# SPDX-FileCopyrightText: 2014 Legrandin # SPDX-License-Identifier: BSD-2-Clause """ Counter with CBC-MAC (CCM) mode. """ from __future__ import annotations from hmac import compare_digest from typing import TYPE_CHECKING, Any, Optional, Union, overload __all__ = ["CcmMode"] import struct from binascii import unhexlify from Crypto import MessageTooLongError from Crypto.Cipher._state_machine import Method from Crypto.Random import get_random_bytes from Crypto.Util._raw_api import is_writeable_buffer from Crypto.Util.number import long_to_bytes from Crypto.Util.strxor import strxor if TYPE_CHECKING: from types import ModuleType Buffer = Union[bytes, bytearray, memoryview] def enum(**enums): return type("Enum", (), enums) MacStatus = enum(NOT_STARTED=0, PROCESSING_AUTH_DATA=1, PROCESSING_PLAINTEXT=2) # Before Crypto.MessageTooLongError (kept for backward compatibility) CCMMessageTooLongError = MessageTooLongError class CcmMode: """Counter with CBC-MAC (CCM). 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. This mode requires a nonce. The nonce shall never repeat for two different messages encrypted with the same key, but it does not need to be random. Note that there is a trade-off between the size of the nonce and the maximum size of a single message you can encrypt. It is important to use a large nonce if the key is reused across several messages and the nonce is chosen randomly. It is acceptable to us a short nonce if the key is only used a few times or if the nonce is taken from a counter. The following table shows the trade-off when the nonce is chosen at random. The column on the left shows how many messages it takes for the keystream to repeat **on average**. In practice, you will want to stop using the key way before that. +--------------------+---------------+-------------------+ | Avg. # of messages | nonce | Max. message | | before keystream | size | size | | repeats | (bytes) | (bytes) | +====================+===============+===================+ | 2^52 | 13 | 64K | +--------------------+---------------+-------------------+ | 2^48 | 12 | 16M | +--------------------+---------------+-------------------+ | 2^44 | 11 | 4G | +--------------------+---------------+-------------------+ | 2^40 | 10 | 1T | +--------------------+---------------+-------------------+ | 2^36 | 9 | 64P | +--------------------+---------------+-------------------+ | 2^32 | 8 | 16E | +--------------------+---------------+-------------------+ This mode is only available for ciphers that operate on 128 bits blocks (e.g. AES but not TDES). See `NIST SP800-38C`_ or RFC3610_. .. _`NIST SP800-38C`: http://csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf .. _RFC3610: https://tools.ietf.org/html/rfc3610 .. _AEAD: http://blog.cryptographyengineering.com/2012/05/how-to-choose-authenticated-encryption.html """ def __init__( self, factory: ModuleType, key: Buffer, nonce: Buffer, mac_len: int, msg_len: Optional[int], assoc_len: Optional[int], cipher_params: dict, ) -> None: self.block_size = factory.block_size """The block size of the underlying cipher, in bytes.""" self.nonce = bytes(nonce) """The nonce originally used to create the object.""" self._factory = factory self._key = bytes(key) self._mac_len = mac_len self._msg_len = msg_len self._assoc_len = assoc_len self._cipher_params = cipher_params self._mac_tag: Optional[bytes] = None # Cache for MAC tag if self.block_size != 16: raise ValueError("CCM mode is only available for ciphers that operate on 128 bits blocks") # MAC tag length (Tlen) if mac_len not in (4, 6, 8, 10, 12, 14, 16): raise ValueError("Parameter 'mac_len' must be even and in the range 4..16 (not %d)" % mac_len) # Nonce value if not (7 <= len(nonce) <= 13): raise ValueError("Length of parameter 'nonce' must be in the range 7..13 bytes") # Message length (if known already) q = 15 - len(nonce) # length of Q, the encoded message length if msg_len and len(long_to_bytes(msg_len)) > q: raise MessageTooLongError("Message too long for a %u-byte nonce" % len(nonce)) # Create MAC object (the tag will be the last block # bytes worth of ciphertext) self._mac = self._factory.new(key, factory.MODE_CBC, iv=b"\x00" * 16, **cipher_params) self._mac_status = MacStatus.NOT_STARTED