is a symmetric block cipher; a single key is used for encryption and decryption. has a block size of 128 bits, and accepts a key of any length up to 256 bits. is fast on both 32-bit and 8-bit CPUs (smart cards, embedded chips, etc.), and in hardware. is flexible; it can be used in network applications where keys are changed frequently and in applications where there is little or no RAM and ROM available.