Re: [quicwg/base-drafts] Remove the explicit packet number gap in favor of one computed (#496)

[Martin Thomson <[e-mail]>]

martinthomson commented on this pull request.



> @@ -1296,6 +1291,22 @@ connection ID, it should expect packets on the new connection ID to start at 18.
 A packet with the new connection ID and a packet number of 17 is discarded as
 being in error.
 
+#### Packet Number Gap
+
+In order to avoid linkage, the packet number gap MUST be externally
+indistinguishable from random. The packet number gap for a connection
+ID with sequence number is computed by encoding the sequence number
+as a 32-bit integer in big-endian format, and then computing:
+
+~~~
+Gap = HKDF-Expand-Label(packet_number_secret,
+                        "QUIC packet sequence gap", sequence, 4)

If we move packet protection to the transport, as planned, then we'll need HKDF, so there's no real issue with taking this change as it is.

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