Implementing-cryptography-using-python

Chapter 5 ■ Stream Ciphers and Block Ciphers 151
Once you have the library installed, you can use the XSalsa20_keystream
to generate a keystream of the desired length, or you can pass any message
(plaintext or ciphertext) to have it XOR'd with the keystream. All values must
be binary strings that include str for Python 2 or the byte for Python 3. Here,
you will see a Python implementation of the salsa20 library:
from salsa20 import XSalsa20_xor
from os import urandom
IV = urandom(24)
KEY = b'*secret**secret**secret**secret*'
ciphertext = XSalsa20_xor(b"IT'S A YELLOW SUBMARINE", IV, KEY)
print(XSalsa20_xor(ciphertext, IV, KEY).decode())
IT'S A YELLOW SUBMARINE
One of the reasons why you should be familiar with Salsa20 is that it is consistently
faster than AES. It is recommended to use Salsa20 for encryption in
typical cryptographic applications.
ChaCha Cipher
ChaCha is a modification of Salsa20 published by Bernstein in 2008. It uses a
new round function that increases diffusion and increases performance on
some architectures. The ChaCha cipher is a stream cipher that uses a 256-bit
key and a 64-bit nonce/IV. Currently the AES block cipher has a dominant role
in secret key encryption; in addition, many systems have started to implement
hardware acceleration for AES, thus adding to its growing adoption. With AES
owning the market share for secret key encryption, it would cause major issues
if AES becomes cryptographically insecure. AES has been shown to be weak
around cache-collision attacks; therefore, Google has proposed ChaCha20 as
an alternative and actively uses it within TLS connections. Currently ChaCha is
three times faster than software-enabled AES and, like Salsa20, is not vulnerable
to timing attacks. ChaCha operates by creating a keystream that is then XOR’d
with the plaintext and has been standardized with RFC 7539.
Some of the key differences between ChaCha and Salsa include improved diffusion
per round, which offers an enhanced resistance to cryptanalysis. ChaCha,
like Salsa20, builds a 4×4 matrix and adds the results to the original matrix to
obtain a 16-word or 64-byte output block. Three additional differences should
be noted. First, the ChaCha series permutes the order of words in the output
block to match the permutation; this is designed to save time on SIMD (single
instruction, multiple data) platforms but does not make any difference on speed
in other platforms. SIMD is a class of parallel computers that can perform the