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A sound cryptographic audit starts by making every assumption explicit: which algorithms and keys exist, what each signature authorizes, where randomness comes from, what a hash is meant to guarantee, and how invalid input fails. Use the OWASP Smart Contract Security Verification Standard (SCSVS) to choose requirements, then use the OWASP Smart Contract Security Testing Guide (SCSTG) to turn those requirements into repeatable tests. Add the broader cryptography controls from the OWASP Application Security Verification Standard (ASVS) where they fit the system, especially in off-chain signers, relayers, oracles, and key-management services.
The workflow below is written for Solidity and EVM systems, while identifying the points that must be adapted for other chains or virtual machines. It follows the cryptographic data from design documentation through deployed bytecode, off-chain services, and operational key custody.
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1. Set the scope and threat model before reading cryptographic code
Begin with a system record, not a list of suspected bugs. Capture the target chain and virtual machine, Solidity compiler and library versions, deployed addresses, deployed bytecode, source-code correspondence, upgrade mechanism, and every external dependency. Record the assets at risk: user balances, governance power, liquidation rights, game outcomes, bridge messages, or any other value whose integrity depends on cryptography.
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SCSVS is aimed primarily at Solidity contracts on EVM-based blockchains. Its requirements are therefore a useful baseline for an EVM review, not a universal rulebook for every chain. SCSTG supplies practical processes, techniques, tools, and test cases that map to SCSVS requirements and SCWE weaknesses. For non-EVM systems, retain the reasoning but replace EVM-specific tests with tests appropriate to that platform.
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Draw the trust boundaries
Make a diagram that separates on-chain code from every off-chain cryptographic component. Mark the signer that creates each message, the actor that submits it, relayers, oracle operators, bridge endpoints, multisignature participants, external contracts, and any service that generates or stores secrets. For each boundary, state what is trusted, what can be delayed or censored, and what can be malicious.
For each signed action, answer four questions in the diagram: who creates the signature, who can submit it, exactly what bytes are signed, and which state change makes the authorization one-time or otherwise bounded. Include chain identifiers, contract addresses, account identities, nonces, deadlines, and action types whenever the protocol relies on them. Their presence must be verified against the actual encoding and contract logic rather than assumed from a user-interface description.
Define adversary capabilities
Write down whether an attacker can observe pending transactions, reorder or delay submissions, control a relayer, influence an oracle callback, compromise one signer, call an external contract, or supply malformed calldata. A randomness design that is adequate against a passive observer may fail when a block producer, transaction-ordering actor, or callback provider can influence inputs. A signature that is unique to one contract may still be replayable across chains if the signed context does not bind the chain.
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OWASP ASVS calls for an up-to-date cryptographic inventory identifying algorithms, keys, certificates, and where keys may be used. Build that inventory from source, bytecode, configuration, deployment scripts, and off-chain repositories; do not rely only on architecture documents.
For every item, record its purpose, input format, output format, owner, exact dependency and version, and trust assumption. Include:
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- Hash functions used for identifiers, Merkle proofs, commitments, storage slots, or signed-message digests.
- Signature schemes, signer-recovery code, verification libraries, and EVM verification precompiles.
- Nonce, deadline, salt, domain, and replay-protection fields.
- Random-value sources, block-derived inputs, oracle callbacks, and any off-chain entropy service.
- Key-generation, derivation, custody, rotation, backup, and revocation performed outside the chain.
- Encryption, commitments, threshold or multisignature constructions, and bridge message authentication.
- Libraries, compiler versions, generated bindings, and serialization code that changes the bytes being authenticated.
Reconcile the inventory with code and dependencies
Search for every cryptographic operation and compare it with the inventory. Trace wrapper functions to the primitive they call; a function named verify may delegate to a precompile, a custom implementation, or an external contract with different failure behavior. Inspect lockfiles and deployment artifacts for the exact version used in production. OWASP’s smart-contract guidance warns against deprecated libraries; determine whether an old primitive is required by an external protocol or is simply historical code that can be removed.
Flag custom cryptographic code for a heightened review. A custom implementation may be unavoidable for protocol compatibility, but it should have a documented rationale, test vectors, review ownership, and a clear statement of which security property it is expected to provide.
