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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>