Re: [quicwg/base-drafts] Change Packet Number Gap to Packet Number Offset (#1307)

[MikkelFJ <[e-mail]>]

I suppose a more elaborate encryption would go like

```
uint32_t key[4];
hkdf(key, context);

uint32_t x;

    x = ((uint32_t)(size_t)PN) ^ key[0];
    x = ((x >> 16) ^ x) * UINT32_C(0x45d9f3b) ^ key[1];
    x = ((x >> 16) ^ x) * UINT32_C(0x45d9f3b) ^ key[2] ;
    x = ((x >> 16) ^ x) * UINT32_C(0x45d9f3b) ^ key[3];
    x = ((x >> 16) ^ x);
    return x;
```
Due to brute force the encryption is not strong, but combined with per path CID and update before 2^32 PN's it should be robust since the PN encryption is not privacy.

To speed things up, one or two multiplication rounds can be taken out.

See also
 http://stackoverflow.com/a/12996028

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