3. Trace every authorization signature end to end
Replay and signature forgery are explicit authentication threats in OWASP’s smart-contract guidance. The practical test is to follow one signed action from the human or service that creates it through serialization, transport, calldata, verification, and state update.
Reconstruct the signed bytes
- Identify the application object that the signer sees and the exact byte sequence produced from it.
- Record field names, types, order, widths, byte-string handling, numeric encoding, and whether optional fields have defaults.
- Locate domain or context data: chain identity, verifying contract, protocol name or version, account, action type, nonce, salt, and expiration.
- Recompute the digest independently and compare it with the digest verified by the contract or precompile.
- Confirm that the signer, relayer, and verifier all use the same encoding for zero values, empty arrays, addresses, and nested structures.
Do not treat a readable JSON representation as proof of what was signed. Two different logical messages can produce the same bytes when fields are concatenated ambiguously, and the same logical message can produce different bytes when serializers disagree.
Probe replay and forgery boundaries
- Submit a valid signature twice and verify that the second attempt fails after the intended nonce or one-time state has been consumed.
- Submit the same signature to a different function, account, contract address, deployment, or chain context. It should fail wherever the authorization is not meant to transfer.
- Change one signed field at a time, including amount, recipient, token, nonce, deadline, operation type, and fee, and confirm verification fails.
- Test expired, not-yet-valid, zero, maximum-width, duplicate, and malformed values.
- Try signatures with altered recovery identifiers, invalid lengths, malformed calldata, and zero or unexpected recovered addresses. Check that failure is deterministic and does not leave partial state changes.
- Submit duplicate requests concurrently when the system has relayers or batching. Verify that nonce consumption and state transitions remain atomic.
Compare the contract’s acceptance condition with the signer’s intended authorization. A recovered address can be correct while the message is still wrong if a field was omitted, interpreted in another unit, or accepted under a broader action type than the signer expected.
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4. Test whether randomness is actually unpredictable
List every outcome whose security depends on unpredictability: lotteries, games, validator or committee selection, secret generation, randomized allocation, and any delay or tie-breaker that claims to be random. For each outcome, identify the entropy source, the time at which it becomes fixed, and every actor that can observe or influence it.
Analyze the source and adversary view
Determine whether the value comes from block data, transaction ordering, timestamps, an external callback, a commit-reveal process, or an off-chain service. Ask whether a block producer, relayer, participant, oracle, or user can predict the value before committing capital or can bias it by choosing timing, withholding a transaction, selecting among submissions, or reverting a callback.
OWASP identifies secure random generation as an essential cryptographic practice. ASVS gives general application guidance that non-guessable random values should use a cryptographically secure random number generator (CSPRNG) with at least 128 bits of entropy. Do not apply that statement mechanically to public blockchains: a contract cannot make public block data secret, and the appropriate mechanism depends on the target chain and threat model. Document why the chosen source remains unpredictable and unbiased for the period in which an attacker can act.
Test timing and influence
- Record when the random input becomes observable to users, relayers, validators, and contract callers.
- Simulate delayed, reordered, duplicated, and reverted submissions around the draw or allocation.
- Vary all caller-controlled and block-derived inputs within their permitted ranges.
- Check whether an operator can discard an unfavorable result, request multiple samples, or choose a favorable callback.
- Verify that a failed or missing randomness response cannot permanently lock funds or silently fall back to a predictable value.
5. Review hashes and commitment encodings
For each hash, write down the required property instead of assuming that every use needs the same strength. The design may require collision resistance, second-preimage resistance, integrity, commitment binding, or a protocol-specific property.
ASVS states that when collision resistance is required, the hash output should be at least 256 bits; when only second-preimage resistance is required, the output should be at least 128 bits. These are general application-security controls, not proof that a particular protocol’s encoding or chain hash semantics are safe. Match the output and primitive to the stated protocol goal.
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Inspect the bytes, not just the hash name
- List every field included in the preimage, its type, width, order, and boundary marker.
- Check that variable-length values, arrays, and adjacent fields cannot be concatenated into the same byte string for two different logical messages.
- Include action, domain, contract, chain, account, version, nonce, and expiry context whenever omitting it would let a digest be reused.
- Verify endianness, padding, address representation, numeric units, and treatment of empty values across every producer and verifier.
- Compare on-chain and off-chain test vectors, including minimum, maximum, zero, empty, and nested values.
