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<concept id="GUID-E7C820D7-7E00-5A95-9847-29A0FB0E78F1" xml:lang="en"><title>What

is Secure Real-Time Transport Protocol?</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>The Secure Real-Time Transport Protocol (SRTP) is defined as a profile

of RTP. SRTP provides confidentiality, message authentication, and replay

protection to the RTP traffic. It also protects the traffic of Real-time Transport

Control Protocol (RTCP). </p>

<p>SRTP is a layer between the RTP and the networking layer. The SRTP protocol

as defined in RFC 3711 provides support for encryption, decryption of RTP

payloads, and integrity protection of the entire packet. </p>

<p>SRTP uses a single master key for generating session keys for encryption

and authentication. RTP is closely related to RTCP(RTP control protocol) which

can be used to control the RTP session. SRTP has a related protocol called

Secure RTCP or SRTCP. SRTCP provides the same security related features to

RTCP, as SRTP provides to RTP. </p>

<p>SRTP is the security layer that resides between the RTP/RTCP application

layer and the transport layer. It generates SRTP packets from the RTP/RTCP

stream and forwards the packets to the receiver. It also transforms incoming

SRTP packets to RTP/RTCP packets and passes these up the stack. </p>

<section id="GUID-EC86AD19-5A80-59C5-A1BE-5196751A62A3"><title>How SRTP Works?</title> <p><b>Symbian

OS Implementation of SRTP</b> </p> <p>The SRTP implementation follows the

mandatory mechanisms defined in RFC 3711: </p> <ul>

<li id="GUID-7F501CFB-A009-5ABA-81ED-442C79691B65"><p>AES-128 Counter mode

for encryption of RTP payloads </p> </li>

<li id="GUID-B18A31C7-C91A-568C-9FA5-BEF66DD5E534"><p>HMAC-SHA1 for authentication/integrity

protection of RTP packets </p> </li>

<li id="GUID-81492123-D4C6-59EC-B6E2-F38B8638E4BC"><p>AES-Counter Mode for

session key derivation </p> </li>

<li id="GUID-1EE2C03C-C4B2-59C2-94EB-6DDAC730E667"><p>NULL mode is supported

for encryption/decryption and authentication. </p> </li>

</ul> <p><b>Functional Model of SRTP</b> </p> <p>The figure below describes

the functional model of SRTP. </p> <fig id="GUID-EC50E1D8-79A0-5088-8DB1-3EA50D63ABE4">

<image href="GUID-26D92AB3-510F-5D29-852A-D55F0C96A798_d0e548281_href.png" placement="inline"/>

</fig> <p>The SRTP Stack is not an active component and is not network aware.

It sits as a processing layer between the RTP stack and the network. The RTP

stack calls for SRTP encryption before it sends the RTP Packets to the network.

At the receiving side, the RTP stack decrypts the data received prior to normal

RTP Processing. </p> <p>The APIs execute within the process boundary of the

RTP Application. The SRTP APIs are generic and can be used by any RTP stack. </p> <p>The

SRTP library implements the following functionalities: </p> <ul>

<li id="GUID-6299069A-B48B-5A0D-A306-77B30EE91423"><p>Encryption of RTP payloads </p> </li>

<li id="GUID-4A8EFCD5-45D4-5DDD-B847-DE1529ECD293"><p>Message authentication

and integrity of the RTP packet </p> </li>

<li id="GUID-18578A1C-7D83-58EE-9E80-B5C0490D9FAD"><p>Replay Protection. </p> </li>

</ul> </section>

<section><title>How to use SRTP?</title> <p>The SRTP stack is offered as a

general purpose DLL. </p> <p>The capability of the DLL is ALL -TCB. </p> <p>The

application should link to srtp.lib. </p> <p><b>Dependency</b> </p> <p>The

SRTP implementation consists of crypto routines using the Symbian cryptographic

libraries namely cryptospi.dll (from OS version 9.5) </p> </section>

</conbody><related-links>

<link href="GUID-06370120-7738-5694-9A8C-3B5B1E3A4352.dita"><linktext>Creating

an SRTP                 Session</linktext></link>

<link href="GUID-425A208C-5D2D-55FD-8623-9E00006E2781.dita"><linktext>Creating

a                 Cryptographic Context</linktext></link>

<link href="GUID-372B7A9F-261E-5F3F-B114-4BE2FE2EF7C7.dita"><linktext>Creating

an SRTP                 Stream</linktext></link>

<link href="GUID-79846EA2-33CD-5D01-8E96-6092C15BD839.dita"><linktext>Encrypting

                RTCP Packet Data</linktext></link>

<link href="GUID-922F292E-9420-593B-B226-2764C5F29DF9.dita"><linktext>Encrypting

RTP                 Packet Data</linktext></link>

<link href="GUID-6E1AC4C9-7CA3-503A-AFC0-BAF29CCA8290.dita"><linktext>Decrypting

RTP                 Packet Data</linktext></link>

<link href="GUID-F9A24A47-7A0B-5F6E-8B0A-B7A7BCC4EAFC.dita"><linktext>Decrypting

                RTCP Packet Data</linktext></link>

<link href="GUID-B9BD2E0A-F214-5344-91A6-E4E99F0D74C8.dita"><linktext>Updating

Master                 Key</linktext></link>

<link href="GUID-F9A24A47-7A0B-5F6E-8B0A-B7A7BCC4EAFC.dita"><linktext>Managing

                Master Key</linktext></link>