Re: [quicwg/base-drafts] Rewrite key update section (#3050)

[Martin Thomson <[e-mail]>]

martinthomson commented on this pull request.

Thanks David.

> @@ -1168,89 +1168,232 @@ anticipation of receiving a ClientHello.
 
 # Key Update
 
-Once the handshake is confirmed, it is possible to update the keys. The
-KEY_PHASE bit in the short header is used to indicate whether key updates
-have occurred. The KEY_PHASE bit is initially set to 0 and then inverted
-with each key update.
+Once the 1-RTT keys are established and confirmed, it is possible to update the

I think that this is fine.  The reason we needed that definition is that we were hanging a lot from it.  This should however be referencing that definition, because it's handshake confirmation that I intended to use (but failed to).

> +
+## Timing of Receive Key Generation {#receive-key-generation}
+
+Endpoints responding to an apparent key update MUST NOT generate a timing
+side-channel signal that might indicate that the Key Phase bit was invalid (see
+{{header-protect-analysis}}).  Endpoints can use dummy packet protection keys in
+place of discarded keys when key updates are not permitted; using dummy keys
+will generate no variation in the timing signal produced by attempting to remove
+packet protection, but all packets with an invalid Key Phase bit will be
+rejected.
+
+The process of creating new packet protection keys for receiving packets could
+reveal that a key update has occurred.  An endpoint MAY perform this process as
+part of packet processing, but this creates a timing signal that can be used by
+an attacker to learn when key updates happen and thus the value of the Key Phase
+bit in certain packets.  Endpoints SHOULD instead defer the creation of the next

This is READ keys, you update write keys immediately.

>  
-In deciding when to update keys, endpoints MUST NOT exceed the limits for use of
-specific keys, as described in Section 5.5 of {{!TLS13}}.
+## Initiating a Key Update {#key-update-initiate}
+
+Endpoints maintain separate read and write secrets for packet protection.  An
+endpoint initiates a key update by updating its packet protection write secret
+and using that to protect new packets.  The endpoint creates a new write secret
+from the existing write secret as performed in Section 7.2 of {{!TLS13}}.  This
+uses the KDF function provided by TLS with a label of "quic ku".  The

See #3054.

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