Block ciphers are more popular than stream ciphers, with several either in active use or recent enough to require supporting for legacy interoperability. I'll talk about the RC2 and DES algorithms today and cover the newer AES algorithm tomorrow.
DES was a widely-used and analyzed cipher algorithm developed for use by the US government. DES is a 64-bit block cipher with a 64-bit key. However, 8 bits of the key are parity bits that are discarded by the algorithm so the effective key strength is 56 bits. DES works by expanding the contents of an input block by duplicating some of the bits, mixing the expanded input block with key bits derived from the encryption key, applying a non-linear transformation to the mixed bits, and finally permuting the output bits of the transformation. This process is called a round and the overall algorithm uses 16 rounds on each block.
Although there's no trivial way of breaking DES, the small key space means that brute forcing a message doesn't require that much computer power. A variant called triple-DES consists of the same algorithm applied with three different keys in succession. Due to some cryptoanalytic properties, triple-DES only doubles the number of effective key strength bits to 112. However, this is still a considerable leap over normal DES. All variants of DES are now deprecated in favor of using AES.
RC2 is another algorithm by Ron Rivest (there's an entire product line of these things) that intends to be very similar in application to DES. RC2 also is a 64-bit block cipher but has a variable key size of up to 128 bits. The variable key size allowed RC2 to be used in a degraded mode that met the US encryption restrictions at the time. RC2 is faster than triple-DES and includes a key salt to prevent precomputing large tables of encryption keys. RC2 has a similar round system to DES but uses two different types of rounds called mix and mash. The details of these rounds were published as RFC 2268. Both RC2 and RC4 were secret algorithms until their source code was anonymously leaked in the mid-1990s.
Next time: Advanced Encryption Standard