Cisco Systems This document specifies Integer Counter Mode (ICM), a mode of operation of a block cipher which defines an indexed keystream generator (which generates a keystream segment given an index). This mode is efficient, parallelizable, and has been proven secure given realistic assumptions about the block cipher. Test vectors are provided for AES. Counter Mode admits many variations. The variant specified in this document is secure and flexible, yet it enables a single implementation of a keystream generator to suffice in different application domains.