A commitment can use a strong hash and still fail if the preimage is ambiguous or lacks context. Record the exact encoding specification in the audit evidence and test it independently from the production serializer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.6. Examine implementations, keys, and failure behavior
Prefer validated primitives
Determine whether each operation uses an industry-validated library, a platform primitive, or an EVM precompile, and record the exact version and call behavior. ASVS calls for industry-validated implementations, constant-time cryptographic operations, and secure failure handling. Constant-time behavior is especially important in off-chain components that process secret keys; do not imply that Solidity can hide on-chain state or make execution private.
Review gas-limit and return-data handling around verification calls. Confirm that a failed external cryptographic call cannot be treated as success because a return value was ignored, truncated, or decoded permissively. Check that malformed input causes a deliberate revert or rejection and cannot leave authorization, nonce, or accounting state partially updated.
Document the key lifecycle
For every signing or encryption key, identify the controller, generation method, storage location, access roles, rotation trigger, backup, revocation, and compromise response. Include keys held by multisignature participants, bridge operators, oracle services, relayers, deployment accounts, and automated bots. ASVS calls for a documented key-management policy and inventory; adapt that general guidance to the contract’s operational model.
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7. Turn the standards into a repeatable test plan
Use SCSVS to select the requirements that apply to the architecture, then map each selected requirement to SCSTG test cases and, where applicable, an SCWE weakness. The SCSTG page was last updated August 19, 2026; record the edition or revision used so another auditor can reproduce the review.
Minimum evidence for each test
- Requirement identifier and related weakness, if one applies.
- Contract address, deployed bytecode or commit, compiler and dependency versions.
- Preconditions: accounts, roles, chain state, keys, fixtures, and oracle or relayer setup.
- Exact input, transaction sequence, signed bytes, and tool or script version.
- Expected security behavior and the observed result, including gas or revert details when relevant.
- Assumptions, environmental limits, untested paths, and the severity rationale.
- Remediation, a regression test, and the evidence that the fix was deployed or verified.
Compare audit coverage, not marketing claims
When evaluating an internal review, external engagement, or testing tool, compare four concrete dimensions: requirement coverage, platform relevance, reproducibility of tests, and coverage of off-chain cryptographic dependencies. A review that checks Solidity verification code but ignores signer software, serialization libraries, relayers, or key custody is incomplete even if every contract function was examined. Conversely, a broad application checklist should not be presented as an EVM-specific requirement when the control does not apply to on-chain execution.
8. Report findings so teams can fix assumptions
State the assumption in one sentence, then show where it enters the system, who can violate it, and what security property is lost. For example: a relayer can submit a valid authorization on another deployment because the signed context omits the verifying contract; the consequence is cross-contract replay of the authorized action. Include the reconstructed bytes, a minimal reproduction, affected versions, and the precise state transition that demonstrates impact.
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Require a regression test for every finding. Re-run the complete signed-message, randomness, hash-encoding, and failure-path suites after remediation, then verify the deployed bytecode and configuration correspond to the reviewed version.
Practical completion checklist
- Scope names the chain, virtual machine, compiler, dependencies, deployed code, upgrade path, and assets at risk.
- Trust boundaries include contracts, signers, relayers, oracles, bridges, multisignature components, and external contracts.
- The inventory covers algorithms, keys, certificates, precompiles, random sources, commitments, encryption, and off-chain custody.
- Every signed action has a reconstructed byte sequence, domain or context analysis, nonce or expiry behavior, and replay tests.
- Malformed, duplicate, expired, and forged inputs fail without partial state changes.
- Randomness analysis covers observability, prediction, bias, timing, retries, and fallback behavior.
- Each hash has a stated security property and an unambiguous, context-bound encoding.
- Implementations use validated primitives with reviewed versions and deliberate failure handling.
- Key generation, storage, rotation, revocation, backup, and compromise response are documented and tested.
- Findings map to applicable SCSVS requirements and SCSTG tests, with reproducible evidence and regression coverage.
The Bottom Line
The audit is complete only when every cryptographic assumption is named, tied to a concrete byte sequence or state transition, tested against the actors who can influence it, and backed by a repeatable control from SCSVS, SCSTG, or an appropriately adapted ASVS requirement.
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