SECURITY REVIEW / Trail of Bits
Project Wycheproof
Loads Project Wycheproof vectors and maps valid, invalid, and acceptable results onto pass, fail, and warning, instead of treating a custom algorithm as in scope.
“Validates cryptographic implementations against Project Wycheproof's test vectors, which encode known attacks and edge cases across AES, RSA, ECDSA, ECDH, and more. Covers loading test vectors, mapping result flags onto pass and fail expectations, and reading a failure. Use when testing a crypto implementation against known attacks, checking a library against standard test vectors, or investigating why two implementations disagree on the same input.”
01 / THE REASONING
Why this made the selection.
- The result field has three values: valid should succeed, invalid should fail, and acceptable may succeed but is called non-ideal.
- A listed mistake is testing only valid cases, or ignoring acceptable, which the file says can hide a subtle bug.
- Timing side channels, unknown bugs, and experimental algorithms are sent to other techniques.
02 / THE REVIEW RECORD
What we actually inspected.
Source review has boundaries.
A clear record is more useful than a “safe” badge.
Material inspected
- plugins/testing-handbook-skills/skills/wycheproof/SKILL.md
- LICENSE
- README.md
Our findings
- The result table defines valid as should succeed, invalid as should fail, and acceptable as allowed to succeed with non-ideal attributes.
- The common-mistakes table says to test all three result types and to treat acceptable as a warning worth investigating.
- Phase 1 prefers a git submodule of github.com/C2SP/wycheproof and recommends testvectors_v1 over the older testvectors tree.
- The harness example fails the test when a valid vector raises, and it also fails when an invalid vector decrypts.
- The guide distinguishes valid, invalid and acceptable vectors and recommends the current testvectors_v1 layout rather than treating every non-valid case identically.
Not established by this review
- The skill was not executed inside a host agent.
- No Wycheproof JSON file was loaded and no crypto harness was run.
The review applies to the material and revision named here. A newer upstream release can change its behavior.
03 / PUT IT TO WORK
Add a skill to your workflow.
DOCUMENTED COMMAND
/plugin marketplace add trailofbits/skillsCopying does not execute this command. It may retrieve a newer version than the reviewed source.
- Add the marketplace, then install the testing-handbook-skills plugin, which contains wycheproof.
- Test valid, invalid and acceptable vectors, not only valid ones.
- Download the complete bundle from https://undominated.ai/resources/skills/trailofbits-wycheproof/bundle.zip and extract it into a new directory. It holds SKILL.md and agents/openai.yaml unchanged at revision 82fe8226, the upstream licence and an attribution file.
- Place the extracted directory in your host's documented skills directory, named wycheproof. Keep SKILL.md and the licence together.
Before you start
- The cryptographic implementation and the Wycheproof vectors
- git (for the submodule) or curl to fetch the vectors; a test runner such as pytest for the example
THE COMPLETE REVIEWED DEFINITION
Read it before you reuse it.
Original source bytes, with attribution.
Review the host-specific setup notes above.
---
name: wycheproof
type: domain
description: "Validates cryptographic implementations against Project Wycheproof's test vectors, which encode known attacks and edge cases across AES, RSA, ECDSA, ECDH, and more. Covers loading test vectors, mapping result flags onto pass and fail expectations, and reading a failure. Use when testing a crypto implementation against known attacks, checking a library against standard test vectors, or investigating why two implementations disagree on the same input."
---
# Wycheproof
Wycheproof is an extensive collection of test vectors designed to verify the correctness of cryptographic implementations and test against known attacks. Originally developed by Google, it is now a community-managed project where contributors can add test vectors for specific cryptographic constructions.
## Background
### Key Concepts
| Concept | Description |
|---------|-------------|
| Test vector | Input/output pair for validating crypto implementation correctness |
| Test group | Collection of test vectors sharing attributes (key size, IV size, curve) |
| Result flag | Indicates if test should pass (valid), fail (invalid), or is acceptable |
| Edge case testing | Testing for known vulnerabilities and attack patterns |
### Why This Matters
Cryptographic implementations are notoriously difficult to get right. Even small bugs can:
- Expose private keys
- Allow signature forgery
- Enable message decryption
- Create consensus problems when different implementations accept/reject the same inputs
Wycheproof has found vulnerabilities in major libraries including OpenJDK's SHA1withDSA, Bouncy Castle's ECDHC, and the elliptic npm package.
## When to Use
**Apply Wycheproof when:**
- Testing cryptographic implementations (AES-GCM, ECDSA, ECDH, RSA, etc.)
