This International Standard specifies six methods for authenticated encryption, i.e. defined ways of processing a data string with the following security objectives: data confidentiality, i.e. protection against unauthorized disclosure of data, data integrity, i.e. protection that enables the recipient of data to verify that it has not been modified, data origin authentication, i.e. protection that enables the recipient of data to verify the identity of the data originator. All six methods specified in this International Standard are based on a block cipher algorithm, and require the originator and the recipient of the protected data to share a secret key for this block cipher. Key management is outside the scope of this standard; key management techniques are defined in ISO/IEC 11770. Four of the mechanisms in this standard, namely mechanisms 1, 3, 4 and 6, allow data to be authenticated which is not encrypted. That is, these mechanisms allow a data string that is to be protected to be divided into two parts, D, the data string that is to be encrypted and integrity-protected, and A (the additional authenticated data) that is integrity-protected but not encrypted. In all cases, the string A may be empty. NOTE Examples of types of data that may need to be sent in unencrypted form, but whose integrity should be protected, include addresses, port numbers, sequence numbers, protocol version numbers, and other network protocol fields that indicate how the plaintext should be handled, forwarded, or processed.