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A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. A block cipher encrypts one block at a time. The block may be of size one byte or more or less. That means we can also encrypt a block of one byte by help of a stream cipher as a stream. So what is the exact difference between a stream cipher and a block cipher? |
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A block cipher is a deterministic and computable function of $k$-bit keys and $n$-bit (plaintext) blocks to $n$-bit (ciphertext) blocks. (More generally, the blocks don't have to be bit-sized, $n$-character-blocks would fit here, too). This means, when you encrypt the same plaintext block with the same key, you'll get the same result. (We normally also want that the function is invertible, i.e. that given the key and the ciphertext block we can compute the plaintext.) To actually encrypt or decrypt a message (of any size), you don't use the block cipher directly, but put it into a mode of operation. The simplest such mode would be electronic code book mode (ECB), which simply cuts the message in blocks, applies the cipher to each block and outputs the resulting blocks. (This is generally not a secure mode, though.) Some early encryption schemes like the one used by Caesar could be categorized as a "block cipher with 1-character blocks in ECB-mode". Or generally, everything that has a code book. We usually use other modes of operation, which include an initialization vector and some kind of feedback, so that every block of every message is encrypted a different way. A stream cipher is a function which directly maps $k$-bit keys and arbitrary length plaintexts to (same arbitrary length) ciphertext, in such a way that prefixes of the plaintext map to prefixes of the ciphertext, i.e. we can compute the starting part of the ciphertext before the trailing part of the plaintext is known. (Often the message sizes might be limited to multiples of some "block size", too, but usually with smaller blocks like whole bytes or such.) If a part of the plaintext repeats, the corresponding ciphertext usually is not the same – different parts of the message will be encrypted in different ways. Often such stream ciphers work by producing a keystream from the actual key (and maybe an initialization vector) and then simply XOR-ing it with the message – these are called synchronous stream ciphers. Other stream ciphers might vary the encryption of future parts of the message depending on previous parts. Some block cipher modes of operation actually create a synchronous stream cipher, like CTR and OFB mode. You should never reuse a key (and IV, if applicable) of a synchronous stream cipher (which includes block ciphers in streaming modes) for different messages, since this can lead to compromises. (And even for the same message it will show that you repeated a message.) Note that in actual usage you will also want a MAC, e.g. integrity protection, for your message. (Some schemes are broken in case of a chosen-ciphertext attack, for example, and such a MAC will prevent this.) |
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