fix(drand): verify pulse randomness matches sha256(signature)

[loom-agent]

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Release Notes

• Fixed a randomness-injection attack on the drand pallet where a valid BLS signature paired with a forged randomness field could bias the pallet's published randomness.

Problem

The drand protocol specifies that randomness = sha256(signature). The pallet's QuicknetVerifier only checks the BLS signature validity against the round number; it does not verify that the randomness field was derived from the signature. An attacker can submit a real drand signature with a forged randomness value, and the pallet will accept and store it, biasing or fully controlling the randomness used by downstream consumers.

Root Cause

In pallets/drand/src/verifier.rs, the QuicknetVerifier::verify method returns Ok(true) based solely on the BLS pairing check. The randomness field is never validated against sha256(signature).

The Fix

Add a second check in QuicknetVerifier::verify: compute sha256(signature) and compare it against pulse.randomness. Return Ok(false) on mismatch, which causes write_pulse to skip storing the forged pulse without erroring the extrinsic.

Testing

Added audit_probe_forged_randomness_rejected which submits a valid drand signature paired with a forged randomness (32 bytes of 0x42). The test verifies that the pulse is NOT stored in Pulses and LastStoredRound is not advanced, confirming the forged pulse was rejected.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

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