Re: [quicwg/base-drafts] GREASE the packet type octet (#311)

[ekr <[e-mail]>]

@huitema points out that we might want to have a keyed permutation because any static permutation is easily degreased. The observation here is that you can have a key that's shared across all of the connections, because we're not really worried about the middlebox connecting to the server and extracting the key for the transformation. With that in mind, here's an alternate keeed proposal.

```
1. Server generates a key K_static which it uses for all connections.
2. Packet has type P and last byte T
3. The server delivers K_static in its first flight.
4. For every packet, you compute P' = P ^(PRF(K, T) & 0x7f) where
    K = 0 for initial packets
    K = K_static for other packets.
```


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