Increase Ciphertext size limit

[wangshan]

Current HPKE Ciphertext size is limited to 2 bytes, about 65KB, many use cases will need more than this, Prio for eg. doesn't have this limit. Can we change this to 4 bytes so the size can be <1..2^32-1>?

struct {
  HpkeConfigId config_id;    // config ID
  opaque enc<1..2^16-1>;     // encapsulated HPKE key
  opaque payload<1..2^16-1>; // ciphertext
} HpkeCiphertext;

[chris-wood]

@wangshan I'm not opposed to bumping this limit since it's a painless change, but what use cases would require individual reports to contain more than 65KB of data? Could you please propose a PR that lifts the limit from 2^16 to 2^32?

[wangshan]

@chris-wood will do.
Use case wise, 65KB can only represent 4B int vector of 16K dimension, if we encode correlations in individual reports then it can easily be breached, for eg. if I want to see histogram of errors by locales, let's say I have 100 error codes and 200 locales, that's already 20K dimension in one hot encoding.

[wangshan]

PR created: #279