[AVTCORE] EKT Problems with RTCP

[John Mattsson <[e-mail]>]

Hi,

While editing the EKT draft I realized that EKT has major problems with RTCP.

+---+ —------ SRTP ---------> +---+
| S | ------- SRTCP SR -----> | R |
+---+ <------ SRTCP RR ------ +---+

Take the above example. The sender S sends RTP and RTCP to the receiver R. R sends RTCP but not RTP to S.

S re-keying: Irrespectively if S sends EKT in SRTP or SRTCP the occurrence of the key change is signalled with ROC || ISN and R has no way to know when to exactly change key for SRTCP (i.e. how ISN maps to the SRTCP index). R is forced to guess and try authenticating with both the old and the new key.

R re-keying: Here ROC || ISN has no meaning at all and S will have to do trial and error with both the old and the new key.

This is not a robust solution and it needs to be fixed. Two suggestions:

- Option 1
One option is to add another field ISI (Initial SRTCP Index) to the EKT_Plaintext. This would then work similar to ISN. The Plaintext could contain both, or one of them. One alternative is that EKT contains both ISN and ISI. Another alternative is that ISN is used in EKT over SRTP and ISI in EKT over SRTCP, forcing EKT to be used in both SRTP and SRTCP.

- Option 2
The current EKT draft says

“MKI is no longer allowed with EKT (as MKI duplicates some of EKT's functions)”.

Its rather EKT that duplicates MKI (RFC 3711) and one simple option would be to simply remove the EKT parts that duplicates MKI and instead mandate use of MKI.

The EKT_Plaintext would then be:
EKT_Plaintext = SRTP_Master_Key || SSRC || ROC || MKI

And the SRT(C)P packets would look like:
+------------+-------------+-----+-----+-----+
| RTP Header | RTP Payload | MKI | TAG | EKT |
+------------+-------------+-----+-----+-----+
+--------------------+-------------+-----+-----+-----+
| RTCP Packet Types  | SRTCP INDEX | MKI | TAG | EKT |
+--------------------+-------------+-----+-----+-----+

This would allow full flexibility in the use of EKT. EKT could be sent in RTP and/or RTCP. Any number of keys could be distributed ahead of time.

If the MKIs are random, this would also make the EKT replay attack (in the case of SSRC collisions) much harder.

MKI could by default be one byte.

For AEAD algorithms MKI is the last field in the SRTP. If AEAD algorithms were mandated for EKT, MKIs with the last bit ‘0’ could be mandated and the short EKT tag would not be needed.


Comments welcome, I would strongly prefer option 2. The more I think about it, the ISN approach duplicates functionality in RFC3711, it is complex, not robust, and vulnerable to replay attacks.


Cheers,
John



JOHN MATTSSON
MSc Engineering Physics, MSc Business Administration and Economics
Ericsson IETF Security Coordinator
Senior Researcher, Security

Ericsson AB
Ericsson Research
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