ZK-011: Add explicit domain separation tags to note commitments

[ANAVHEOBA]

Wave: PrivacyLayer ZK Wave 1
Issue Key: ZK-011
Area: commitment
Priority: High
Drips Complexity: High

Summary

Commitments should be domain-separated from Merkle node hashes and nullifier hashes so the protocol does not rely on positional conventions alone.

Relevant Code

• circuits/lib/src/hash/commitment.nr
• circuits/lib/src/hash/nullifier.nr
• circuits/lib/src/hash/pair.nr
• circuits/commitment/src/main.nr
• sdk/src/note.ts

Scope

• Define a domain-separation scheme for note commitments inside the hash input layout.
• Update the Noir hash helper and SDK commitment path together.
• Add regression tests that prove commitment, nullifier, and Merkle hashing occupy distinct domains.

Acceptance Criteria

• Commitment hashing uses an explicit domain-separated preimage format.
• Cross-stack fixtures are updated to the new encoding.
• Regression tests catch accidental reuse of the old hash domain.

Out of Scope

• Soroban contract changes
• Frontend and wallet UI work

Dependencies

• ZK-001
• ZK-009

References

• circuits/lib/src/hash/commitment.nr
• circuits/lib/src/hash/nullifier.nr
• circuits/lib/src/hash/pair.nr
• circuits/commitment/src/main.nr
• sdk/src/note.ts

[snakefood3232]

Ensuring domain separation for note commitments is crucial to eliminate reliance on positional conventions. I'd handle this by defining a domain-separation scheme, updating the Noir hash helper, and revising the SDK commitment path to meet the new criteria. With regression tests, we'll verify distinct domains for commitment, nullifier, and Merkle hashing. Can have a PR up in 3 days. I've previously implemented similar domain-separation schemes in cryptographic protocols, which aligns well with this task.

[soomtochukwu]

@soomtochukwu has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hello, you can trust me to take care of this ASAP

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