0x730. Symmetric Encryption

Symmetric ciphers are cryptosystems that use the same key to encrypt and decrypt messages. The encryption and decryption process is generally faster than with asymmetric encryption, but key distribution can be difficult.

These ciphers are generally either block ciphers or stream ciphers. A block cipher operates on blocks of a fixed size, usually 64 or 128 bits. The same block of plaintext will always encrypt to the same ciphertext block, using the same key. DES, Blowfish, and AES (Rijndael) are all block ciphers. Stream ciphers generate a stream of pseudo-random bits, usually either one bit or byte at a time. This is called the keystream, and it is XORed with the plaintext. This is useful for encrypting continuous streams of data. RC4 and LSFR are examples of popular stream ciphers. RC4 will be discussed in depth in "Wireless 802.11b Encryption" on Section 0x770.