Implementing-cryptography-using-python

164 Chapter 5 ■ Stream Ciphers and Block Ciphers
be combined with the counter using any invertible operation (XOR, addition,
or concatenation) to produce a unique counter block for encryption. If the IV
is not random, such as a packet counter, the IV and counter should be concatenated.
Figure 5.14 illustrates how the counter block is incremented during the
decryption process.
Nonce
c59bcf35...
Counter
00000000
Nonce
c59bcf35...
Counter
00000001
Nonce
c59bcf35...
Counter
00000002
Key
block cipher
encryption
Key
block cipher
encryption
Key
block cipher
encryption
Ciphertext
Ciphertextt
Ciphertext
Plaintext Plaintext Plaintext
Figure 5.14: Counter (CTR) mode decryption
Take caution with how you implement the counter; adding or XORing the
IV and counter into a single value could break the security under a chosenplaintext
attack; this is due to the attacker being able to manipulate the entire
IV–counter pair to cause a collision. Once an attacker controls the IV–counter
pair and plaintext, XOR of the ciphertext with the known plaintext would yield
a value that, when XOR’d with the ciphertext of the other block sharing the same
IV–counter pair, would decrypt that block. An example using the CTR mode
would look similar to the following:
from Crypto.Util import Counter
Counter.new(128, initial_value = int(binascii.hexlify('Not very
random.'), 16))
Tricks with Stream Modes
Notice that both the CTR and OFB modes act like stream ciphers (the “infinite
one-time pad” concept). That is, the block cipher is used to generate a sequence
of pseudorandom blocks that are XOR’d with the message. This ensures that
the message does not impact the random blocks.
These modes have some strengths and some weaknesses:
Strengths:
■■
The random stream can be precomputed on the encryption side
■■
We never have to decrypt a block cipher (so any one-way function can do
the job)