext: bytes,
                                associated_data: bytes) -> bytes:
    if len(ciphertext) > core.crypto_format.NON_RAW_PREFIX_SIZE:
      prefix = ciphertext[:core.crypto_format.NON_RAW_PREFIX_SIZE]
      ciphertext_no_prefix = ciphertext[core.crypto_format.NON_RAW_PREFIX_SIZE:]
      for entry in self._primitive_set.primitive_from_identifier(prefix):
        try:
          return entry.primitive.decrypt_deterministically(ciphertext_no_prefix,
                                                           associated_data)
        except core.TinkError as e:
          logging.info(
              'ciphertext prefix matches a key, but cannot decrypt: %s', e)
    # Let's try all RAW keys.
    for entry in self._primitive_set.raw_primitives():
      try:
        return entry.primitive.decrypt_deterministically(ciphertext,
                                                         associated_data)
      except core.TinkError as e:
        pass
    # nothing works.
    raise core.TinkError('Decryption failed.')


class DeterministicAeadWrapper(
    core.PrimitiveWrapper[_deterministic_aead.DeterministicAead,
                          _deterministic_aead.DeterministicAead]):
  """DeterministicAeadWrapper is a PrimitiveWrapper for DeterministicAead.

  The created primitive works with a keyset (rather than a single key). To
  encrypt a plaintext, it uses the primary key in the keyset, and prepends to
  the ciphertext a certain prefix associated with the primary key. To decrypt,
  the primitive uses the prefix of the ciphertext to efficiently select the
  right key in the set. If the keys associated with the prefix do not work, the
  primitive tries all keys with OutputPrefixType RAW.
  """

  def wrap(
      self, pset: core.PrimitiveSet) -> _deterministic_aead.DeterministicAead:
    return _WrappedDeterministicAead(pset)

  def primitive_class(self) -> Type[_deterministic_aead.DeterministicAead]:
    return _deterministic_aead.DeterministicAead

  def input_primitive_class(
      self) -> Type[_deterministic_aead.DeterministicAead]:
    return _deterministic_aead.DeterministicAead