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# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.
import binascii
import math
import struct
from pathlib import Path
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms
from tests.utils import load_nist_vectors
BLOCK_SIZE = 64
MAX_COUNTER = 2**64 - 1
def encrypt(
key: bytes, nonce: bytes, initial_block_counter: int, plaintext: bytes
) -> bytes:
full_nonce = struct.pack("<Q", initial_block_counter) + nonce
encryptor = Cipher(
algorithms.ChaCha20(key, full_nonce), mode=None
).encryptor()
plaintext_len_blocks = math.ceil(len(plaintext) / BLOCK_SIZE)
blocks_until_overflow = MAX_COUNTER - initial_block_counter + 1
if plaintext_len_blocks <= blocks_until_overflow:
return binascii.hexlify(encryptor.update(plaintext))
else:
bytes_until_overflow = min(blocks_until_overflow * 64, len(plaintext))
first_batch = binascii.hexlify(
encryptor.update(plaintext[:bytes_until_overflow])
)
# We manually handle the overflow by resetting the counter to zero once
# we surpass MAX_COUNTER blocks. This way we can check the vectors are
# correct without relying on the same logic that generated them.
full_nonce = struct.pack("<Q", 0) + nonce
encryptor = Cipher(
algorithms.ChaCha20(key, full_nonce), mode=None
).encryptor()
second_batch = binascii.hexlify(
encryptor.update(plaintext[bytes_until_overflow:])
)
return first_batch + second_batch
def verify_vectors(filename: Path):
with open(filename) as f:
vector_file = f.read().splitlines()
vectors = load_nist_vectors(vector_file)
for vector in vectors:
key = binascii.unhexlify(vector["key"])
nonce = binascii.unhexlify(vector["nonce"])
ibc = int(vector["initial_block_counter"])
pt = binascii.unhexlify(vector["plaintext"])
computed_ct = encrypt(key, nonce, ibc, pt)
assert computed_ct == vector["ciphertext"]
overflow_path = Path(
"vectors/cryptography_vectors/ciphers/ChaCha20/counter-overflow.txt"
)
verify_vectors(overflow_path)
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