code: public void deriveSrtcpKeys() {
          // compute the session encryption key
          byte label = 3;
          computeIv(label);
  
          KeyParameter encryptionKey = new KeyParameter(masterKey);
          cipher.init(true, encryptionKey);
          Arrays.fill(masterKey, (byte)0);
  
          cipherCtr.getCipherStream(cipher, encKey, policy.getEncKeyLength(), ivStore);
  
          if (authKey != null) {
              label = 4;
              computeIv(label);
              cipherCtr.getCipherStream(cipher, authKey, policy.getAuthKeyLength(), ivStore);
  
              switch ((policy.getAuthType())) {
  	            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 = 5;
          computeIv(label);
          cipherCtr.getCipherStream(cipher, saltKey, policy.getSaltKeyLength(), 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);
      }
docs: Derives the srtcp session keys from the master key.