This memo defines the NULL encryption algorithm and its use with the IPsec Encapsulating Security Payload (ESP). NULL does nothing to alter plaintext data. In fact@ NULL@ by itself@ does nothing. NULL provides the means for ESP to provide authentication and integrity without confidentiality. Further information on the other components necessary for ESP implementations is provided by [ESP] and [ROAD].
RFC 2410-1998 history
1998RFC 2410-1998 The NULL Encryption Algorithm and Its Use With IPsec