protected virtual void PrepareSrtpSharedSecret()
{
//Set master secret back to security parameters (only works in old bouncy castle versions)
//mContext.SecurityParameters.MasterSecret = masterSecret;
SrtpParameters srtpParams =
SrtpParameters.GetSrtpParametersForProfile(clientSrtpData.ProtectionProfiles[0]);
int keyLen = srtpParams.GetCipherKeyLength();
int saltLen = srtpParams.GetCipherSaltLength();
srtpPolicy = srtpParams.GetSrtpPolicy();
srtcpPolicy = srtpParams.GetSrtcpPolicy();
srtpMasterClientKey = new byte[keyLen];
srtpMasterServerKey = new byte[keyLen];
srtpMasterClientSalt = new byte[saltLen];
srtpMasterServerSalt = new byte[saltLen];
// 2* (key + salt length) / 8. From http://tools.ietf.org/html/rfc5764#section-4-2
// No need to divide by 8 here since lengths are already in bits
byte[] sharedSecret = GetKeyingMaterial(2 * (keyLen + saltLen));
/*
*
* See: http://tools.ietf.org/html/rfc5764#section-4.2
*
* sharedSecret is an equivalent of :
*
* struct {
* client_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
* server_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
* client_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];
* server_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];
* } ;
*
* Here, client = local configuration, server = remote.
* NOTE [ivelin]: 'local' makes sense if this code is used from a DTLS SRTP client.
* Here we run as a server, so 'local' referring to the client is actually confusing.
*
* l(k) = KEY length
* s(k) = salt lenght
*
* So we have the following repartition :
* l(k) 2*l(k)+s(k)
* 2*l(k) 2*(l(k)+s(k))
* +------------------------+------------------------+---------------+-------------------+
* + local key | remote key | local salt | remote salt |
* +------------------------+------------------------+---------------+-------------------+
*/
Buffer.BlockCopy(sharedSecret, 0, srtpMasterClientKey, 0, keyLen);
Buffer.BlockCopy(sharedSecret, keyLen, srtpMasterServerKey, 0, keyLen);
Buffer.BlockCopy(sharedSecret, 2 * keyLen, srtpMasterClientSalt, 0, saltLen);
Buffer.BlockCopy(sharedSecret, (2 * keyLen + saltLen), srtpMasterServerSalt, 0, saltLen);
}
private IPacketTransformer GenerateTransformer(SDPSecurityDescription securityDescription, bool isRtp)
{
var srtpParams = SrtpParameters.GetSrtpParametersForProfile((int)securityDescription.CryptoSuite);
var engine = new SrtpTransformEngine(securityDescription.KeyParams[0].Key,
securityDescription.KeyParams[0].Salt,
srtpParams.GetSrtpPolicy(),
srtpParams.GetSrtcpPolicy());
if (isRtp)
{
return(engine.GetRTPTransformer());
}
else
{
return(engine.GetRTCPTransformer());
}
}