Reduce ChaCha rounds for Turbine from 20 to 8

Summary

Reduce the number of ChaCha rounds for turbine weighted shuffle calculation from 20 to 8 in order to reduce the amount of compute time spent on this.

Motivation

Cheap, easy performance win for Turbine. This doesn't need to be ultra secure. Just needs to be random enough to prevent malicious nodes from censoring blocks.

New Terminology

None

Detailed Design

• Add a feature for controlling switching to 8 ChaCha rounds.
• Pull feature state to determine if we should perform 20 rounds (legacy) or 8 rounds (after feature activation) of ChaCha
  • Broadcast stage
  • Retransmit stage
  • Retransmit signature check
• Use the specified ChaCha rounds for computing turbine tree weighted shuffle of staked peers before determining parent/children.

Alternatives Considered

What alternative designs were considered and what pros/cons does this feature have relative to them?

Impact

Should be unnoticeable by most. Small performance improvement in ability to send block data out.

Security Considerations

If someone could predictably grind and influence turbine tree to ensure some malicious minority of stake could censor unrecoverable portions of a block, this would be a problem. Today, the grinding can be done, which means further ChaCha rounds provide very limited beenfit, but the ability to influence the generated tree is clamped down because the input data includes:

• shred index
• shred slot
• shred type (coding or data)
• leader node