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<task id="GUID-7B6073BF-6D8C-5F00-98EC-966573777677" xml:lang="en"><title>Managing
Master Key</title><abstract><p>The Secure Real-Time Transport Protocol (SRTP) stream requires
the sender and receiver to maintain the cryptographic context of the packets.
SRTP uses two types of keys: master keys and session keys for packet encryption
and decryption. Master key is a random bit string. The session keys are derived
from the master key. The Session key is used directly in cryptographic transform
for encryption or message authentication.  </p><p> A single master key provides
keying material for confidentiality and integrity protection of both SRTP
and the corresponding SRTCP stream. For each master key, a count of processed
packets is maintained. There is a limit on the number of packets processed
by a single master key. SRTCP maintains a separate counter for master key
even if the master key is the same as that for SRTP. The counter counts the
number of SRTCP packets processed with the key.   </p><p>The SRTP stack checks
for the count of packet processed when the RTP/RTCP packets are encrypted
or decrypted. If the number of packets processed is maximum, the master key
status is identified as stale. The SRTP notifies the SRTP client of the master
key status. This is Notifying the Master Key Expiry.  </p><p> The SRTP stack
can request for a new master key(ReKey) even before the master key status
is identified as stale.This is ReKey request.</p></abstract><prolog><metadata><keywords/></metadata></prolog><taskbody>
<prereq id="GUID-E3BEB74D-79C7-59A9-9366-CCD8B52DB7F8-GENID-1-7-1-19-1-1-6-1-5-1-10-1-3-1"><p>An SRTP session
must be created and initialized before an application requests for a RTP/RTCP
packet decryption and encryption. </p> <p>The two variants of the API <xref href="GUID-4301A1D8-5A9B-39F6-B4F0-84A7A1698617.dita"><apiname>SRTPMasterKeyStaleEvent()</apiname></xref> must be
implemented by the application. </p> </prereq>
<steps id="GUID-FB88B32D-6FD6-504B-A39F-D2882FF355FB">
<step id="GUID-B7B460C7-C6C2-5AB0-957B-2ABA30877680"><cmd>An RTP/RTCP packet
is encrypted or decrypted . </cmd>
</step>
<step id="GUID-67F4846F-16C8-5DB1-ABB2-3886993AF5A4"><cmd>The SRTP uses the
current master key to check the count of packet processed. If the number of
packets processed is not the maximum permissible value, the count is increased.
If the number of packets processed has reached maximum permissible value,
the SRTP DLL notifies the expiry(stale) state of the current master key. </cmd>
</step>
<step id="GUID-BB007F89-6FC5-58D0-9C89-5A04643C508F"><cmd>The SRTP DLL notifies
the master key status using the API's <xref href="GUID-4301A1D8-5A9B-39F6-B4F0-84A7A1698617.dita"><apiname>SRTPMasterKeyStaleEvent()</apiname></xref> of
the class <xref href="GUID-4301A1D8-5A9B-39F6-B4F0-84A7A1698617.dita"><apiname>MSRTPReKeyingObserver</apiname></xref> .The
notification can be initiated by MSRTPReKeyingObserver or CSRTPSession. </cmd>
<substeps id="GUID-7DD25E11-8593-53E4-AFFB-1DEFBA4EB9EC">
<substep id="GUID-69A703E5-6CA8-5BC1-A910-533981F9374C"><cmd/>
<info>The prototypes for both the variants of <xref href="GUID-4301A1D8-5A9B-39F6-B4F0-84A7A1698617.dita"><apiname>SRTPMasterKeyStaleEvent()</apiname></xref> are
described below: </info>
<info><ul>
<li id="GUID-B9D9F267-48AD-55A6-AD8A-54B3C85C8764"><codeblock id="GUID-4373AEA4-66E2-5968-B6C9-13E300C1617B" xml:space="preserve"> virtual void SRTPMasterKeyStaleEvent(const CSRTPStream&amp; aStream)=0;
                     
                  </codeblock> <p>This API is called
by the SRTP Stream initiated with MSRTPReKeyingObserver, if a master key is
identified as stale and needs to be refreshed. </p> <p>aStream is the input
parameter. It contains the stream of the master key that needs to be refreshed. </p> </li>
</ul> </info>
<info><ul>
<li id="GUID-A652F1E7-E2F9-59CB-B050-8FB3221FD7FA"><codeblock id="GUID-12C8EF78-DA37-5BA4-A62E-162B32EC301A" xml:space="preserve"> virtual void SRTPMasterKeyStaleEvent(const CSRTPSession&amp; aSession)=0;
                     
                  </codeblock> <p>This API is called
by SRTP Stream initiated with CSRTPSession if a master key is identified as
stale and needs to be refreshed. </p> <p>aSession is the input parameter.
It contains the session of the master key that needs to be refreshed. </p> </li>
</ul> </info>
</substep>
</substeps>
</step>
<step id="GUID-4A10324C-9356-5083-B109-C1CE1EEDEBED"><cmd>The SRTP DLL requests
for a new master key using the API <xref href="GUID-4301A1D8-5A9B-39F6-B4F0-84A7A1698617.dita"><apiname> 
               ReKeyNeeded()</apiname></xref> of the class<xref href="GUID-4301A1D8-5A9B-39F6-B4F0-84A7A1698617.dita"><apiname>CSRTPSession</apiname></xref>. </cmd>
<substeps id="GUID-56DEF32A-0762-5D93-9F5D-D025DBB03533">
<substep id="GUID-876D52C4-F774-569F-9A09-BEAD80557D07"><cmd/>
<info>The prototype of the API is as described below: </info>
<info><ul>
<li id="GUID-06E4B681-D1C0-52C3-AE2B-A5BA4F5A037D"><codeblock id="GUID-B005930E-F049-5144-AF9F-2AD38DCF1F50" xml:space="preserve">void ReKeyNeeded(const CSRTPStream&amp; aStream, TBool aIsStrmCrypto);
                     
                  </codeblock> <p>This API requests
for ReKey. </p> <p>aStream is the input parameter that needs the Rekey. </p> <p>aIsStrmCrypto
is the boolean input parameter. It contains true if stream has its own cryptographic
context. Else the value is false. </p> </li>
</ul> </info>
</substep>
</substeps>
</step>
</steps>
<result><p>SRTP stack notifies the SRTP client of the master key expiry state. </p> <p>SRTP
stack requests for ReKey. </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-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>
</related-links></task>