fix DSC Key Smuggling via Unsigned TBS Data [LA-A]

[x-senpai-x]

Summary

This PR addresses the DSC Key Smuggling via Unsigned TBS Data audit finding. It includes:

1. Zero-padding constraint (1300-byte path)

• Adds check_zero_padding(tbs_certificate, tbs_certificate_len) in t_add_dsc_verify_1300 immediately after the length bounds check
• Imports utils::check_zero_padding, which already exists and is used in the 720-byte path

2. Constrained ASN.1 length validation (both DSC circuits)

• Adds get_asn1_element_length constrained utility to utils/utils/src/lib.nr — computes total TLV length from a DER element header without unconstrained/unsafe
• Both t_add_dsc_720 and t_add_dsc_verify_1300 now validate the prover-supplied tbs_certificate_len against the value derived from the DER SEQUENCE header, preventing a malicious prover from lying about the signed length

3. Pubkey signed-length bound check

• verify_rsa_pubkey_in_tbs now accepts tbs_certificate_len and asserts pubkey_offset + DSC_KEY_SIZE <= tbs_certificate_len, ensuring the key lies within the authenticated region of the TBS, not just within the fixed-size buffer
• Both t_add_id_data_720 and t_add_id_data_1300 compute tbs_certificate_len from the DER header and pass it through

4. DER SubjectPublicKeyInfo structural validation

• verify_rsa_pubkey_in_tbs now validates that the modulus is wrapped in a proper DER INTEGER (tag, long-form length, optional leading zero) and that the INTEGER is the first child of an RSAPublicKey SEQUENCE (30 82)
• Encoding form (leading-zero vs. no-leading-zero) is inferred from TBS bytes rather than from dsc_pubkey, preventing a prover from manipulating which path is taken

5. Modify ProverInputs and remove signed_attributes_size field

• Matched the inputs according to the ones generated using passport-input-gen library as they follow ASN.1 parsing.

---

Background

The 1300-byte DSC registration circuit splits SHA-256 hashing across two sub-circuits (t_add_dsc_hash_1300 + t_add_dsc_verify_1300) using a pre-computed hash. Unlike the 720-byte path — which calls sig_check_rsa::verify_signature → sha256_and_check_data_to_sign → check_zero_padding automatically — the 1300-byte path passes a pre-computed msg_hash: [u8; 32] directly to fragmented_sig_check_rsa::verify_rsa_signature, which never sees the raw TBS buffer. This meant the zero-padding guard was never applied.

---

Note

The following items from the audit are deferred to subsequent PRs:

Binding tbs_certificate_len into the commitment chain (if needed)
Audit suggestion 1 (separate PR)

[Copilot]

Pull request overview

This PR closes a security gap in the 1300-byte DSC verification circuit by ensuring the padded portion of the fixed-size TBS buffer is constrained to zero, preventing attacker-controlled trailing bytes from influencing downstream commitments while not being covered by the signature/hash path.

Changes:

• Import and apply utils::check_zero_padding to tbs_certificate in t_add_dsc_verify_1300 right after the tbs_certificate_len bounds check.
• Add an explanatory comment clarifying why the constraint is required for commitment safety.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.