SIMD-0317: Enforce 32 data + 32 coding shreds (#317)

* SIMD-0317: Enforce 32 data + 32 coding shreds

* address comments

proposals/0317-enforce-32-data-shreds.md

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+---
+simd: '0317'
+title: Enforce 32 data + 32 coding shreds
+authors:
+  - Emanuele Cesena
+category: Standard
+type: Core
+status: Review
+created: 2025-07-08
+feature:
+---
+
+## 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.