self._t = None # Allowed transitions after initialization self._next = [Method.UPDATE, Method.ENCRYPT, Method.DECRYPT, Method.DIGEST, Method.VERIFY] # Cumulative lengths self._cumul_assoc_len = 0 self._cumul_msg_len = 0 # Cache for unaligned associated data/plaintext. # This is a list with byte strings, but when the MAC starts, # it will become a binary string no longer than the block size. self._cache: Any = [] # Start CTR cipher, by formatting the counter (A.3) self._cipher = self._factory.new( key, self._factory.MODE_CTR, nonce=struct.pack("B", q - 1) + self.nonce, **cipher_params ) # S_0, step 6 in 6.1 for j=0 self._s_0 = self._cipher.encrypt(b"\x00" * 16) # Try to start the MAC if None not in (assoc_len, msg_len): self._start_mac() def _start_mac(self): assert self._mac_status == MacStatus.NOT_STARTED assert None not in (self._assoc_len, self._msg_len) assert isinstance(self._cache, list) # Formatting control information and nonce (A.2.1) q = 15 - len(self.nonce) # length of Q, the encoded message length (2..8) flags = (self._assoc_len > 0) << 6 flags |= ((self._mac_len - 2) // 2) << 3 flags |= q - 1 b_0 = struct.pack("B", flags) + self.nonce + long_to_bytes(self._msg_len, q) # Formatting associated data (A.2.2) # Encoded 'a' is concatenated with the associated data 'A' assoc_len_encoded = b"" if self._assoc_len > 0: if self._assoc_len < (2**16 - 2**8): enc_size = 2 elif self._assoc_len < (2**32): assoc_len_encoded = b"\xff\xfe" enc_size = 4 else: assoc_len_encoded = b"\xff\xff" enc_size = 8 assoc_len_encoded += long_to_bytes(self._assoc_len, enc_size) # b_0 and assoc_len_encoded must be processed first self._cache.insert(0, b_0) self._cache.insert(1, assoc_len_encoded) # Process all the data cached so far first_data_to_mac = b"".join(self._cache) self._cache = b"" self._mac_status = MacStatus.PROCESSING_AUTH_DATA self._update(first_data_to_mac) def _pad_cache_and_update(self): assert self._mac_status != MacStatus.NOT_STARTED assert len(self._cache) < self.block_size # Associated data is concatenated with the least number # of zero bytes (possibly none) to reach alignment to # the 16 byte boundary (A.2.3) len_cache = len(self._cache) if len_cache > 0: self._update(b"\x00" * (self.block_size - len_cache)) def update(self, assoc_data: Buffer) -> CcmMode: """Protect associated data If there is any associated data, the caller has to invoke this function one or more times, before using ``decrypt`` or ``encrypt``. By *associated data* it is meant any data (e.g. packet headers) that will not be encrypted and will be transmitted in the clear. However, the receiver is still able to detect any modification to it. In CCM, the *associated data* is also called *additional authenticated data* (AAD). If there is no associated data, this method must not be called. The caller may split associated data in segments of any size, and invoke this method multiple times, each time with the next segment. Args: assoc_data (bytes/bytearray/memoryview): A piece of associated data. There are no restrictions on its size. """ 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._cumul_assoc_len += len(assoc_data) if self._assoc_len is not None and self._cumul_assoc_len > self._assoc_len: raise MessageTooLongError("Associated data longer than declared") self._update(assoc_data) return self def _update(self, assoc_data_pt=b""): """Update the MAC with associated data or plaintext (without FSM checks)""" # If MAC has not started yet, we just park the data into a list. # If the data is mutable, we create a copy and store that instead. if self._mac_status == MacStatus.NOT_STARTED: if is_writeable_buffer(assoc_data_pt): assoc_data_pt = bytes(assoc_data_pt) self._cache.append(assoc_data_pt) return assert len(self._cache) < self.block_size if len(self._cache) > 0: filler = min(self.block_size - len(self._cache), len(assoc_data_pt)) self._cache += bytes(assoc_data_pt[:filler]) assoc_data_pt = bytes(assoc_data_pt[filler:]) if len(self._cache) < self.block_size: return # The cache is exactly one block self._t = self._mac.encrypt(self._cache) self._cache = b"" update_len = len(assoc_data_pt) // self.block_size * self.block_size self._cache = bytes(assoc_data_pt[update_len:]) if update_len > 0: self._t = self._mac.encrypt(assoc_data_pt[:update_len])[-16:] @overload def encrypt(self, plaintext: Buffer) -> bytes: ... @overload def encrypt(self, plaintext: Buffer, output: Union[bytearray, memoryview]) -> None: ... @overload def encrypt( self, plaintext: Buffer, output: Optional[Union[bytearray, memoryview]] = None ) -> Optional[bytes]: ... def encrypt( self, plaintext: Buffer, output: Optional[Union[bytearray, memoryview]] = None ) -> Optional[bytes]: """Encrypt data with the key set at initialization. A cipher object is stateful: once you have encrypted a message you cannot encrypt (or decrypt) another message using the same object. This method can be called only **once** if ``msg_len`` was not passed at initialization. If ``msg_len`` was given, the data to encrypt can be broken up in two or more pieces and `encrypt` can be called multiple times. That is, the statement: >>> c.encrypt(a) + c.encrypt(b) is equivalent to: >>> c.encrypt(a+b) This function does not add any padding to the plaintext. Args: plaintext (bytes/bytearray/memoryview): The piece of data to encrypt. It can be of any length. Keyword Args: output (bytearray/memoryview): The location where the ciphertext must be written to. If ``None``, the ciphertext is returned. Returns: If ``output`` is ``None``, the ciphertext as ``bytes``. Otherwise, ``None``. """ if Method.ENCRYPT not in self._next: raise TypeError("encrypt() can only be called after initialization or an update()") self._next = [Method.ENCRYPT, Method.DIGEST] # No more associated data allowed from now if self._assoc_len is None: assert isinstance(self._cache, list) self._assoc_len = sum([len(x) for x in self._cache]) if self._msg_len is not None: self._start_mac() else: if self._cumul_assoc_len < self._assoc_len: raise ValueError("Associated data is too short") # Only once piece of plaintext accepted if message length was # not declared in advance if self._msg_len is None: q = 15 - len(self.nonce) if len(long_to_bytes(len(plaintext))) > q: raise MessageTooLongError("Message too long for a %u-byte nonce" % len(self.nonce)) self._msg_len = len(plaintext) self._start_mac() self._next = [Method.DIGEST] self._cumul_msg_len += len(plaintext) if self._cumul_msg_len > self._msg_len: msg = "Message longer than declared for (%u bytes vs %u bytes" % ( self._cumul_msg_len, self._msg_len, ) raise MessageTooLongError(msg) if self._mac_status == MacStatus.PROCESSING_AUTH_DATA: # Associated data is concatenated with the least number # of zero bytes (possibly none) to reach alignment to # the 16 byte boundary (A.2.3) self._pad_cache_and_update() self._mac_status = MacStatus.PROCESSING_PLAINTEXT self._update(plaintext) return self._cipher.encrypt(plaintext, output=output) @overload def decrypt(self, ciphertext: Buffer) -> bytes: ... @overload def decrypt(self, ciphertext: Buffer, output: Union[bytearray, memoryview]) -> None: ... @overload def decrypt( self, ciphertext: Buffer, output: Optional[Union[bytearray, memoryview]] = None ) -> Optional[bytes]: ... def decrypt( self, ciphertext: Buffer, output: Optional[Union[bytearray, memoryview]] = None ) -> Optional[bytes]: """Decrypt data with the key set at initialization. A cipher object is stateful: once you have decrypted a message you cannot decrypt (or encrypt) another message with the same object. This method can be called only **once** if ``msg_len`` was not passed at initialization. If ``msg_len`` was given, the data to decrypt can be broken up in two or more pieces and `decrypt` can be called multiple times. That is, the statement: >>> c.decrypt(a) + c.decrypt(b) is equivalent to: >>> c.decrypt(a+b) 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. Keyword Args: output (bytearray/memoryview): The location where the plaintext must be written to. If ``None``, the plaintext is returned. Returns: If ``output`` is ``None``, the plaintext as ``bytes``. Otherwise, ``None``. """ if Method.DECRYPT not in self._next: raise TypeError("decrypt() can only be called after initialization or an update()") self._next = [Method.DECRYPT, Method.VERIFY] # No more associated data allowed from now if self._assoc_len is None: assert isinstance(self._cache, list) self._assoc_len = sum([len(x) for