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The Secure Real-time Transport Protocol (or SRTP) defines a profile of RTP (Real-time Transport Protocol), intended to provide encryption, message authentication and integrity, and replay protection to the RTP data in both unicast and multicast applications. It was developed by a small team of IP protocol and cryptographic experts from Cisco and Ericsson including David Oran, David McGrew, Mark Baugher, Mats Naslund, Elisabetta Carrara, Karl Norman, and Rolf Blom. It was first published by the IETF in March 2004 as RFC 3711. Since RTP is closely related to RTCP (Real Time Control Protocol) which can be used to control the RTP session, SRTP also has a sister protocol, called Secure RTCP (or SRTCP); SRTCP provides the same security-related features to RTCP, as the ones provided by SRTP to RTP. Utilization of SRTP or SRTCP is optional to the utilization of RTP or RTCP; but even if SRTP/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 indispensably required when using SRTCP. == Data flow encryption == For encryption and decryption of the data flow (and hence for providing confidentiality of the data flow), SRTP (together with SRTCP) utilizes AES as the default cipher. There are two cipher modes defined which allow the original block cipher AES 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 long 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 encryption key length 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 UMTS 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 the second supported cipher (or the third supported cipher mode in sum). In fact, the NULL cipher does not perform any encryption (i.e. the encryption algorithm functions as though the key stream contains only zeroes, and copies the input stream to the output stream without any changes). It is mandatory for this cipher mode to be implemented in any SRTP-compatible system. As such, it can be used when the confidentiality guarantees ensured by SRTP are not required, while other SRTP features (such as authentication and message integrity) may be used. Though technically SRTP can easily accommodate new encryption algorithms, the SRTP standard states that new encryption algorithms besides those described cannot simply be added in some implementation of SRTP protocol. The only legal way to add a new encryption algorithm, while still claiming the compatibility with SRTP standard, is to publish a new companion standard track RFC which must clearly define the new algorithm. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Secure Real-time Transport Protocol」の詳細全文を読む スポンサード リンク
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