Implementing-cryptography-using-python

162 Chapter 5 ■ Stream Ciphers and Block Ciphers
CFB offers two advantages over CBC mode with the stream cipher modes
OFB and CTR: first, the block cipher is only used in the encryption process,
and second, the message does not need to be padded to a multiple of the cipher
block size.
OFB Mode
The OFB mode, formally named Output Feedback, makes a block cipher into
a synchronous stream cipher. OFB mode generates keystream blocks that are
XOR’d with plaintext blocks to get the ciphertext. See Figure 5.11 for a visual
representation. As with other stream ciphers, modifying a bit in the ciphertext
will result in a flipped bit in the produced plaintext at the same location. This
characteristic allows many error-correcting codes to function normally. The
encryption and decryption methods are the same.
Initialization Vector (IV)
Key
block cipher
encryption
Key
block cipher
encryption
Key
block cipher
encryption
Plaintext
Plaintext
Plaintext
Ciphertext Ciphertext Ciphertext
Figure 5.11: Output Feedback (OFB) mode encryption
As with the CBC mode, the OFB cipher operation depends on the previous
blocks and therefore cannot be performed in parallel. One notable difference
is that because the ciphertext and plaintext are only used for the final XOR,
the block cipher operations may be performed in advance. This property will
allow the final step to be performed in parallel once the cipher or plaintext is
available. See Figure 5.12 for a review of OFB mode decryption.
One thing to note is that you can obtain an OFB mode keystream by using
CBC mode with a constant string of zeros as input. This property can be useful
as it allows the usage of fast hardware implementations of CBC mode for OFB
mode encryption.