- Validating that crypto code handles edge cases correctly
- Verifying implementations against known attack vectors
- Setting up CI/CD for cryptographic libraries
- Auditing third-party crypto code for correctness
**Consider alternatives when:**
- Testing for timing side-channels (use constant-time testing tools instead)
- Finding new unknown bugs (use fuzzing instead)
- Testing custom/experimental cryptographic algorithms (Wycheproof only covers established algorithms)
## Quick Reference
| Scenario | Recommended Approach | Notes |
|----------|---------------------|-------|
| AES-GCM implementation | Use `aes_gcm_test.json` | 316 test vectors across 44 test groups |
| ECDSA verification | Use `ecdsa_*_test.json` for specific curves | Tests signature malleability, DER encoding |
| ECDH key exchange | Use `ecdh_*_test.json` | Tests invalid curve attacks |
| RSA signatures | Use `rsa_*_test.json` | Tests padding oracle attacks |
| ChaCha20-Poly1305 | Use `chacha20_poly1305_test.json` | Tests AEAD implementation |
## Testing Workflow
```
Phase 1: Setup Phase 2: Parse Test Vectors
┌─────────────────┐ ┌─────────────────┐
│ Add Wycheproof │ → │ Load JSON file │
│ as submodule │ │ Filter by params│
└─────────────────┘ └─────────────────┘
↓ ↓
Phase 4: CI Integration Phase 3: Write Harness
┌─────────────────┐ ┌─────────────────┐
│ Auto-update │ ← │ Test valid & │
│ test vectors │ │ invalid cases │
└─────────────────┘ └─────────────────┘
```
## Repository Structure
The Wycheproof repository is organized as follows:
```text
┣ 📜 README.md : Project overview
┣ 📂 doc : Documentation
┣ 📂 java : Java JCE interface testing harness
┣ 📂 javascript : JavaScript testing harness
┣ 📂 schemas : Test vector schemas
┣ 📂 testvectors : Test vectors
┗ 📂 testvectors_v1 : Updated test vectors (more detailed)
```
The essential folders are `testvectors` and `testvectors_v1`. While both contain similar files, `testvectors_v1` includes more detailed information and is recommended for new integrations.
## Supported Algorithms
Wycheproof provides test vectors for a wide range of cryptographic algorithms:
| Category | Algorithms |
|----------|------------|
| **Symmetric Encryption** | AES-GCM, AES-EAX, ChaCha20-Poly1305 |
| **Signatures** | ECDSA, EdDSA, RSA-PSS, RSA-PKCS1 |
| **Key Exchange** | ECDH, X25519, X448 |
| **Hashing** | HMAC, HKDF |
| **Curves** | secp256k1, secp256r1, secp384r1, secp521r1, ed25519, ed448 |
## Test File Structure
Each JSON test file tests a specific cryptographic construction. All test files share common attributes:
```json
"algorithm" : The name of the algorithm tested
"schema" : The JSON schema (found in schemas folder)
"generatorVersion" : The version number
"numberOfTests" : The total number of test vectors in this file
"header" : Detailed description of test vectors
"notes" : In-depth explanation of flags in test vectors
"testGroups" : Array of one or multiple test groups
```
### Test Groups
Test groups group sets of tests based on shared attributes such as:
- Key sizes
- IV sizes
- Public keys
- Curves
This classification allows extracting tests that meet specific criteria relevant to the construction being tested.
