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<task id="GUID-6E1AC4C9-7CA3-503A-AFC0-BAF29CCA8290" xml:lang="en"><title>Decrypting

RTP Packet Data</title><abstract><p>SRTP generates SRTP packets from the RTP stream and forwards

the packets to the receiver. SRTP transforms incoming SRTP packets to RTP

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><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 RTP payload decryption. </p> </prereq>

<steps id="GUID-AC6FB890-F2EF-593F-962C-6CEE085B65E8">

<step id="GUID-3256EEBB-F4B9-53BD-98E9-82CCE18A9AE3"><cmd>An application requests

for decryption of RTP packet. </cmd>

</step>

<step id="GUID-898D6551-1AEF-58B7-80FD-DF3E99E8F07C"><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-B61784E0-3579-524B-A5E4-7CC0FB31EBA9"><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-FC44022E-8761-5897-B8A4-CC89C3797542"><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-441EBC28-ABE2-5E21-B98C-F30480373BD6"><cmd/>

<info>The packet is decrypted using <xref href="GUID-8B875E75-DD09-3E4D-9FC6-152091869BA2.dita"><apiname>UnprotectRTPL()</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-D7CDB11F-54A5-5040-A37F-6AEB84D96E6A">

<substep id="GUID-90A3D943-B8B5-5735-B493-32399CD91279"><cmd/>

<info>The prototype for the API <codeph>UnprotectRTPL()</codeph> is as described

below. </info>

<stepxmp><codeblock id="GUID-F1298F1A-59CB-58AE-96C6-C1828B6E8BE3" xml:space="preserve">

                     IMPORT_C HBufC8* UnprotectRTPL(TUint aSSRC, const TDesC8& 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 of the stream of RTP packets

identified by a 32 bit numeric. SSRC identifier is a part of the RTP 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-1879AA8F-BC4B-558A-8AF8-9836ACA5FCBE"><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-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>