fix: increase MAX_SIG_STRUCTURE_SIZE to 8192

[newtsjamie]

Summary

• Increase MAX_SIG_STRUCTURE_SIZE from 5120 to 8192 in all 3 proof_b circuits
• CapCut videos have sig_structure of 5241 bytes (RSA-4096 GlobalSign intermediate), exceeding the 5120 limit

Rationale

8192 chosen to handle:

• Current: CapCut ~5241, Adobe ~4400, ChatGPT ~2700
• Future: RSA-4096 3-cert chains up to ~6400 bytes
• Enterprise: 4-cert chains up to ~8000 bytes

sha256_var() only hashes actual bytes — unused buffer costs zero constraints. Only signers with larger chains pay extra proving cost.

Test plan

• Compile all 3 circuits with nargo compile
• Verify existing signers (Adobe, ChatGPT, Leica, Google) still prove correctly
• Verify CapCut video (5241 byte sig_structure) proves successfully

🤖 Generated with Claude Code

Summary by CodeRabbit

• Enhancements
  • Increased maximum buffer capacity for signature structures to support larger certificate chains and complex cryptographic configurations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a74fbc3c-36ad-415b-afa4-42b372bebdf3

📥 Commits

Reviewing files that changed from the base of the PR and between 8e7db67 and 7ac06dd.

📒 Files selected for processing (3)

• proof_b/src/main.nr
• proof_b_es256/src/main.nr
• proof_b_ps256/src/main.nr

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📝 Walkthrough

Walkthrough

Three Noir modules increased the MAX_SIG_STRUCTURE_SIZE constant from 5120 to 8192 bytes, expanding the buffer capacity for CBOR-encoded COSE Sig_structure. Comments were updated to reflect the new sizing basis. No logic, assertions, or interface signatures were altered.

Changes

Cohort / File(s)	Summary
Buffer Size Configuration proof_b/src/main.nr, proof_b_es256/src/main.nr, proof_b_ps256/src/main.nr	Increased MAX_SIG_STRUCTURE_SIZE from 5120 to 8192 bytes and updated associated documentation to reflect revised sizing rationale based on DER-encoded certificate chain dimensions.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 Buffers grow in threes today,
5120 hops to 8192's way,
More room for certs and sig's embrace,
A hoppy change with breathing space! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately captures the main change: increasing MAX_SIG_STRUCTURE_SIZE to 8192 across all three proof_b circuits. It is specific, concise, and clearly communicates the primary modification.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/increase-sig-structure-size

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