### Test Vector Attributes
#### Shared Attributes
All test vectors contain four common fields:
- **tcId**: Unique identifier for the test vector within a file
- **comment**: Additional information about the test case
- **flags**: Descriptions of specific test case types and potential dangers (referenced in `notes` field)
- **result**: Expected outcome of the test
The `result` field can take three values:
| Result | Meaning |
|--------|---------|
| **valid** | Test case should succeed |
| **acceptable** | Test case is allowed to succeed but contains non-ideal attributes |
| **invalid** | Test case should fail |
#### Unique Attributes
Unique attributes are specific to the algorithm being tested:
| Algorithm | Unique Attributes |
|-----------|-------------------|
| AES-GCM | `key`, `iv`, `aad`, `msg`, `ct`, `tag` |
| ECDH secp256k1 | `public`, `private`, `shared` |
| ECDSA | `msg`, `sig`, `result` |
| EdDSA | `msg`, `sig`, `pk` |
## Implementation Guide
### Phase 1: Add Wycheproof to Your Project
**Option 1: Git Submodule (Recommended)**
Adding Wycheproof as a git submodule ensures automatic updates:
```bash
git submodule add https://github.com/C2SP/wycheproof.git
```
**Option 2: Fetch Specific Test Vectors**
If submodules aren't possible, fetch specific JSON files:
```bash
#!/bin/bash
TMP_WYCHEPROOF_FOLDER=".wycheproof/"
TEST_VECTORS=('aes_gcm_test.json' 'aes_eax_test.json')
BASE_URL="https://raw.githubusercontent.com/C2SP/wycheproof/master/testvectors_v1/"
# Create wycheproof folder
mkdir -p $TMP_WYCHEPROOF_FOLDER
# Request all test vector files if they don't exist
for i in "${TEST_VECTORS[@]}"; do
if [ ! -f "${TMP_WYCHEPROOF_FOLDER}${i}" ]; then
curl -o "${TMP_WYCHEPROOF_FOLDER}${i}" "${BASE_URL}${i}"
if [ $? -ne 0 ]; then
echo "Failed to download ${i}"
exit 1
fi
fi
done
```
### Phase 2: Parse Test Vectors
Identify the test file for your algorithm and parse the JSON:
**Python Example:**
```python
import json
def load_wycheproof_test_vectors(path: str):
testVectors = []
try:
with open(path, "r") as f:
wycheproof_json = json.loads(f.read())
except FileNotFoundError:
print(f"No Wycheproof file found at: {path}")
return testVectors
# Attributes that need hex-to-bytes conversion
convert_attr = {"key", "aad", "iv", "msg", "ct", "tag"}
for testGroup in wycheproof_json["testGroups"]:
# Filter test groups based on implementation constraints
if testGroup["ivSize"] < 64 or testGroup["ivSize"] > 1024:
continue
for tv in testGroup["tests"]:
# Convert hex strings to bytes
for attr in convert_attr:
if attr in tv:
tv[attr] = bytes.fromhex(tv[attr])
testVectors.append(tv)
return testVectors
```
**JavaScript Example:**
```javascript
const fs = require('fs').promises;
async function loadWycheproofTestVectors(path) {
const tests = [];
try {
const fileContent = await fs.readFile(path);
const data = JSON.parse(fileContent.toString());
data.testGroups.forEach(testGroup => {
testGroup.tests.forEach(test => {
// Add shared test group properties to each test
test['pk'] = testGroup.publicKey.pk;
tests.push(test);
});
});
} catch (err) {
console.error('Error reading or parsing file:', err);
throw err;
}
return tests;
}
```
### Phase 3: Write Testing Harness
Create test functions that handle both valid and invalid test cases.
**Python/pytest Example:**
```python
import pytest
from cryptography.hazmat.primitives.ciphers.aead import AESGCM
tvs = load_wycheproof_test_vectors("wycheproof/testvectors_v1/aes_gcm_test.json")
@pytest.mark.parametrize("tv", tvs, ids=[str(tv['tcId']) for tv in tvs])
def test_encryption(tv):
try:
aesgcm = AESGCM(tv['key'])
ct = aesgcm.encrypt(tv['iv'], tv['msg'], tv['aad'])
except ValueError as e:
# Implementation raised error - verify test was expected to fail
assert tv['result'] != 'valid', tv['comment']
return
if tv['result'] == 'valid':
assert ct[:-16] == tv['ct'], f"Ciphertext mismatch: {tv['comment']}"
assert ct[-16:] == tv['tag'], f"Tag mismatch: {tv['comment']}"
elif tv['result'] == 'invalid' or tv['result'] == 'acceptable':
assert ct[:-16] != tv['ct'] or ct[-16:] != tv['tag']
@pytest.mark.parametrize("tv", tvs, ids=[str(tv['tcId']) for tv in tvs])
def test_decryption(tv):
try:
aesgcm = AESGCM(tv['key'])
decrypted_msg = aesgcm.decrypt(tv['iv'], tv['ct'] + tv['tag'], tv['aad'])
except ValueError:
assert tv['result'] != 'valid', tv['comment']
return
except InvalidTag:
assert tv['result'] != 'valid', tv['comment']
assert 'ModifiedTag' in tv['flags'], f"Expected 'ModifiedTag' flag: {tv['comment']}"
