Intel TruSecure This document describes the use of the AES Cipher Algorithm with 128 bit key in Counter (CTR) Mode, with an implicit counter, as a confidentiality mechanism within the context of the IPsec Encapsulating Security Payload [ESP]. CTR is a parallelizable block-cipher mode of operation. It uses the underlying block cipher as a stream cipher. Accordingly, great care must be exercised to utilize it appropriately within IPsec.