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

[Jana Iyengar <[e-mail]>]

janaiyengar approved this pull request.

A few nits. I've asked @kazuho if he could take another look as well.

> +corresponding key and IV are created from that secret as defined in
+{{protection-keys}}.  The header protection key is not updated.
+
+For example, to update write keys with TLS 1.3, HKDF-Expand-Label is used as:
+
+~~~
+secret_<n+1> = HKDF-Expand-Label(secret_<n>, "quic ku",
+                                 "", Hash.length)
+~~~
+
+The endpoint toggles the value of the Key Phase bit and uses the updated key and
+IV to protect all subsequent packets.
+
+An endpoint MUST NOT initiate a key update prior to having received an
+acknowledgment for a packet that it sent protected with keys from the current
+key phase.  This ensures that keys are available to both peers before another

```suggestion
key phase.  This ensures that keys are available to both peers before another key update
```

> +receiving packets.  These keys will be needed to process packets the peer sends
+after updating.
+
+An endpoint SHOULD retain old keys so that packets sent by its peer prior to
+receiving the key update can be processed.  Discarding old keys too early can
+cause delayed packets to be discarded.  Discarding packets will be interpreted
+as packet loss by the peer and could adversely affect performance.
+
+
+## Responding to a Key Update
+
+A peer is permitted to initiate a key update after receiving an acknowledgement
+of a packet in the current key phase.  If a packet is received with a key phase
+that differs from the value the endpoint used to protect the last packet it
+sent, the endpoint uses the next packet protection keys for reading and the
+corresponding key and IV; see {{receive-key-generation}} for considerations

It might be kinder to the reader if you could split this 3-line sentence into two sentences.

> +its peer has updated keys twice without awaiting confirmation.  An endpoint MAY
+treat consecutive key updates as a connection error of type KEY_UPDATE_ERROR.
+
+An endpoint that receives an acknowledgement that is carried in a packet
+protected with old keys where any acknowledged packet was protected with newer
+keys MAY treat that as a connection error of type KEY_UPDATE_ERROR.  This
+indicates that a peer has received and acknowledged a packet that initiates a
+key update, but has not updated keys in response.
+
+
+## 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 yet permitted; using dummy keys

```suggestion
place of discarded keys when key updates are not yet permitted. Using dummy keys
```

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