The reliable way to audit cryptography in a smart-contract system is to turn every security claim into a documented, testable assumption. Start with the OWASP Smart Contract Security Verification Standard (SCSVS) to choose applicable requirements, use the OWASP Smart Contract Security Testing Guide (SCSTG) to design repeatable tests, and supplement them with the broader OWASP Application Security Verification Standard (ASVS) cryptography controls where the architecture includes off-chain services. The result should show which primitives, keys, signatures, hashes and randomness sources exist; what each is supposed to protect; who controls them; and what happens when inputs are malformed, replayed or unavailable.
What a cryptographic-assumption audit actually examines
A cryptographic audit is not a search for a particular “approved” algorithm. It asks whether the protocol’s security promises follow from the exact primitives, encodings, key handling and trust boundaries implemented in code.
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For every cryptographic operation, write down four things:
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- The mechanism: the hash, signature scheme, commitment, random-value source, encryption operation, precompile or library call.
- The claimed property: for example, collision resistance, second-preimage resistance, signer authenticity, message integrity, unpredictability or replay prevention.
- The boundary: which contract, account, relayer, oracle, bridge, signer or off-chain service is trusted to perform or verify it.
- The failure consequence: what value, authorization, payout, vote or state transition becomes possible if the assumption fails.
This approach catches problems that an algorithm checklist misses. A strong hash can still protect the wrong bytes. A valid signature can still authorize the wrong contract or chain. A source of random-looking block data can still be predictable to an actor who can influence transaction ordering. A secure off-chain key can still be used with a replayable message.
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Use the standards in the right order
OWASP’s SCSVS is primarily a requirements baseline for Solidity contracts deployed on EVM-based blockchains. Select the SCSVS requirements that match the system, then use the SCSTG as the practical testing manual. OWASP describes the SCSTG as mapping tests to SCSVS requirements and to weaknesses catalogued as SCWE. The SCSTG page was last updated on August 19, 2026.
The OWASP ASVS cryptography chapter is broader application-security guidance. It is useful for signers, APIs, custody services, key-management systems and other off-chain components, but its controls should not be presented as EVM-specific rules. Adapt each control to the chain, virtual machine and threat model.
| Material | Use it for | Important limitation |
|---|---|---|
| SCSVS | Selecting smart-contract security requirements and architecture checks. | Written mainly for Solidity and EVM-based deployments; another virtual machine requires adaptation. |
| SCSTG | Repeatable manual processes, techniques, tools and test cases for verifying SCSVS requirements. | A test must still be adapted to the actual chain, contract design and deployment. |
| SCWE weakness catalog | Classifying findings consistently when a weakness matches an entry. | Not every protocol-specific cryptographic bug will have an exact catalog entry. |
| ASVS 5.0 cryptography controls | Prompts for inventories, validated implementations, random generation, key lifecycle, constant-time operations and secure failure handling. | General application guidance, not a complete smart-contract encoding or randomness specification. |
Keep a traceability column in your workpapers: SCSVS requirement, SCSTG test, relevant SCWE weakness (if any), code location, deployment version and result. This prevents a review from becoming an unstructured list of impressions.
Step 1: Establish scope and the threat model
Freeze the system version before testing. Record the chain and network, virtual machine, Solidity compiler and optimizer settings, dependency versions, deployed bytecode, verified source correspondence, proxy or upgrade model, and all external contracts called by the system. Include the assets at risk: funds, collateral, voting power, identity claims, randomness outcomes, bridge messages or any other value whose integrity depends on cryptography.
Then draw two boundaries on one diagram. The first is the on-chain boundary: contract storage, calldata, logs, block fields, precompiles and calls to external contracts. The second is the off-chain boundary: wallets, signing services, relayers, oracles, bridge agents, key-management systems, APIs and monitoring. Mark every point where data or authority crosses between them.
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Questions the diagram must answer
- Which actor creates each signature, and which actor is allowed to submit it?
- What exact data is signed, and where is it encoded before signing?
- Which contract, account, chain identifier, nonce, deadline or domain value binds the signature to its intended context?
- What state change makes an authorization one-time, and can a failed transaction consume or preserve that state?
- Can a relayer, oracle, bridge or external contract alter the response that the verifier trusts?
- Who can upgrade the verifier or replace a dependency, and what cryptographic assumptions change when that happens?
Include multi-signature components, bridges and oracle callbacks in scope. A contract may verify a signature correctly while still trusting an unvalidated external response or an upgrade administrator who can replace the verification logic.
