Your payment system's cryptographic tools, like hash functions and block ciphers, rely on assumptions about mathematical hardness. When these assumptions fail, so does your PCI DSS compliance. A recent analysis revealed a full-round collision in a Poseidon hash function instance, showing how adaptive parameter selection can weaken security. The attack manipulated the relationship between fixed round constants and a chosen MDS (Maximum Distance Separable) matrix, achieving collisions across all sixteen output coordinates.
This is not just theoretical. Hash function collisions let attackers substitute malicious inputs for legitimate ones while producing identical outputs, compromising digital signatures, integrity checks, and encryption schemes that protect payment transactions.
What This Checklist Covers
This checklist helps you assess if your cryptographic implementations are vulnerable to parameter-selection attacks. It applies to systems using hash functions, permutations, or block ciphers in payment processing, tokenization engines, or PCI DSS Requirement 3.4 cryptographic key management.
Prerequisites
Before starting this audit:
- Inventory your cryptographic primitives: Document every hash function, block cipher, and permutation in your Cardholder Data Environment (CDE), including libraries, versions, and parameter sets.
- Identify construction-time decisions: Determine if your implementations allow runtime parameter selection, especially for linear layers, round constants, or S-box configurations.
- Map cryptographic dependencies: Trace which payment functions depend on each primitive, such as signature verification, HMAC generation, key derivation, and tokenization.
Cryptographic Collision Prevention Checklist
1. Parameter Selection Audit
Done when: You've verified that all cryptographic parameters were selected using a transparent, deterministic process before deployment.
□ Document the provenance of every MDS matrix in deployed hash functions
□ Confirm round constants were generated using a verifiable random process (e.g., digits of π)
□ Verify the parameter selection order: round constants must be fixed before linear layers are chosen
□ Review any custom or modified cryptographic implementations for adaptive parameter choices
Good looks like: A design document showing round constants derived from SHA-256 output of a public seed, with MDS matrix selection following a published algorithm that doesn't depend on those constants.
2. Collision Resistance Verification
Done when: You've confirmed your hash functions maintain collision resistance under your actual deployment parameters.
□ Test that your hash function produces different outputs for intentionally similar inputs
□ Verify no known collision attacks exist for your specific parameter set
□ Check whether your implementation uses parameter combinations documented in cryptographic literature
□ Review whether scalar constructions in your implementation reduce to low-degree equations that could admit parameter families
Good looks like: FIPS 186-4 approved hash functions (SHA-256, SHA-384) with no custom parameter modifications, or formally verified implementations of newer constructions.
3. Linear Layer Independence Check
Done when: You've established that your MDS matrices or linear layers cannot be manipulated to create prescribed collision trajectories.
□ Verify MDS matrices were not chosen to satisfy specific image constraints
□ Confirm linear layers don't allow "midpoint reset" patterns where one prescribed image cancels state against round constants
□ Check that half-difference trajectories through your permutation don't return to one-dimensional subspaces
□ Audit any feed-forward mechanisms for scalar recurrence patterns that could close collision differences
Good looks like: Standard constructions where the MDS matrix comes from a fixed mathematical structure with no degrees of freedom after round constants are set.
4. PCI DSS Cryptographic Control Mapping
Done when: You've verified that potential collision vulnerabilities don't undermine your PCI DSS Requirement 3 controls.
□ Identify which PCI DSS 4.0 requirements depend on collision resistance
□ Document fallback controls if your primary hash function is compromised
□ Verify your Hashed Index Table (HIT) implementation doesn't rely solely on collision resistance for PAN truncation security
□ Review whether your tokenization system could generate duplicate tokens if hash collisions occur
Good looks like: Layered controls where PAN truncation uses both hashing and additional entropy, and tokenization includes collision detection before token issuance.
5. Key Derivation Function (KDF) Review
Done when: You've confirmed that hash function collisions can't produce identical Data Encryption Keys (DEKs) from different inputs.
□ Audit KDF implementations for dependence on collision-resistant hash functions
□ Verify that Key Encryption Keys (KEKs) aren't derived using potentially vulnerable hash constructions
□ Check whether your HMAC implementations could produce identical authentication tags under collision scenarios
□ Test that different input key material produces cryptographically distinct output keys
Good looks like: NIST SP 800-108 KDF using SHA-256 in counter mode, with distinct context strings preventing cross-protocol key reuse even if collisions exist.
6. Digital Signature Validation
Done when: You've verified that collision attacks can't forge valid signatures on payment messages.
□ Confirm your signature schemes hash messages before signing
□ Test that identical hash outputs from different messages would be detected by your validation logic
□ Verify certificate chains in your payment gateway don't use deprecated hash functions
□ Review ISO 8583 message authentication for collision-resistant HMAC constructions
Good looks like: RSA-PSS or ECDSA signatures with SHA-256, and payment message authentication using HMAC-SHA-256 with unique keys per session.
7. Third-Party Cryptographic Library Assessment
Done when: You've evaluated all external cryptographic dependencies for collision vulnerabilities.
□ Run Software Composition Analysis (SCA) to identify cryptographic library versions
□ Check NIST's Cryptographic Algorithm Validation Program (CAVP) for your library versions
□ Verify your libraries don't implement custom Poseidon, Rescue, or other algebraic hash functions with adaptive parameters
□ Review whether your libraries allow runtime selection of MDS matrices or round constants
Good looks like: OpenSSL 3.x or BoringSSL with only FIPS 140-3 validated modules enabled, and no custom cryptographic implementations outside validated boundaries.
Common Mistakes
Assuming algorithm-level security implies implementation-level security: A hash function family may be secure in general while specific parameter choices create vulnerabilities. The KoalaBear Poseidon instance demonstrates this: the broader Poseidon construction isn't broken, but this specific parameter set with adaptive MDS matrix selection enabled a full-round collision.
Treating matrix-only checks as sufficient validation: Verifying that an MDS matrix meets distance properties doesn't capture adaptive correlations with round constants. Your validation must consider the interaction between all parameters.
Ignoring parameter selection order: If your implementation allows the MDS matrix to be chosen after round constants are fixed, you've created the exact vulnerability that enabled the collision attack. This ordering matters.
Relying on collision resistance alone for PCI DSS compliance: PCI DSS Requirement 3.4 requires strong cryptography, but collision-resistant hashing is just one component. Your compliance posture must include key management, access controls, and monitoring, not just cryptographic primitives.
Next Steps
If you've identified potential vulnerabilities:
Immediate: Disable any custom hash function implementations with adaptive parameter selection until you can verify their security.
Short-term (30 days): Migrate high-risk functions (PAN hashing, key derivation, signature generation) to FIPS 140-3 validated implementations with fixed, published parameters.
Long-term (90 days): Establish a cryptographic review process requiring formal security proofs for any non-standard constructions, and mandate that parameter selection follows transparent, deterministic procedures.
Your payment security doesn't rest on a single hash function. But when cryptographic primitives fail, they fail silently until an attacker exploits the collision you didn't know existed. This checklist helps you find those weaknesses before someone else does.
