Encrypted Key Transport for DTLS and Secure RTP
draft-ietf-perc-srtp-ekt-diet-13

Adam Roach's comments were addressed in:
https://github.com/ietf/perc-wg/pull/180

Re-sending my initial comments, as only one of the 17 were addressed
in subsequent revisions. While no single comment rises to the level of
a DISCUSS-worthy issue, several of these are moderately severe. I would
appreciate either a response to each comment, or a corresponding
change in the document.

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Thanks to the work that everyone has put in on getting an EKT mechanism
specified and finalized. I have a handful of comments that I would like to see
considered prior to publication of the document.

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§1:

>  EKT provides a way for an SRTP session participant, to securely
>  transport its SRTP master key and current SRTP rollover counter to
>  the other participants in the session.

Nit: "...participant to securely..."

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§4.1:

>   EKTMsgTypeExtension = %x03-FF

Shouldn't this be "%x01 / %x03-ff" ?

>   SRTPMasterKeyLength = BYTE
>   SRTPMasterKey = 1*256BYTE

I think this either needs to be "1*255BYTE", or we need text that explicitly
indicates that an SRTPMasterKeyLength value of 0x00 means "256 bytes." Probably
the former.

I think this is even further constrained by the fact that EKTCiphertext is
limited to 256 bytes, and contains the SRTPMasterKeyLength, SRTPMasterKey,
SSRC, and ROC (and is not compressed) -- which means the SRTPMasterKeyLength
can't be more than (256 - 1 - 4 - 4 =) 247 bytes. So perhaps "1*247BYTE" is
more appropriate?

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§4.2.1:

>  The creation of the EKTField MUST precede the normal SRTP
>  packet processing.

Why? This seems unnecessary and unnecessarily complicated. If the order of
operations has an impact on the bits on the wire (I don't see how it does?),
then please include some explanatory text here that clarifies the reason for
this constraint.

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§4.2.1:

>  When a packet is sent with the ShortEKTField, the ShortEKFField is
>  simply appended to the packet.

Nit: s/ShortEKFField/ShortEKTField/

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§4.2.1:

>  5.  If the SSRC in the EKTPlaintext does not match the SSRC of the
>      SRTP packet received, then all the information from this
>      EKTPlaintext MUST be discarded and the following steps in this
>      list are skipped.

I can see implementors easily interpreting this as requiring them to discard
the RTP payload as well. If that's not the intention (I don't think it is),
consider adding text like "The FullEKTField is removed from the packet then
normal SRTP or SRTCP processing occurs."

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§4.3:

>  Section 4.2.1 recommends that SRTP senders continue using an old key
>  for some time after sending a new key in an EKT tag.

This is the first appearance of the phrase "EKT tag," which never seems to be
properly defined. I presume this is meant to be the combination of the EKT
Ciphertext and the SPI?

In any case, please clearly define this term somewhere, preferably before using
it the first time.

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§4.3:

>  cannot be used and they also need to create a counter that keeps
>  track of how many times the key has been used to encrypt data to
>  ensure it does not exceed the T value for that cipher (see ).

The parenthetical phrase appears to be missing something here.


>  If
>  either of these limits are exceeded, the key can no longer be used

Nit: "...either... is exceeded..."

>  for encryption.  At this point implementation need to either use the

Nit: "...implementations need..."

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§4.5:

>  If a source has its EKTKey changed by the key management, it MUST
>  also change its SRTP master key

I suppose it's not terribly important for interop, but the implication that this
change takes place immediately seems to contradict the 250 ms period specified
in §4.2.1. Perhaps a few words here about how these two normative statements
are intended to interact would save implementors a bit of grief.

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§4.6:

>  This document defines the use of EKT with SRTP.  Its use with SRTCP
>  would be similar, but is reserved for a future specification.

After reading this far, I was quite surprised to find this qualification. If
this is the intention for this document, please adjust the rest of the text to
match. Some examples follow.