Step 2: Build a complete cryptographic inventory
Do not rely on architecture diagrams alone. Search the source, generated artifacts, deployment scripts, configuration, dependency lockfiles and off-chain services. Compare what is documented with what is deployed and executed.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems| Inventory field | What to record |
|---|---|
| Primitive and implementation | Algorithm, curve or parameter set, precompile, library function or custom code; include the exact dependency and version. |
| Purpose and property | Authentication, integrity, collision resistance, second-preimage resistance, commitment binding, unpredictability, confidentiality or another stated goal. |
| Input format | Exact bytes, ABI types, field order, length prefixes, domain or context fields, canonicalization and handling of empty values. |
| Output and consumer | Digest, signature, recovered address, random value, commitment or ciphertext; identify every contract or service that consumes it. |
| Key information | Key owner, generation method, storage location, rotation authority, expiry, recovery path and compromise response. |
| Trust assumption | Who must remain honest or available, and what an attacker gains if that assumption fails. |
| Lifecycle and version | When the primitive was introduced, whether it is upgradeable, and whether old data remains valid after migration. |
Inventory verification should identify deprecated libraries, copied cryptographic code and protocol-mandated legacy primitives. A legacy choice required for interoperability needs a compensating control and a migration plan; a legacy choice left over from an earlier design should be removed rather than normalized.
Include operations performed entirely off-chain. Key derivation, custody, encryption, signature aggregation and random-value generation can determine whether an on-chain check is meaningful even when no secret appears in Solidity.
Step 3: Trace every signature and authorization
For each signed action, follow the message from the business event to the signer, through serialization and transport, into the contract verifier, and finally into the state update. Document the message as a byte-level structure, not just as a list of field names.
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- Identify the authorization statement. State exactly what the signer intends: a transfer, order, vote, permit, withdrawal, bridge message or administrative change.
- Capture the canonical encoding. Record field types, order, widths, lengths, nested structures, integer interpretation, text normalization and any type or domain separator.
- Check context binding. Verify that the signed data is bound to the intended account, contract, function or action, chain and time window. A signature valid in one context should not silently authorize another.
- Compare signer and verifier logic. Ensure the off-chain encoder and on-chain decoder construct identical bytes and apply the same rules for omitted, zero, negative, oversized or duplicate values.
- Inspect recovery and validity checks. Test malformed signatures, invalid points or parameters where applicable, zero addresses, unexpected recovery results, duplicate signers and signatures with unusual but technically accepted values.
- Follow nonce and state timing. Determine when a nonce is consumed, whether duplicate submissions are rejected, and whether a revert leaves the authorization usable as intended.
- Test failure paths. Confirm that invalid signatures, missing signers, expired deadlines and malformed calldata cannot trigger a partial state change or an unsafe fallback.
Replay scenarios to test explicitly
- Submitting the same signed message twice to the same contract.
- Submitting it to a different contract or proxy instance.
- Replaying it on another chain, fork or network with a different chain identifier.
- Using an authorization for a different account, function, token, asset or recipient.
- Submitting it after its deadline, after a nonce has advanced, or after a signer has been removed.
- Reusing a bridge or oracle message after a partial failure or relayer replacement.
Replay and signature forgery are authentication threats identified by OWASP. The practical encoding checks above are audit inferences: the applicable signature standard and implementation still determine which canonicalization and malleability rules apply.
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List every outcome that depends on unpredictability: lotteries, game results, validator or committee selection, secret generation, randomized allocation, reveal ordering and anti-front-running mechanisms. For each, identify when the random value is fixed, who can observe it, and who can influence the inputs before it is fixed.
Threat questions for an on-chain source
- Can a block producer influence or withhold the block data used by the calculation?
- Can a trader or relayer choose transaction ordering, timing or inclusion to improve its result?
- Can an external callback, oracle or bridge response be delayed, replaced or selectively withheld?
- Does the contract expose a value before all parties have committed, allowing prediction or selective reveal?
- What happens if a participant never reveals, a callback fails, or the randomness provider becomes unavailable?
OWASP treats secure random-number generation as an essential cryptographic practice. The ASVS 5.0 general control says non-guessable random values should use a cryptographically secure random number generator and have at least 128 bits of entropy. That is not a mechanical recipe for public blockchains: a blockchain protocol needs a randomness mechanism appropriate to its chain, timing model and adversary. State precisely whether the design seeks unpredictability, bias resistance, liveness, or all three.
Test the complete commit, reveal, callback or beacon protocol, including aborted rounds and adversarial scheduling. A source can be unpredictable to ordinary users yet biasable by an actor who controls a validator, relayer or reveal decision.
Step 5: Review hashes and commitments at the byte level
For every hash, write the required property beside the call. Collision resistance, second-preimage resistance, integrity checking and commitment binding are different claims. A digest length that is adequate for one claim may not meet another, and a strong digest cannot repair ambiguous input encoding.
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OWASP ASVS 5.0 states that when collision resistance is required, the output must be at least 256 bits; when only second-preimage resistance is required, the output must be at least 128 bits. These are general cryptographic controls, not proof that a particular protocol’s encoding or chain-specific hash semantics are safe.
Encoding checks
- Enumerate the exact bytes hashed, including field boundaries, array lengths, type information and context.
- Look for ambiguous concatenation in which two different logical tuples produce the same byte string.
- Check whether addresses, integers, signed values and variable-length data are represented canonically.
- Confirm that a commitment binds every value that later affects settlement, ownership, payout or authorization.
- Verify that a hash is not being treated as encryption or as a source of fresh randomness.
- Test empty, maximum-length, duplicate and reordered inputs, plus values that differ only in type or leading zeros.
