/**
* Transform a SRTCP packet into a RTCP packet.
* This method is called when a SRTCP packet was received.
*
* Operations done by the this operation include:
* Authentication check, Packet replay check and decryption.
*
* Both encryption and authentication functionality can be turned off
* as long as the SRTPPolicy used in this SRTPCryptoContext requires no
* encryption and no authentication. Then the packet will be sent out
* untouched. However this is not encouraged. If no SRTCP feature is enabled,
* then we shall not use SRTP TransformConnector. We should use the original
* method (RTPManager managed transportation) instead.
*
* @param pkt the received RTCP packet
* @return true if the packet can be accepted
* false if authentication or replay check failed
*/
public bool ReverseTransformPacket(RawPacket pkt)
{
bool decrypt = false;
int tagLength = policy.AuthTagLength;
int indexEflag = pkt.GetSRTCPIndex(tagLength);
if ((indexEflag & 0x80000000) == 0x80000000)
{
decrypt = true;
}
int index = (int)(indexEflag & ~0x80000000);
/* Replay control */
if (!CheckReplay(index))
{
return(false);
}
/* Authenticate the packet */
if (policy.AuthType != SrtpPolicy.NULL_AUTHENTICATION)
{
// get original authentication data and store in tempStore
pkt.ReadRegionToBuff(pkt.GetLength() - tagLength, tagLength, tempStore);
// Shrink packet to remove the authentication tag and index
// because this is part of authenticated data
pkt.shrink(tagLength + 4);
// compute, then save authentication in tagStore
AuthenticatePacket(pkt, indexEflag);
for (int i = 0; i < tagLength; i++)
{
if ((tempStore[i] & 0xff) == (tagStore[i] & 0xff))
{
continue;
}
else
{
return(false);
}
}
}
if (decrypt)
{
/* Decrypt the packet using Counter Mode encryption */
if (policy.EncType == SrtpPolicy.AESCM_ENCRYPTION || policy.EncType == SrtpPolicy.TWOFISH_ENCRYPTION)
{
ProcessPacketAESCM(pkt, index);
}
/* Decrypt the packet using F8 Mode encryption */
else if (policy.EncType == SrtpPolicy.AESF8_ENCRYPTION || policy.EncType == SrtpPolicy.TWOFISHF8_ENCRYPTION)
{
ProcessPacketAESF8(pkt, index);
}
}
Update(index);
return(true);
}