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<task id="GUID-F9A24A47-7A0B-5F6E-8B0A-B7A7BCC4EAFC" xml:lang="en"><title>Decrypting
RTCP Packet Data</title><abstract>SRTP generates SRTP packets from the RTP or RTCP stream and forwards
the packets to the receiver. SRTP transforms incoming SRTP packets to RTP/RTCP
packets and passes them up the stack. To ensure the confidentiality of the
packet content, data encryption is used. Encrypted RTP packet data is encapsulated
for transmission in a single lower-layer packet as a unit. The presence of
encryption and the correct usage of key are confirmed by the receiver through
header and payload validity checks.   <p/>At the receiving end, the packet
is authenticated first. Authentication is followed by is decoding the encrypted
payload. This process is known as decryption. Decryption requires a decryption
key.<p/></abstract><prolog><metadata><keywords/></metadata></prolog><taskbody>
<prereq><p>An SRTP session must be created and initialized before an application
requests for a RTCP payload decryption. </p> </prereq>
<steps id="GUID-CFD08369-E326-5C0D-B3D1-20B1C8C7BEE2">
<step id="GUID-41BF395B-F3A0-5F32-9779-452E83CCDBD3"><cmd>An application requests
for decryption of RTCP packet. </cmd>
</step>
<step id="GUID-9F86B8B4-D036-5414-BD50-0CBAAF6D4F0E"><cmd>The SRTP DLL checks
for the cryptographic context of the input stream. If a match is not found,
SRTP DLL checks for the cryptographic context of the session. </cmd>
</step>
<step id="GUID-3E04E9F0-EB72-5225-9769-9F58018F2D22"><cmd>The SRTP DLL checks
the replay list for the packet index. If a packet with the same index is found
in the replay list the packet is dropped. </cmd>
</step>
<step id="GUID-DFD6C096-6EAD-5217-9B58-FA4531FFF2C7"><cmd>The SRTP DLL computes
the authentication tag for the packet. The computed tag is matched with the
tag in the packet. If the tags match, the packet is accepted and the packet
index is added to the replay list. </cmd>
</step>
<step id="GUID-32C9E93C-7AAA-5039-B705-FD8A665E69EA"><cmd/>
<info>The packet is decrypted using <xref href="GUID-8B875E75-DD09-3E4D-9FC6-152091869BA2.dita"><apiname>UnprotectRTCPL()</apiname></xref> API
of the class <xref href="GUID-8B875E75-DD09-3E4D-9FC6-152091869BA2.dita"><apiname>CSRTPSession</apiname></xref>.The session
keys in the cryptographic context of the current SRTP session are used for
decryption. </info>
<substeps id="GUID-64F100C5-B100-5362-B196-053822365927">
<substep id="GUID-CC7F0063-A04A-5878-BCCB-380946857800"><cmd/>
<info>The prototype for the <codeph>API UnprotectRTCPL()</codeph> is as described
below. </info>
<stepxmp><codeblock id="GUID-6B0B89C1-9085-5674-8C5F-8E3C6A297AE5" xml:space="preserve">
                     IMPORT_C HBufC8* UnprotectRTCPL(TUint aSSRC, const TDesC8&amp; aPacket);
                  </codeblock> </stepxmp>

<info>This API is invoked if the session finds a match stream and unprotect
stream. </info>
<info>aSSRC is the input parameter that contains the Synchronisation Source(SSRC).
Synchronisation Source is the source for the stream of RTP packets identified
by a 32 bit numeric. SSRC identifier is a part of the RTCP packet header. </info>
<stepxmp>aPacket is the input parameter that contains packet to be unprotected.
The length of descriptor should match the actual data length. A mismatch in
the length descriptor and actual length may lead to incorrect encryption. </stepxmp>
<info>HBufC8 is the output parameter that contains the decrypted packet. </info>
<stepresult><p>The API leaves, if matching stream is not found. </p> <p>In
late binding cases, the API tries to find a stream with SSRC is equal to zero
and unprotect the packet. </p> </stepresult>
</substep>
</substeps>
</step>
<step id="GUID-13C77140-79F2-5D9B-8483-823C1414889C"><cmd>The SRTP DLL updates
the count of packets decrypted. </cmd>
</step>
</steps>
<result><p>On successful execution of the API, decrypted data is returned. </p> <p>If
any of the above mentioned checks fail, the packet is discarded. </p> </result>
</taskbody><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-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>
</related-links></task>