# SPDX-FileCopyrightText: 2014 Legrandin # SPDX-License-Identifier: BSD-2-Clause """ Synthetic Initialization Vector (SIV) mode. """ from __future__ import annotations from hmac import compare_digest from typing import TYPE_CHECKING, Optional, Union, overload __all__ = ["SivMode"] from binascii import unhexlify from Crypto.Cipher._state_machine import Method from Crypto.Protocol.KDF import _S2V from Crypto.Util._raw_api import is_buffer from Crypto.Util.number import bytes_to_long if TYPE_CHECKING: from types import ModuleType Buffer = Union[bytes, bytearray, memoryview] class SivMode: """Synthetic Initialization Vector (SIV). This is an Authenticated Encryption with Associated Data (`AEAD`_) mode. It provides both confidentiality and authenticity. The header of the message may be left in the clear, if needed, and it will still be subject to authentication. The decryption step tells the receiver if the message comes from a source that really knows the secret key. Additionally, decryption detects if any part of the message - including the header - has been modified or corrupted. Unlike other AEAD modes such as CCM, EAX or GCM, accidental reuse of a nonce is not catastrophic for the confidentiality of the message. The only effect is that an attacker can tell when the same plaintext (and same associated data) is protected with the same key. The length of the MAC is fixed to the block size of the underlying cipher. The key size is twice the length of the key of the underlying cipher. This mode is only available for AES ciphers. +--------------------+---------------+-------------------+ | Cipher | SIV MAC size | SIV key length | | | (bytes) | (bytes) | +====================+===============+===================+ | AES-128 | 16 | 32 | +--------------------+---------------+-------------------+ | AES-192 | 16 | 48 | +--------------------+---------------+-------------------+ | AES-256 | 16 | 64 | +--------------------+---------------+-------------------+ See `RFC5297`_ and the `original paper`__. .. _RFC5297: https://tools.ietf.org/html/rfc5297 .. _AEAD: http://blog.cryptographyengineering.com/2012/05/how-to-choose-authenticated-encryption.html .. __: http://www.cs.ucdavis.edu/~rogaway/papers/keywrap.pdf """ def __init__(self, factory: ModuleType, key: Buffer, nonce: Buffer, kwargs: dict) -> None: self.block_size = factory.block_size """The block size of the underlying cipher, in bytes.""" self._factory = factory self._cipher_params = kwargs if len(key) not in (32, 48, 64): raise ValueError("Incorrect key length (%d bytes)" % len(key)) if nonce is not None: if not is_buffer(nonce): raise TypeError("When provided, the nonce must be bytes, bytearray or memoryview") if len(nonce) == 0: raise ValueError("When provided, the nonce must be non-empty") self.nonce = bytes(nonce) """Public attribute is only available in case of non-deterministic encryption.""" subkey_size = len(key) // 2 self._mac_tag: Optional[bytes] = None # Cache for MAC tag self._kdf = _S2V(key[:subkey_size], ciphermod=factory, cipher_params=self._cipher_params) self._subkey_cipher = key[subkey_size:] # Purely for the purpose of verifying that cipher_params are OK factory.new(key[:subkey_size], factory.MODE_ECB, **kwargs) # Allowed transitions after initialization self._next = [Method.UPDATE, Method.ENCRYPT, Method.DECRYPT, Method.DIGEST, Method.VERIFY] def _create_ctr_cipher(self, v): """Create a new CTR cipher from V in SIV mode""" v_int = bytes_to_long(v) q = v_int & 0xFFFFFFFFFFFFFFFF7FFFFFFF7FFFFFFF return self._factory.new( self._subkey_cipher, self._factory.MODE_CTR, initial_value=q, nonce=b"", **self._cipher_params ) def update(self, component: Buffer) -> SivMode: """Protect one associated data component For SIV, the associated data is a sequence (*vector*) of non-empty byte strings (*components*). This method consumes the next component. It must be called once for each of the components that constitue the associated data. Note that the components have clear boundaries, so that: >>> cipher.update(b"builtin") >>> cipher.update(b"securely") is not equivalent to: >>> cipher.update(b"built") >>> cipher.update(b"insecurely") If there is no associated data, this method must not be called. Args: component (bytes/bytearray/memoryview): The next associated data component. """ if Method.UPDATE not in self._next: raise TypeError("update() can only be called immediately after initialization") self._next = [Method.UPDATE, Method.ENCRYPT, Method.DECRYPT, Method.DIGEST, Method.VERIFY] self._kdf.update(component) return self def encrypt(self, plaintext: Buffer) -> bytes: """ For SIV, encryption and MAC authentication must take place at the same point. This method shall not be used. Use `encrypt_and_digest` instead. """ raise TypeError("encrypt() not allowed for SIV mode. Use encrypt_and_digest() instead.") def decrypt(self, ciphertext: Buffer) -> bytes: """ For SIV, decryption and verification must take place at the same point. This method shall not be used. Use `decrypt_and_verify` instead. """ raise TypeError("decrypt() not allowed for SIV mode. Use decrypt_and_verify() instead.") def digest(self) -> bytes: """Compute the *binary* MAC tag. The caller invokes this function at the very end. This method returns the MAC that shall be sent to the receiver, together with the ciphertext. Returns: the MAC, as