Block Cipher Modes

A block cipher mode, or mode, for short, is an algorithm that features the use of a symmetric key block cipher algorithm to provide an information service, such as confidentiality or authentication.

With the advent of new block ciphers, such as the Advanced Encryption Standard (AES), there is a need to update long-standing modes of operation and an opportunity to consider the development of new modes.

Currently, NIST has approved eight modes of the approved block ciphers in a series of special publications. As summarized on the Current Modes page, there are five confidentiality modes (ECB, CBC, OFB, CFB, and CTR), one authentication mode (CMAC), and two combined modes for confidentiality and authentication (CCM and GCM).

NIST initiated a 90-day period of public comment, ending September 3, 2008, on the proposal to approve an additional confidentiality mode, XTS-AES, by reference to IEEE Std. 1619-2007. See the Request for Public Comment on XTS for additional information, including a link to the relevant excerpt of the standard which is available free during the public comment period.

Several other modes have been submitted to NIST for consideration; the submission documents are available at the Modes Development page. NIST welcomes public input on whether to approve any of these modes. Comments may be submitted to EncryptionModes@nist.gov. The comments that NIST has received are available at the Public Comment page.