TECH SBH

Block Cipher The basic scheme of a block cipher is depicted as follows − A block cipher takes a block of plaintext bits and generates a block of ciphertext bits, generally of same size. The size of block is fixed in the given scheme. The choice of block size does not directly affect to the strength of encryption scheme. The strength of cipher depends up on the key length. Block Size Though any size of block is acceptable, following aspects are borne in mind while selecting a size of a block. Avoid very small block size  − Say a block size is m bits. Then the possible plaintext bits combinations are then 2 m . If the attacker discovers the plain text blocks corresponding to some previously sent ciphertext blocks, then the attacker can launch a type of ‘dictionary attack’ by building up a dictionary of plaintext/ciphertext pairs sent using that encryption key. A larger block size makes attack harder as the dictionary needs to be larger. Do not have very large block siz