Implementing-cryptography-using-python

Chapter 5 ■ Stream Ciphers and Block Ciphers 147
at the first register. The output of register 1 is the XOR of bit 66 (1) and bit 93
(0), which is 1. Register 2’s output is bit 162 XOR bit 177, which is also 1. Finally,
register 3 is the XOR of bit 288 (1) and bit 243 (0), so also a 1. Thus, the first
output is 1. Now everything must slide, so let’s look at the feedback. The input
to register 2 is the XOR of bit 171 (1) and the XOR of that first register’s output
bit (1) and the product of bits 91 and 92 (0), so 1 + 1 + 0 = 0 is the new input bit
to register 2. All the other bits in register 2 slide one to the right. (Register 2 is
now 0 in bit 94, 1 in bits 95−174, and 0 everywhere else.) Likewise, you can trace
that the new input bit of register 3 is a 1. The new input bit for register 1 is a 0.
Now that we have introduced stream ciphers, you can examine some additional
stream ciphers, namely, ARC4, Vernam, Salsa20, and the ChaCha20 ciphers.
Both the Salsa20 and the ChaCha20 are similar in nature as they were created
by the same author.
ARC4
The RC4 stream cipher was created by Ron Rivest in 1987. RC4 was classified as
a trade secret by RSA Security but was eventually leaked to a message board
in 1994. RC4 was originally trademarked by RSA Security so it is often referred
to as ARCFOUR or ARC4 to avoid trademark issues. ARC4 would later become
commonly used in a number of encryption protocols and standards such as SSL,
TLS, WEP, and WPA. In 2015, it was prohibited for all versions of TLS by RFC
7465. ARC4 has been used in many hardware and software implementations.
One of the main advantages of ARC4 is its speed and simplicity, which you will
notice in the following code:
"""
Implement the ARC4 stream cipher. - Chapter 5
"""
def arc4crypt(data, key):
x = 0
box = range(256)
for i in range(256):
x = (x + box[i] + ord(key[i % len(key)])) % 256
# swap range objects
box = list(box)
box[i], box[x] = box[x], box[i]
x = 0
y = 0
out = []
for char in data:
x = (x + 1) % 256
y = (y + box[x]) % 256