>  The following shows the syntax of the EKTField expressed in ABNF
>  [RFC5234].  The EKTField is added to the end of an SRTP or SRTCP
>  packet.
-----
>  Rollover Counter (ROC): On the sender side, this is set to the
>  current value of the SRTP rollover counter in the SRTP/SRTCP context
>  associated with the SSRC in the SRTP or SRTCP packet.
-----
>  1.  The final byte is checked to determine which EKT format is in
>      use.  When an SRTP or SRTCP packet contains a ShortEKTField, the
>      ShortEKTField is removed from the packet then normal SRTP or
>      SRTCP processing occurs.
-----
>      The reason for
>      using the last byte of the packet to indicate the type is that
>      the length of the SRTP or SRTCP part is not known until the
>      decryption has occurred.
-----
>  7.  At this point, EKT processing has successfully completed, and the
>      normal SRTP or SRTCP processing takes place.
-----
>  This allows
>  those peers to process EKT keying material in SRTP (or SRTCP) and
>  retrieve the embedded SRTP keying material.

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§4.7:

>     To accommodate packet loss, it is
>     RECOMMENDED that three consecutive packets contain the
>     FullEKTField be transmitted.

Nit: "...containing..." (alternately, remove "be transmitted" -- both make a
grammatically correct sentance)

More substantially -- under "New sender:", I'm a little surprised that there
isn't any mention of other senders re-keying in response to a new sender
joining. In the vast majority of conferences, when a sender joins, that same
entity generally will also be a receiver. It seems this should trigger other
senders to include the key in their next packet.

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§4.7:

>  Rekey:
>     By sending EKT tag over SRTP, the rekeying event shares fate with
>     the SRTP packets protected with that new SRTP master key.

Is this actually true? Going back to the 250 ms period specified in §4.2.1, it
seems that the master key is sent out in packets pretty far removed from those
it actually protects.

Between this and the inconsistency I mention in §4.5 above, this increasingly
feels like maybe there were two different ways of reasoning about the timing
of sending a master key versus the timing of actually using it. Does the text
in §4.2.1 perhaps represent an outdated notion of how this is intended to
work?

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§4.7:

>     If sending audio and video, the RECOMMENDED
>     frequency is the same as the rate of intra coded video frames.  If
>     only sending audio, the RECOMMENDED frequency is every 100ms.

Is this "100ms" correct?  Assuming, say, the use of Opus at voice quality with
20 ms packets, this is taking packets on the order of 40 bytes in length and
tacking on something like 20 to 30 bytes to every fifth packet. That's an
increase in overall stream size on the order of roughly 15% to 20%.

At the same time, when using real-time video, intra frames are going to happen
roughly every 500 ms to 1500 ms. If a cadence on that order is okay for
audiovisual streams, I have to imagine it's okay for audio streams.

So, to clarify: is this "100ms" a typo for "1000 ms"?

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§7.2:

>                  +----------+-------+---------------+
>                  | Name     | Value | Specification |
>                  +----------+-------+---------------+
>                  | AESKW128 |     1 | RFCAAAA       |
>                  | AESKW256 |     2 | RFCAAAA       |
>                  | Reserved |   255 | RFCAAAA       |
>                  +----------+-------+---------------+
>
>                        Table 3: EKT Cipher Types

Section 5.2.1 reserves "0" as well. I suspect we want to replicate that
reservation in this table.

Note to self: make sure editors look/respond to Adam’s and Ben’s comments.


Old comments preserved for posterity. I didn't check if they still apply:

I share Benjamin's concern about extensibility.

In 4.4.1:

   The default EKT Cipher is the Advanced Encryption Standard (AES) Key
   Wrap with Padding [RFC5649] algorithm.  It requires a plaintext
   length M that is at least one octet, and it returns a ciphertext with
   a length of N = M + (M mod 8) + 8 octets.

I started looking at RFC 5649. Maybe I was tired and my math was wrong, but I couldn't figure out how you came up with the N value above.
In particular, where is the "+ 8" coming from?

In 6:

   An attacker who tampers with the bits in FullEKTField can prevent the
   intended receiver of that packet from being able to decrypt it.  This
   is a minor denial of service vulnerability.  Similarly the attacker
   could take an old FullEKTField from the same session and attach it to
   the packet.  The FullEKTField would correctly decode and pass
   integrity checks.  However, the key extracted from the FullEKTField ,
   when used to decrypt the SRTP payload, would be wrong and the SRTP
   integrity check would fail.  Note that the FullEKTField only changes
   the decryption key and does not change the encryption key.  None of
   these are considered significant attacks as any attacker that can
   modify the packets in transit and cause the integrity check to fail.

The last sentence seems to be incomplete. Did you mean "can" instead of the last "and"?