When a protocol uses a hash as a commitment, document when the committed value becomes public, how the opening is checked, whether a salt is required, and what happens if an opener refuses to reveal. A collision-resistant hash does not by itself guarantee fairness or liveness.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Step 6: Examine implementations, keys and failure behavior
Prefer a recognized, industry-validated library, platform primitive or precompile over custom cryptographic code. Record exact call behavior, parameter validation, return values and version constraints. OWASP ASVS calls for industry-validated implementations, constant-time cryptographic operations and secure failure handling. Constant-time behavior is especially relevant to off-chain components handling secrets; on-chain execution is public and deterministic, so a Solidity contract cannot conceal state merely by rearranging operations.
Key-management questions
- How is each key generated, and which entropy source is used?
- Where is it stored: a signer service, hardware-backed system, deployment account, validator, operator workstation or contract storage?
- Who can use, rotate, revoke or recover it, and are those powers separated?
- What prevents an old key from remaining authorized after rotation?
- How are backups protected, tested and destroyed when no longer needed?
- What is the documented procedure for suspected compromise, lost access, emergency pause and contract upgrade?
Maintain a key inventory and a written lifecycle policy. Adapt general ASVS lifecycle guidance to the contract’s operational model: an immutable contract may need an explicit signer-set transition, while an upgradeable system must protect both the implementation and the authority that changes it.
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Exercise invalid lengths, malformed signatures, unsupported curves or parameters, zero values, duplicate entries, out-of-range integers, failed precompile calls, empty return data and unexpected revert behavior. Confirm that errors fail closed and do not leave partially updated authorization, nonce or accounting state. Test dependency failure as well as dependency success: an external verifier, oracle or bridge call that returns an unexpected value must not be interpreted as approval.
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Step 7: Turn the review into reproducible tests and findings
Use SCSTG test cases as a repeatable plan, then record enough detail for another auditor to reproduce each result. Every test record should contain:
- Requirement and weakness mapping: the SCSVS requirement, SCSTG test and SCWE entry when applicable.
- Code identity: repository commit, deployed bytecode or verified source, compiler and dependency versions.
- Preconditions: chain, accounts, signer keys or test fixtures, contract state and external-service assumptions.
- Procedure: the transaction sequence, signed bytes, calldata, block conditions and expected failure or success.
- Observed result: event logs, return values, state changes, gas or revert data relevant to the finding.
- Impact and exploit conditions: the asset or authorization affected and the actor capabilities required.
- Limitation: what was not tested, such as unavailable production keys, proprietary signer code or a different deployment.
Separate a cryptographic defect from an unverified assumption. “The contract uses a 256-bit hash” is an observation; “the commitment is binding” is a claim that also depends on encoding, salt handling, reveal rules and the threat model. Report the evidence and the remaining assumption.
How to compare audit approaches
If you are choosing between an internal review, a specialist assessment or a standards-driven test campaign, compare the method rather than a marketing label. The OWASP materials support four useful axes:
| Axis | What good coverage looks like | Warning sign |
|---|---|---|
| Requirement coverage | Each applicable SCSVS requirement has an owner, test or justified exclusion. | A report lists tools or vulnerabilities without showing which requirements were examined. |
| Platform relevance | Tests reflect the actual chain, virtual machine, precompiles, upgrade pattern and transaction model. | Generic EVM assumptions are applied unchanged to a different platform. |
| Reproducibility | Signed bytes, fixtures, versions, preconditions and expected outcomes are preserved. | Results depend on an undocumented script, private key or one-time environment. |
| Boundary coverage | On-chain contracts and off-chain signers, relayers, oracles, bridges and custody are reviewed together. | The contract verifier is tested while message construction and key operations are assumed trustworthy. |
Neither SCSVS nor SCSTG provides a comparative ranking of audit firms or commercial tools. A standards mapping improves completeness and accountability; it does not replace protocol-specific reasoning.
Final cryptographic-audit checklist
- The chain, virtual machine, compiler, dependencies, deployment and upgrade model are recorded.
- Every hash, signature, random source, commitment, encryption operation, precompile and custom primitive appears in an inventory.
- Each inventory item has a stated security property, exact inputs and outputs, owner, version and trust assumption.
- Every signed action has a canonical byte representation and explicit account, contract, chain, action, nonce and time context where required.
- Replay, forgery, malformed input, duplicate submission, failed validation and partial-state scenarios have been executed.
- Randomness tests address prediction, bias, transaction ordering, block influence, selective reveal and provider failure.
- Hash tests distinguish collision resistance from second-preimage resistance and verify unambiguous encoding.
- Key generation, storage, rotation, revocation, backup, recovery and compromise response are documented.
- Recognized implementations and exact versions are identified; custom or deprecated cryptography has a justified reason and mitigation.
- Failure paths fail closed, and external responses are validated before they affect authorization or accounting.
- Findings map to SCSVS and SCSTG material where applicable and include reproducible evidence, impact and limitations.
A completed audit should leave the protocol team able to answer one practical question for every cryptographic assumption: “What exact bytes, key, actor, state and failure behavior make this security claim true?” If any part of that answer is undocumented or untested, the assumption remains an audit item rather than a security guarantee.
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