Implementing-cryptography-using-python

152 Chapter 5 ■ Stream Ciphers and Block Ciphers
same operation on multiple data points simultaneously. Most modern-day CPU
designs include SIMD instruction to improve the performance of multimedia.
The second difference is that ChaCha builds the initial matrix with all attackercontrolled
input words at the bottom:
Constant Constant Constant Constant
Key Key Key Key
Key Key Key Key
Input Input Input Input
These constants are the same as in Salsa20. The first round of ChaCha adds
keys into the constants. The key words are copied in order; the input words are
the block counter followed by the nonce.
The third difference is that ChaCha processes through rows in the same order
in every round. The first round modifies first, fourth, third, second, first, fourth,
third, second along columns, and the second round modifies first, fourth, third,
second, first, fourth, third, second along southeast diagonals:
quarter_round (x0,x4,x8,x12)
quarter_round (x1,x5,x9,x13)
quarter_round (x2,x6,x10,x14)
quarter_round (x3,x7,x11,x15)
quarter_round (x0,x5,x10,x15)
quarter_round (x1,x6,x11,x12)
quarter_round (x2,x7,x8,x13)
quarter_round (x3,x4,x9,x14)
The four quarter-round words are always in top-to-bottom order in the matrix.
A couple libraries are available that will offer ChaCha as an encryption scheme.
You may also elect to use a pure Python implementation. To implement ChaCha
using Python, use the following recipe:
"""
Implement the ChaCha20 stream cipher.
"""
import struct
import sys, os, binascii
from base64 import b64encode
def yield_chacha20_xor_stream(key, iv, position=0):
# Generate the xor stream with the ChaCha20 cipher."""
if not isinstance(position, int):
raise TypeError
if position & ~0xffffffff:
raise ValueError('Position is not uint32.')
if not isinstance(key, bytes):