Advanced Encryption Standard
The Advanced Encryption Standard (AES) specifies a FIPS-approved cryptographic algorithm that can be used to protect electronic data. The AES algorithm is a symmetric block cipher that can encrypt (encipher) and decrypt (decipher) information. Encryption converts data to an unintelligible form called ciphertext; decrypting the ciphertext converts the data back into its original form, called plaintext.
The AES algorithm is capable of using cryptographic keys of 128, 192, and 256 bits to encrypt and decrypt data in blocks of 128 bits. The key size of 256 bits is approved for Top-Secret communications by the NSA.
Additional information regarding the use of AES can be found in CNSS Policy No. 15, Fact Sheet No. 1 National Policy on the Use of the Advanced Encryption Standard (AES) to Protect National Security Systems and National Security Information, June 2003.
Complete information on the AES development effort is available here here.