Construction of Stream Ciphers from Block Ciphers and their Security

With well-established encryption algorithms like DES or AES at hand, one could have the impression that most of the work for building a cryptosystem -for example a suite of algorithms for the transmission of encrypted data over the internet - is already done. But the task of a cipher is very specific: to encrypt or decrypt a data block of a specified length. Given an plaintext of arbitrary length, the most simple approach would be to break it down to blocks of the desired length and to use padding for the final block. Each block is encrypted separately with the same key, which results in identical ciphertext blocks for identical plaintext blocks. This is known as Electronic Code Book (ECB) mode of operation, and is not recommended in many situations because it does not hide data patterns well. Furthermore, ciphertext blocks are independent from each other, allowing an attacker to substitute, delete or replay blocks unnoticed. The feedback modes in fact turn the block cipher into a stream cipher by using the algorithm as a keystream generator. Since every mode may yield different usage and security properties, it is necessary to analyse them in detail. For the traditional modes like Output Feedback (OFB), Cipher Feedback (CFB), Counter (CTR) and their variants, this has been done thoroughly, but there are also researches on rarely used modes like Plaintext or Key Feedback mode and combination modes like CTR-OFB and CTR-CFB. In cases where buffering is limited or when characters must be processed as they are received (e.g. in terminals) it is useful or even mandatory to use a stream cipher for en-/decryption. Furthermore, many stream ciphers are superior to block ciphers concerning error propagation. Hence building stream ciphers from block ciphers can be useful alternative to other stream ciphers. This paper aims to give an overview on these modes of operation and their security, as their understanding is imperative for any cryptosystem that is build on top of them.