Stream Ciphers

Question 1

How does a Stream Cipher differ from a Block Cipher?

Answer 1

A block cipher encrypts and decrypts in fixed size chunks, while a stream cipher does this one element at a time (e.g. bytes)

Question 2

What do we call the random byte generator used in stream ciphers, and how are these used?

Answer 2

They are called keystreams, and we XOR them with the plaintext bytes. To decrypt we require the same keystream sequence.

Question 3

What are some of the considerations for stream ciphers?

Answer 3

Long period before repetition. Keystream should appear random, keys should be sufficiently long (e.g. 128 bits)

Question 4

Compare the advantages and disadvantages of stream and block ciphers.

Answer 4

Stream ciphers use less code and are faster. However, block ciphers allow for reuse of keys.

Question 5

What is RC4?

Answer 5

A variable sized stream cipher, with a period of over 10^100. Only requires 8 to 16 operations per byte, so very fast.
First few bytes of keystream are non-random(depend on how keys are generated)

Question 6

When is RC4 less reliable?

Answer 6

When used with WEP wireless protocol.