/**
* Derives the srtp session keys from the master key
*
* @param index
* the 48 bit SRTP packet index
*/
public void DeriveSrtpKeys(long index)
{
// compute the session encryption key
long label = 0;
ComputeIv(label, index);
KeyParameter encryptionKey = new KeyParameter(masterKey);
cipher.Init(true, encryptionKey);
Arrays.Fill(masterKey, (byte)0);
cipherCtr.GetCipherStream(cipher, encKey, policy.EncKeyLength, ivStore);
// compute the session authentication key
if (authKey != null)
{
label = 0x01;
ComputeIv(label, index);
cipherCtr.GetCipherStream(cipher, authKey, policy.AuthKeyLength, ivStore);
switch ((policy.AuthType))
{
case SrtpPolicy.HMACSHA1_AUTHENTICATION:
KeyParameter key = new KeyParameter(authKey);
mac.Init(key);
break;
default:
break;
}
}
Arrays.Fill(authKey, (byte)0);
// compute the session salt
label = 0x02;
ComputeIv(label, index);
cipherCtr.GetCipherStream(cipher, saltKey, policy.SaltKeyLength, ivStore);
Arrays.Fill(masterSalt, (byte)0);
// As last step: initialize cipher with derived encryption key.
if (cipherF8 != null)
{
SrtpCipherF8.DeriveForIV(cipherF8, encKey, saltKey);
}
encryptionKey = new KeyParameter(encKey);
cipher.Init(true, encryptionKey);
Arrays.Fill(encKey, (byte)0);
}