return
assert tv['result'] == 'valid', f"No invalid test case should pass: {tv['comment']}"
assert decrypted_msg == tv['msg'], f"Decryption mismatch: {tv['comment']}"
```
**JavaScript/Mocha Example:**
```javascript
const assert = require('assert');
function testFactory(tcId, tests) {
it(`[${tcId + 1}] ${tests[tcId].comment}`, function () {
const test = tests[tcId];
const ed25519 = new eddsa('ed25519');
const key = ed25519.keyFromPublic(toArray(test.pk, 'hex'));
let sig;
if (test.result === 'valid') {
sig = key.verify(test.msg, test.sig);
assert.equal(sig, true, `[${test.tcId}] ${test.comment}`);
} else if (test.result === 'invalid') {
try {
sig = key.verify(test.msg, test.sig);
} catch (err) {
// Point could not be decoded
sig = false;
}
assert.equal(sig, false, `[${test.tcId}] ${test.comment}`);
}
});
}
// Generate tests for all test vectors
for (var tcId = 0; tcId < tests.length; tcId++) {
testFactory(tcId, tests);
}
```
### Phase 4: CI Integration
Ensure test vectors stay up to date by:
1. **Using git submodules**: Update submodule in CI before running tests
2. **Fetching latest vectors**: Run fetch script before test execution
3. **Scheduled updates**: Set up weekly/monthly updates to catch new test vectors
## Common Vulnerabilities Detected
Wycheproof test vectors are designed to catch specific vulnerability patterns:
| Vulnerability | Description | Affected Algorithms | Example CVE |
|---------------|-------------|---------------------|-------------|
| Signature malleability | Multiple valid signatures for same message | ECDSA, EdDSA | CVE-2024-42459 |
| Invalid DER encoding | Accepting non-canonical DER signatures | ECDSA | CVE-2024-42460, CVE-2024-42461 |
| Invalid curve attacks | ECDH with invalid curve points | ECDH | Common in many libraries |
| Padding oracle | Timing leaks in padding validation | RSA-PKCS1 | Historical OpenSSL issues |
| Tag forgery | Accepting modified authentication tags | AES-GCM, ChaCha20-Poly1305 | Various implementations |
### Signature Malleability: Deep Dive
**Problem:** Implementations that don't validate signature encoding can accept multiple valid signatures for the same message.
**Example (EdDSA):** Appending or removing zeros from signature:
```text
Valid signature: ...6a5c51eb6f946b30d
Invalid signature: ...6a5c51eb6f946b30d0000 (should be rejected)
```
**How to detect:**
```python
# Add signature length check
if len(sig) != 128: # EdDSA signatures must be exactly 64 bytes (128 hex chars)
return False
```
**Impact:** Can lead to consensus problems when different implementations accept/reject the same signatures.
**Related Wycheproof tests:**
- EdDSA: tcId 37 - "removing 0 byte from signature"
- ECDSA: tcId 06 - "Legacy: ASN encoding of r misses leading 0"
## Case Study: Elliptic npm Package
This case study demonstrates how Wycheproof found three CVEs in the popular elliptic npm package (3000+ dependents, millions of weekly downloads).
### Overview
The [elliptic](https://www.npmjs.com/package/elliptic) library is an elliptic-curve cryptography library written in JavaScript, supporting ECDH, ECDSA, and EdDSA. Using Wycheproof test vectors on version 6.5.6 revealed multiple vulnerabilities:
- **CVE-2024-42459**: EdDSA signature malleability (appending/removing zeros)
- **CVE-2024-42460**: ECDSA DER encoding - invalid bit placement
- **CVE-2024-42461**: ECDSA DER encoding - leading zero in length field
### Methodology
1. **Identify supported curves**: ed25519 for EdDSA
2. **Find test vectors**: `testvectors_v1/ed25519_test.json`
3. **Parse test vectors**: Load JSON and extract tests
4. **Write test harness**: Create parameterized tests
5. **Run tests**: Identify failures
6. **Analyze root causes**: Examine implementation code
7. **Propose fixes**: Add validation checks
### Key Findings
**EdDSA Issue (CVE-2024-42459):**
- Missing signature length validation
- Allowed trailing zeros in signatures
- Fix: Add `if(sig.length !== 128) return false;`
**ECDSA Issue 1 (CVE-2024-42460):**
- Missing check for first bit being zero in DER-encoded r and s values
- Fix: Add `if ((data[p.place] & 128) !== 0) return false;`
**ECDSA Issue 2 (CVE-2024-42461):**
- DER length field accepted leading zeros
- Fix: Add `if(buf[p.place] === 0x00) return false;`
### Impact
All three vulnerabilities allowed multiple valid signatures for a single message, leading to consensus problems across implementations.
