In cryptography, a block cipher is a symmetric key cipher which operates on a groups of bits of fixed length, called blocks, using an exact transformation. During encryption, a block cipher algorithm might take (for example) a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext. The exact transformation is controlled using a second input — the secret key. Decryption is similar: the decryption algorithm takes, in this example, a 128-bit block of ciphertext together with the secret key, and output the original 128-bit block of plaintext.
One of the early block cipher designs was the Data Encryption Standard (DES), developed at IBM and published as a standard in 1977. A successor to DES, the Advanced Encryption Standard (AES); also using block cipher, was adopted in 2001. RC5 is a simple symmetric-key made by Ronald Rivest in 1994. RC6 is another made from RC5. RC stands for "Rivest Cipher", or alternatively, "Ron's Code".