Pages on Security

   Stream ciphers are symmetric-key ciphers (i.e., encryption and decryption use the same key) that perform bit-by-bit encryption. In a stream cipher, any bit of the ciphertext depends on a single bit of the plaintext (and the encryption key). Stream ciphers differ from the other big class of symmetric ciphers - the  block ciphers,  which encrypt block-by-block. In a block cipher, the bits in a ciphertext block depend on (ideally) all bits in the corresponding plaintext block. To the extreme, a stream cipher is a particular type of block cipher with the block's length equal to 1. Compared to block ciphers, stream ciphers are usually faster, but the test of time shows them less secure in general. A stream cipher can be seen as the analog of the One Time Pad (OTP) (see Perfect Secrecy and the One Time Pad (OTP) ) in computational security. Recall OTP, for which encryption is simply a bitwise XOR between the plaintext and the encryption key. The same holds for the str...

I have explained unconditional  (or  information-theoretical)  security in my previous post (see Unconditional vs. Conditional Security ). As I have mentioned there, we can refer to unconditional security in the context of various cryptographic primitives, among which the  encryption schemes (see Symmetric vs. Asymmetric Encryption ). An encryption scheme that is information-theoretically secure provides  perfect secrecy (see, e.g., [1]) ,  because the  ciphertext  perfectly  hides  the  plaintext.  In other words, the adversary has the same probability to correctly indicate the message  m  regardless if he/she knows the corresponding ciphertext  c.  Hence, the knowledge of the ciphertext gives no new information about the plaintext. We ignore the length of the message - which, of course, is exposed - and assume that all the possible messages are equally long. More rigorously, the  a-posteriori ...