Block Cipher Modes Flashcards
How does electronic cookbook mode work?
ECB splits the plaintext into blocks and runs the block cipher on each block.
Why is ECB not considered secure?
ECB gives information about the plaintext away. As the same message with the same key will always produce the same ciphertext.
How does cipher block chaining mode work?
CBC adds a random initialisation vector (IV) that during encryption the first block of the plaintext is XOR’d with before running the block encryption. The next block is XOR’d with this ciphertext before running the block encryption and so on.
What is the problem with CBC mode?
Encryption of each block requires the result of the previous block, therefore it can not be executed in parallel.
How does counter mode work?
In counter mode a random 32 bit or 64 bit nonce is generated. For each block, the nonce is combined with a counter (if the nonce is 32 bit it is appended, if the nonce is 64 bit it is added), which is the index of the block (starting at 0). This is then encrypted, the output is then XOR’d with the plaintext to get the ciphertext.
What is the problem with CTR?
It causes the block cipher to lose its property of diffusion. As a single change in the plaintext will only cause a single bit to change in the ciphertext.
