The Secure Real-time Transport Protocol (SRTP) is a profile for Real-time Transport Protocol (RTP) intended to provide encryption, message authentication and integrity, and replay attack protection to the RTP data in both unicast and multicast applications. It was developed by a small team of Internet Protocol and cryptographic experts from Cisco and Ericsson. It was first published by the IETF in March 2004 as . Since RTP is accompanied by the RTP Control Protocol (RTCP) which is used to control an RTP session, SRTP has a sister protocol, called Secure RTCP (SRTCP); it securely provides the same functions to SRTP as the ones provided by RTCP to RTP. Utilization of SRTP or SRTCP is optional in RTP or RTCP applications; but even if SRTP or SRTCP are used, all provided features (such as encryption and authentication) are optional and can be separately enabled or disabled. The only exception is the message authentication feature which is indispensable and required when using SRTCP. Data flow encryption SRTP and SRTCP use Advanced Encryption Standard (AES) as the default cipher. There are two cipher modes defined which allow the AES block cipher to be used as a stream cipher: Segmented Integer Counter Mode A typical counter mode, which allows random access to any blocks, which is essential for RTP traffic running over unreliable network with possible loss of packets. In the general case, almost any function can be used in the role of counter, assuming that this function does not repeat for a large number of iterations. But the standard for encryption of RTP data is just a usual integer incremental counter. AES running in this mode is the default encryption algorithm, with a default key size of 128 bits and a default session salt key length of 112 bits. f8-mode A variation of output feedback mode, enhanced to be seekable and with an altered initialization function. The default values of the encryption key and salt key are the same as for AES in counter mode. (AES running in this mode has been chosen to be used in 3G mobile networks.) Besides the AES cipher, SRTP allows the ability to disable encryption outright, using the so-called NULL cipher, which can be assumed as an alternate supported cipher. In fact, the NULL cipher does not perform any encryption; The encryption algorithm functions as the identity function, and copies the input stream to the output stream