What is a stream cipher?
Q: What is a stream cipher?
A: A stream cipher is a symmetric key cipher where plaintext bits are combined with a pseudorandom cipher bit stream (keystream) using an exclusive-or (xor) operation.
Q: How does it differ from block ciphers?
A: Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware requirements. Block ciphers operate on large blocks of fixed length, while stream ciphers encrypt digits one at a time and the transformation of successive digits varies during the encryption state.
Q: What type of keys does it use?
A: Stream ciphers make use of much smaller and more convenient cryptographic keys, for example 128 bits keys.
Q: How does it generate the keystream?
A: The keystream is generated based on the cryptographic key used, in a similar way to the one-time pad encryption algorithm. However, because the keystream is pseudorandom and not truly random, the security associated with the one-time pad cannot be applied.
Q: Why must you never use the same starting state twice?
A: Using the same starting state twice can lead to serious security problems as it makes it easier for attackers to decrypt data without knowing or having access to your cryptographic key.
Q: Is there any risk associated with using stream ciphers?
A: Yes, if used incorrectly or without taking proper precautions then there is risk associated with using stream ciphers as they can be completely insecure if not handled properly.
