Re: [quicwg/base-drafts] Define an anti-forgery limit (#3620)

David Schinazi <> Fri, 08 May 2020 22:49 UTC

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Date: Fri, 08 May 2020 15:48:57 -0700
From: David Schinazi <>
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Subject: Re: [quicwg/base-drafts] Define an anti-forgery limit (#3620)
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@DavidSchinazi commented on this pull request.

This feels like playing with fire to me. I think I'd almost prefer to remove CCM from the spec entirely, and add it as an extension document once there exists a peer-reviewed analysis that doesn't have the 2^14 record size caveat.

> +AEAD_AES_128_CCM, but the analysis in {{ccm-bounds}} shows that a limit of 2^23
+packets can be used to obtain the same confidentiality protection as the limits
+specified in TLS.
+The usage limits defined in TLS 1.3 exist to provide protection against attacks
+on confidentiality and apply to successful applications of AEAD protection. The
+integrity protections in authenticated encryption also depend on limiting the
+number of attempts to forge packets. TLS achieves this by closing connections
+after any record fails an authentication check. In comparison, QUIC ignores any
+packet that cannot be authenticated, allowing multiple attempts at defeating
+integrity protection.
+Endpoints MUST count the number of packets that are received but cannot be
+authenticated.  If the number of packets that fail authentication exceeds a
+limit that is specific to the AEAD in use, the endpoint MUST immediately close
+the connection.  Endpoints MUST initiate a key update before reaching this

This text feels unclear to me. It could be interpreted as "you MUST close the connection and initiate key rotation" which is silly. I think what's missing is the fact that the counter of invalid packets received is rest on key rotation. But if we have a "MUST initiate rotation", what's the point of the "MUST close the connection"?

> +successfully forge a packet; see {{AEBounds}} and {{ROBUST}}.
+For AEAD_AES_128_GCM, AEAD_AES_256_GCM, and AEAD_CHACHA20_POLY1305, the limit on
+the number of packets that fail authentication is 2^36.  Note that the analysis
+in {{AEBounds}} supports a higher limit for the AEAD_AES_128_GCM and
+AEAD_AES_256_GCM, but this specification recommends a lower limit.  For
+AEAD_AES_128_CCM, the limit on the number of packets that fail authentication
+is 2^23.5; see {{ccm-bounds}}.
+: These limits were originally calculated using assumptions about the
+  limits on TLS record size. The maximum size of a TLS record is 2^14 bytes.
+  In comparison, QUIC packets can be up to 2^16 bytes.  However, it is
+  expected that QUIC packets will generally be smaller than TLS records.
+  Where packets might be larger than 2^14 bytes in length, smaller limits might

This sounds scary to me. It feels like the spec is saying `Computing the limits for larger packet sizes is left as an exercise to the reader.` As a reader of this specification, I personally do not feel competent to get that right.

> @@ -2029,6 +2079,104 @@ ffff00001b0008f067a5502a4262b574 6f6b656ea523cb5ba524695f6569f293
+# Analysis of Limits on AEAD_AES_128_CCM Usage {#ccm-bounds}

Would it make sense to move this to an appendix? Ideally I would prefer for this to not be in the draft at all, and instead have a reference to a peer-reviewed paper, but I suspect that doesn't exist today.

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