The mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext.
Correct Answer: C
Output feedback (OFB) https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Output_feedback_(OFB) The output feedback (OFB) mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext. Just as with other stream ciphers, flipping a bit in the ciphertext produces a flipped bit in the plaintext at the same location. This property allows many error-correcting codes to function normally even when applied before encryption. Incorrect answers: Cipher feedback (CFB) - mode, a close relative of CBC, makes a block cipher into a self-synchronizing stream cipher. Electronic codebook (ECB) - the simplest of the encryption modes (named after conventional physical codebooks). The message is divided into blocks, and each block is encrypted separately. Cipher-block chaining (CBC) - Ehrsam, Meyer, Smith and Tuchman invented the cipher block chaining (CBC) mode of operation in 1976. In CBC mode, each block of plaintext is XORed with the previous ciphertext block before being encrypted. This way, each ciphertext block depends on all plaintext blocks processed up to that point. To make each message unique, an initialization vector must be used in the first block.
Question 67
Which of the following statements is most true regarding binary operations and encryption?
Correct Answer: C
They can form a part of viable encryption methods for example - https://en.wikipedia.org/wiki/XOR_cipher The XOR operator is extremely common as a component in more complex ciphers. By itself, using a constant repeating key, a simple XOR cipher can trivially be broken using frequency analysis. If the content of any message can be guessed or otherwise known then the key can be revealed. Its primary merit is that it is simple to implement, and that the XOR operation is computationally inexpensive. A simple repeating XOR (i.e. using the same key for xor operation on the whole data) cipher is therefore sometimes used for hiding information in cases where no particular security is required. The XOR cipher is often used in computer malware to make reverse engineering more difficult.
