Enforce 32 data + 32 coding shreds

Summary

All clients currently send 32 data + 32 coding shreds, however Turbine still allows to send and receive a variable number of shreds. With this change we will enforce sending and receiving only 32 data + 32 coding shreds.

Motivation

It is inconvinient to support many combinations of data + coding shreds. Even the logic to validate if a shred index is valid or not is complex because it requires to receive a coding shred for the FEC set to know the index boundaries. With fixed 32 data + 32 coding shreds this logic becomes trivial.

There's no security benefit in variable number of data + coding shreds, and in fact security is reduced when the number of shreds is low.

With fixed 32 data + 32 coding shreds, equivocation detection is simplified because it's sufficient to receive any two shreds in the same FEC set with different Merkle roots and valid signatures.

New Terminology

N/A

Detailed Design

A sender should always produce 32 data shreds + 32 coding shreds per FEC set (this is currently already happening).

Receivers currently accept FEC sets with variable number of shreds.

If enforce_32_data_shreds: <PUBKEY> is active, then any FEC set with a number of shreds different than 32 data + 32 coding will be dropped on ingest.

As a result, the FEC set payload must be exactly equal to 31840 bytes (with 995 bytes of payload per data shred).

Alternatives Considered

Leave as is.

Impact

Clients will no longer accept FEC sets (thus blocks) with any number of shreds different than 32 data + 32 coding.

Security Considerations

Security is improved since the (minimum) number of shreds is now 32 + 32, validating shred indexes is trivial and equivocation detection is simplified.

Backwards Compatibility

This feature is not backwards compatible.