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# Copyright 2020 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
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# See the License for the specific language governing permissions and
# limitations under the License.
"""Module for envelope encryption with KMS."""
from __future__ import absolute_import
from __future__ import division
# Placeholder for import for type annotations
from __future__ import print_function
import struct
from tink.proto import tink_pb2
from tink import core
from tink.aead import _aead
class KmsEnvelopeAead(_aead.Aead):
"""Implements envelope encryption.
Envelope encryption generates a data encryption key (DEK) which is used
to encrypt the payload. The DEK is then send to a KMS to be encrypted and
the encrypted DEK is attached to the ciphertext. In order to decrypt the
ciphertext, the DEK first has to be decrypted by the KMS, and then the DEK
can be used to decrypt the ciphertext. For further information see
The ciphertext structure is as follows:
* Length of the encrypted DEK: 4 bytes (big endian)
* Encrypted DEK: variable length, specified by the previous 4 bytes
* AEAD payload: variable length
# Defines in how many bytes the DEK length will be encoded.
def __init__(self, key_template: tink_pb2.KeyTemplate, remote: _aead.Aead):
self.key_template = key_template
self.remote_aead = remote
def encrypt(self, plaintext: bytes, associated_data: bytes) -> bytes:
# Get new key from template
dek = core.Registry.new_key_data(self.key_template)
dek_aead = core.Registry.primitive(dek, _aead.Aead)
# Encrypt plaintext
ciphertext = dek_aead.encrypt(plaintext, associated_data)
# Wrap DEK key values with remote
encrypted_dek = self.remote_aead.encrypt(dek.value, b'')
# Construct ciphertext, DEK length encoded as big endian
enc_dek_len = struct.pack('>I', len(encrypted_dek))
return enc_dek_len + encrypted_dek + ciphertext
def decrypt(self, ciphertext: bytes, associated_data: bytes) -> bytes:
ct_len = len(ciphertext)
# Recover DEK length
if ct_len < self.DEK_LEN_BYTES:
raise core.TinkError
dek_len = struct.unpack('>I', ciphertext[0:self.DEK_LEN_BYTES])[0]
# Basic check if DEK length can be valid.
if dek_len > (ct_len - self.DEK_LEN_BYTES) or dek_len < 0:
raise core.TinkError
# Decrypt DEK with remote AEAD
encrypted_dek_bytes = ciphertext[self.DEK_LEN_BYTES:self.DEK_LEN_BYTES +
dek_bytes = self.remote_aead.decrypt(encrypted_dek_bytes, b'')
# Get AEAD primitive based on DEK
dek = tink_pb2.KeyData()
dek.type_url = self.key_template.type_url
dek.value = dek_bytes
dek.key_material_type = tink_pb2.KeyData.KeyMaterialType.SYMMETRIC
dek_aead = core.Registry.primitive(dek, _aead.Aead)
# Extract ciphertext payload and decrypt
ct_bytes = ciphertext[self.DEK_LEN_BYTES + dek_len:]
return dek_aead.decrypt(ct_bytes, associated_data)