a byte string. """ if Method.DIGEST not in self._next: raise TypeError("digest() cannot be called when decrypting or validating a message") self._next = [Method.DIGEST] if self._mac_tag is None: self._mac_tag = self._kdf.derive() return self._mac_tag def hexdigest(self) -> str: """Compute the *printable* MAC tag. This method is like `digest`. Returns: the MAC, as a hexadecimal string. """ return self.digest().hex() def verify(self, received_mac_tag: Buffer) -> None: """Validate the *binary* MAC tag. The caller invokes this function at the very end. This method checks if the decrypted message is indeed valid (that is, if the key is correct) and it has not been tampered with while in transit. Args: received_mac_tag (bytes/bytearray/memoryview): This is the *binary* MAC, as received from the sender. Raises: ValueError: if the MAC does not match. The message has been tampered with or the key is incorrect. """ if Method.VERIFY not in self._next: raise TypeError("verify() cannot be called when encrypting a message") self._next = [Method.VERIFY] if self._mac_tag is None: self._mac_tag = self._kdf.derive() if not compare_digest(self._mac_tag, received_mac_tag): raise ValueError("MAC check failed") def hexverify(self, hex_mac_tag: str) -> None: """Validate the *printable* MAC tag. This method is like `verify`. Args: hex_mac_tag (string): This is the *printable* MAC, as received from the sender. Raises: ValueError: if the MAC does not match. The message has been tampered with or the key is incorrect. """ self.verify(unhexlify(hex_mac_tag)) @overload def encrypt_and_digest(self, plaintext: Buffer) -> tuple[bytes, bytes]: ... @overload def encrypt_and_digest( self, plaintext: Buffer, output: Union[bytearray, memoryview] ) -> tuple[None, bytes]: ... def encrypt_and_digest( self, plaintext: Buffer, output: Optional[Union[bytearray, memoryview]] = None ) -> tuple[Optional[bytes], bytes]: """Perform encrypt() and digest() in one step. Args: plaintext (bytes/bytearray/memoryview): The piece of data to encrypt. Keyword Args: output (bytearray/memoryview): The location where the ciphertext must be written to. If ``None``, the ciphertext is returned. Returns: The tuple ``(ciphertext, tag)``, both ``bytes``. The ciphertext is ``None`` when the ``output`` parameter specified a location for the result. """ if Method.ENCRYPT not in self._next: raise TypeError("encrypt() can only be called after initialization or an update()") self._next = [Method.DIGEST] # Compute V (MAC) if hasattr(self, "nonce"): self._kdf.update(self.nonce) self._kdf.update(plaintext) self._mac_tag = self._kdf.derive() cipher = self._create_ctr_cipher(self._mac_tag) return cipher.encrypt(plaintext, output=output), self._mac_tag @overload def decrypt_and_verify(self, ciphertext: Buffer, mac_tag: Buffer) -> bytes: ... @overload def decrypt_and_verify( self, ciphertext: Buffer, mac_tag: Buffer, output: Union[bytearray, memoryview] ) -> None: ... def decrypt_and_verify( self, ciphertext: Buffer, mac_tag: Buffer, output: Optional[Union[bytearray, memoryview]] = None ) -> Optional[bytes]: """Perform decryption and verification in one step. A cipher object is stateful: once you have decrypted a message you cannot decrypt (or encrypt) another message with the same object. You cannot reuse an object for encrypting or decrypting other data with the same key. This function does not remove any padding from the plaintext. Args: ciphertext (bytes/bytearray/memoryview): The piece of data to decrypt. It can be of any length. mac_tag (bytes/bytearray/memoryview): This is the *binary* MAC, as received from the sender. Keyword Args: output (bytearray/memoryview): The location where the plaintext must be written to. If ``None``, the plaintext is returned. Returns: the plaintext as ``bytes`` or ``None`` when the ``output`` parameter specified a location for the result. Raises: ValueError: if the MAC does not match. The message has been tampered with or the key is incorrect. """ if Method.DECRYPT not in self._next: raise TypeError("decrypt() can only be called after initialization or an update()") self._next = [Method.VERIFY] # Take the MAC and start the cipher for decryption self._cipher = self._create_ctr_cipher(mac_tag) plaintext = self._cipher.decrypt(ciphertext, output=output) if hasattr(self, "nonce"): self._kdf.update(self.nonce) self._kdf.update(plaintext if output is None else output) self.verify(mac_tag) return plaintext def _create_siv_cipher(factory, **kwargs): """Create a new block cipher, configured in Synthetic Initializaton Vector (SIV) mode. Args: factory (object): A symmetric cipher module from `Crypto.Cipher` (like `Crypto.Cipher.AES`). Keyword Args: key (bytes/bytearray/memoryview): The secret key to use in the symmetric cipher. It must be 32, 48 or 64 bytes long. If AES is the chosen cipher, the variants *AES-128*, *AES-192* and or *AES-256* will be used internally. nonce (bytes/bytearray/memoryview): For deterministic encryption, it is not present. Otherwise, it is a value that must never be reused for encrypting message under this key. There are no restrictions on its length, but it is recommended to use at least 16 bytes. """ try: key = kwargs.pop("key") except KeyError as e: raise TypeError("Missing parameter: " + str(e)) nonce = kwargs.pop("nonce", None) return SivMode(factory, key, nonce, kwargs)