Implementing-cryptography-using-python

Chapter 5 ■ Stream Ciphers and Block Ciphers 145
hexnonce = binascii.hexlify(nonce)
oursecret = 54321
concatenated_hex = hexnonce + format(oursecret, 'x')
even_length = concatenated_hex.rjust(len(concatenated_hex) +
len(concatenated_hex) % 2, '0')
hexhash = hashlib.sha256(binascii.unhexlify(even_length)).hexdigest()
newseed = (int(hexhash, 16)) % 2**32
print(newseed)
To get a consistent key on both the sending and receiving side, we directly
pass the nonce. To test this, replace the previous nonce with “cc4304c09aee” and
keep the original seed of 54321. The new seed that generates should equate to
“3336748862.” Now you need a system for sending the nonce in your ciphertext.
If you have the first six bytes as nonce bytes and the remaining bytes as the
ciphertext, you can pass everything in the same message. Since the underlying
generator is C’s rand function, the encryption is still not strong enough to protect
any secrets, but it is much stronger than it was.
Knowing the first six bytes are nonce bytes, here is a new challenge for you:
Secret Key: 61983
Message: 3e08816f1377f89f1c596fc197dd52946c92577bfd7c25c3
If you get stuck, you can find the solution in the ch5_decrypt2 file on this
book’s website.
Answer: Seed is 42847799; the message is 'this is a message.'
Ideally, by now, you are gaining an understanding of how stream ciphers
work. Stream ciphers are generally not as secure or well-understood as block
ciphers (which we study next). In software, you will most likely deal with
block ciphers, though there are tools such as Wireguard which will use stream
ciphers. Wireguard is a software VPN protocol that uses the ChaCha20 stream
cipher; you will learn more about ChaCha20 later in this chapter. So, while some
tools may use stream ciphers, stream ciphers play a more significant role in the
hardware ecosystem. With space-constrained devices that need encrypted data
streams, we need fast hardware implementations that encrypt bit-by-bit. So,
as you code these, imagine the hardware version. As outlined in the previous
section, the encryption schemes are still not strong enough to protect any classified
data. In this part, you examine a full-strength encryption scheme called
Trivium. Bart Preneel and Christophe De Cannière created it and submitted
it to the eSTREAM competition. eSTREAM is a project that was organized by
the EU Ecrypt network to help identify new stream ciphers that may be suitable
for widespread adoption. The project began in November 2004 and completed
in April 2008. It is designed to provide a reasonably efficient software
encryption implementation and is specified as an International Standard under
ISO/IEC 29192-3.