**Lessons learned:**
- Wycheproof catches subtle encoding bugs
- Reusable test harnesses pay dividends
- Test vector comments and flags help diagnose issues
- Even popular libraries benefit from systematic test vector validation
## Advanced Usage
### Tips and Tricks
| Tip | Why It Helps |
|-----|--------------|
| Filter test groups by parameters | Focus on test vectors relevant to your implementation constraints |
| Use test vector flags | Understand specific vulnerability patterns being tested |
| Check the `notes` field | Get detailed explanations of flag meanings |
| Test both encrypt/decrypt and sign/verify | Ensure bidirectional correctness |
| Run tests in CI | Catch regressions and benefit from new test vectors |
| Use parameterized tests | Get clear failure messages with tcId and comment |
### Common Mistakes
| Mistake | Why It's Wrong | Correct Approach |
|---------|----------------|------------------|
| Only testing valid cases | Misses vulnerabilities where invalid inputs are accepted | Test all result types: valid, invalid, acceptable |
| Ignoring "acceptable" result | Implementation might have subtle bugs | Treat acceptable as warnings worth investigating |
| Not filtering test groups | Wastes time on unsupported parameters | Filter by keySize, ivSize, etc. based on your implementation |
| Not updating test vectors | Miss new vulnerability patterns | Use submodules or scheduled fetches |
| Testing only one direction | Encrypt/sign might work but decrypt/verify fails | Test both operations |
## Related Skills
### Tool Skills
| Skill | Primary Use in Wycheproof Testing |
|-------|-----------------------------------|
| **pytest** | Python testing framework for parameterized tests |
| **mocha** | JavaScript testing framework for test generation |
| **constant-time-testing** | Complement Wycheproof with timing side-channel testing |
| **cryptofuzz** | Fuzz-based crypto testing to find additional bugs |
### Technique Skills
| Skill | When to Apply |
|-------|---------------|
| **coverage-analysis** | Ensure test vectors cover all code paths in crypto implementation |
| **property-based-testing** | Test mathematical properties (e.g., encrypt/decrypt round-trip) |
| **fuzz-harness-writing** | Create harnesses for crypto parsers (complements Wycheproof) |
### Related Domain Skills
| Skill | Relationship |
|-------|--------------|
| **crypto-testing** | Wycheproof is a key tool in comprehensive crypto testing methodology |
| **fuzzing** | Use fuzzing to find bugs Wycheproof doesn't cover (new edge cases) |
## Skill Dependency Map
```
┌─────────────────────┐
│ wycheproof │
│ (this skill) │
└──────────┬──────────┘
│
┌───────────────────┼───────────────────┐
│ │ │
▼ ▼ ▼
┌─────────────────┐ ┌─────────────────┐ ┌─────────────────┐
│ pytest/mocha │ │ constant-time │ │ cryptofuzz │
│ (test framework)│ │ testing │ │ (fuzzing) │
└────────┬────────┘ └────────┬────────┘ └────────┬────────┘
│ │ │
└───────────────────┼───────────────────┘
│
▼
┌──────────────────────────┐
│ Technique Skills │
│ coverage, harness, PBT │
└──────────────────────────┘
```
## Resources
### Official Repository
**[Wycheproof GitHub Repository](https://github.com/C2SP/wycheproof)**
The official repository contains:
- All test vectors in `testvectors/` and `testvectors_v1/`
- JSON schemas in `schemas/`
- Reference implementations in Java and JavaScript
- Documentation in `doc/`
### Real-World Examples
**[pycryptodome](https://pypi.org/project/pycryptodome/)**
The pycryptodome library integrates Wycheproof test vectors in their test suite, demonstrating best practices for Python crypto implementations.
### Community Resources
- [C2SP Community](https://c2sp.org/) - Cryptographic specifications and standards community maintaining Wycheproof
- Wycheproof issues tracker - Report bugs in test vectors or suggest new constructions
## Summary
Wycheproof is an essential tool for validating cryptographic implementations against known attack vectors and edge cases. By integrating Wycheproof test vectors into your testing workflow:
1. Catch subtle encoding and validation bugs
2. Prevent signature malleability issues
3. Ensure consistent behavior across implementations
4. Benefit from community-contributed test vectors
5. Protect against known cryptographic vulnerabilities
The investment in writing a reusable testing harness pays dividends through continuous validation as new test vectors are added to the Wycheproof repository.
The download contains SKILL.md. Extract the whole bundle; the supporting files are required. Inspect the included MANIFEST.json for file hashes.
By Trail of Bits. Exact upstream source ↗ · Licence · Attribution
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04 / FOLLOW THE EVIDENCE
The source trail.
Our notes are separate from the original resource.
Check upstream before adopting a new version.
- Skill definition ↗Checked
https://github.com/trailofbits/skills/blob/82fe8226252622fa807643bdca1710901198553a/plugins/testing-handbook-skills/skills/wycheproof/SKILL.md
Supports: Workflow and stated scope
- Repository licence ↗Checked
https://github.com/trailofbits/skills/blob/82fe8226252622fa807643bdca1710901198553a/LICENSE
Supports: Redistribution terms