Use SRTP_MAX_SRTCP_TRAILER_LEN for protect_rtcp

Applying protection to RTCP packets can add more bytes to the packet than
RTP does.

Author: vopatek

src/pylibsrtp/__init__.py

@@ -39,8 +39,12 @@
 ]
 
 
+# SRTP_MAX_TAG_LEN + SRTP_MAX_MKI_LEN
 SRTP_MAX_TRAILER_LEN = 16 + 128
 
+# SRTP_SRCTP_INDEX_LEN + SRTP_MAX_TAG_LEN + SRTP_MAX_MKI_LEN
+SRTP_MAX_SRTCP_TRAILER_LEN = 4 + 16 + 128
+
 
 class Error(Exception):
     """
@@ -238,7 +242,7 @@ def protect_rtcp(self, packet: bytes) -> bytes:
         :param packet: :class:`bytes`
         :rtype: :class:`bytes`
         """
-        return self.__process(packet, lib.srtp_protect_rtcp, SRTP_MAX_TRAILER_LEN)
+        return self.__process(packet, lib.srtp_protect_rtcp, SRTP_MAX_SRTCP_TRAILER_LEN)
 
     def unprotect(self, packet: bytes) -> bytes:
         """

tests/test_session.py

@@ -2,6 +2,7 @@
 import secrets
 from unittest import TestCase
 
+import pylibsrtp
 from pylibsrtp import Error, Policy, Session
 
 RTP = (
@@ -207,7 +208,7 @@ def test_rtp_any_ssrc(self):
 
                 # bad length
                 with self.assertRaises(ValueError) as cm:
-                    tx_session.protect(b"0" * 1500)
+                    tx_session.protect(b"0" * (1501 - pylibsrtp.SRTP_MAX_TRAILER_LEN))
                 self.assertEqual(str(cm.exception), "packet is too long")
 
                 # unprotect RTP
@@ -244,7 +245,9 @@ def test_rtcp_any_ssrc(self):
 
                 # bad length
                 with self.assertRaises(ValueError) as cm:
-                    tx_session.protect_rtcp(b"0" * 1500)
+                    tx_session.protect_rtcp(
+                        b"0" * (1501 - pylibsrtp.SRTP_MAX_SRTCP_TRAILER_LEN)
+                    )
                 self.assertEqual(str(cm.exception), "packet is too long")
 
                 # unprotect RTCP