x in self._cache]) if self._msg_len is not None: self._start_mac() else: if self._cumul_assoc_len < self._assoc_len: raise ValueError("Associated data is too short") # Only once piece of ciphertext accepted if message length was # not declared in advance if self._msg_len is None: q = 15 - len(self.nonce) if len(long_to_bytes(len(ciphertext))) > q: raise MessageTooLongError("Message too long for a %u-byte nonce" % len(self.nonce)) self._msg_len = len(ciphertext) self._start_mac() self._next = [Method.VERIFY] self._cumul_msg_len += len(ciphertext) if self._cumul_msg_len > self._msg_len: msg = "Message longer than declared for (%u bytes vs %u bytes" % ( self._cumul_msg_len, self._msg_len, ) raise MessageTooLongError(msg) if self._mac_status == MacStatus.PROCESSING_AUTH_DATA: # Associated data is concatenated with the least number # of zero bytes (possibly none) to reach alignment to # the 16 byte boundary (A.2.3) self._pad_cache_and_update() self._mac_status = MacStatus.PROCESSING_PLAINTEXT # Encrypt is equivalent to decrypt with the CTR mode plaintext = self._cipher.encrypt(ciphertext, output=output) if output is None: self._update(plaintext) else: self._update(output) return plaintext 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] return self._digest() def _digest(self): if self._mac_tag: return self._mac_tag if self._assoc_len is None: assert isinstance(self._cache, list) self._assoc_len = sum([len(x) for x in self._cache]) if self._msg_len is not None: self._start_mac() else: if self._cumul_assoc_len < self._assoc_len: raise ValueError("Associated data is too short") if self._msg_len is None: self._msg_len = 0 self._start_mac() if self._cumul_msg_len != self._msg_len: raise ValueError("Message is too short") # Both associated data and payload are concatenated with the least # number of zero bytes (possibly none) that align it to the # 16 byte boundary (A.2.2 and A.2.3) self._pad_cache_and_update() # Step 8 in 6.1 (T xor MSB_Tlen(S_0)) self._mac_tag = strxor(self._t, self._s_0)[: self._mac_len] 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 not compare_digest(self._digest(), 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. """ return self.encrypt(plaintext, output=output), self.digest() @overload def decrypt_and_verify(self, ciphertext: Buffer, received_mac_tag: Buffer) -> bytes: ... @overload def decrypt_and_verify( self, ciphertext: Buffer, received_mac_tag: Buffer, output: Union[bytearray, memoryview] ) -> None: ... def decrypt_and_verify( self, ciphertext: Buffer, received_mac_tag: Buffer, output: Optional[Union[bytearray, memoryview]] = None, ) -> Optional[bytes]: """Perform decrypt() and verify() in one step. Args: ciphertext (bytes/bytearray/memoryview): The piece of data to decrypt. received_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. """ plaintext = self.decrypt(ciphertext, output=output) self.verify(received_mac_tag) return plaintext def _create_ccm_cipher(factory, **kwargs): """Create a new block cipher, configured in CCM mode. Args: factory (module): 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. nonce (bytes/bytearray/memoryview): A value that must never be reused for any other encryption. Its length must be in the range ``[7..13]``. 11 or 12 bytes are reasonable values in general. Bear in mind that with CCM there is a trade-off between nonce length and maximum message size. If not specified, a 11 byte long random string is used. mac_len (integer): Length of the MAC, in bytes. It must be even and in the range ``[4..16]``. The default is 16. msg_len (integer): Length of the message to (de)cipher. If not specified, ``encrypt`` or ``decrypt`` may only be called once. assoc_len (integer): Length of the associated data. If not specified, all data is internally buffered. """ try: key = key = kwargs.pop("key") except KeyError as e: raise TypeError("Missing parameter: " + str(e)) nonce = kwargs.pop("nonce", None) # N if nonce is None: nonce = get_random_bytes(11) mac_len = kwargs.pop("mac_len", factory.block_size) msg_len = kwargs.pop("msg_len", None) # p assoc_len = kwargs.pop("assoc_len", None) # a cipher_params = dict(kwargs) return CcmMode(factory, key, nonce, mac_len, msg_len, assoc_len, cipher_params)