Wycheproof
Trail of BitsOfficialPublished by Trail of Bits
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- Security
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Written by AutoGineer from the skill’s own files, not by the author
Wycheproof is a community-managed project providing extensive test vectors to verify the correctness of cryptographic implementations. It covers a wide range of algorithms, including AES-GCM, ECDSA, ECDH, and RSA, by providing input/output pairs that encode known vulnerabilities and edge cases. By integrating these vectors into a testing harness, developers can ensure their implementations handle invalid inputs, signature malleability, and other common cryptographic flaws correctly, preventing consensus issues and security vulnerabilities.
What it does
- Provides standardized test vectors for symmetric encryption, signatures, key exchange, and hashing.
- Defines expected outcomes (valid, invalid, or acceptable) for specific cryptographic inputs.
- Supports integration via git submodules or direct fetching of JSON test files.
- Enables automated validation of cryptographic libraries against known attack patterns.
- Categorizes tests into groups based on parameters like key size, IV size, and elliptic curves.
When to use it
- When testing the correctness of cryptographic implementations like AES-GCM or ECDSA.
- When auditing third-party cryptographic code for potential vulnerabilities.
- When setting up CI/CD pipelines to prevent regressions in cryptographic libraries.
- When investigating discrepancies between different implementations handling the same input.
Advantages
- Reduces the risk of common vulnerabilities like signature malleability and padding oracles.
- Leverages community-maintained test vectors that evolve with new security findings.
- Facilitates consistent behavior across different cryptographic implementations.
- Provides clear, structured test data that simplifies the creation of test harnesses.
Usage examples
- “How do I integrate Wycheproof test vectors into my Python project?”
- The skill provides a guide to adding the repository as a git submodule and using a Python function to parse JSON test vectors, filter them by parameters, and execute them via pytest.
- “What kind of vulnerabilities can Wycheproof help me detect?”
- It helps detect issues such as signature malleability, invalid DER encoding, invalid curve attacks, padding oracles, and tag forgery.
Install
Download the skill as a ZIP:
https://autogineer.com/agent-skills/trailofbits/wycheproof/downloadUnzip it into the cross-client skills folder of your project:
mkdir -p .agents/skills && unzip wycheproof.zip -d .agents/skills
Documentation by Trail of Bits
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:
┣ 📜 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:
"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 groupsTest 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
notesfield)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:
git submodule add https://github.com/C2SP/wycheproof.gitOption 2: Fetch Specific Test Vectors
If submodules aren't possible, fetch specific JSON files:
#!/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
donePhase 2: Parse Test Vectors
Identify the test file for your algorithm and parse the JSON:
Python Example:
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 testVectorsJavaScript Example:
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:
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:
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:
Using git submodules: Update submodule in CI before running tests
Fetching latest vectors: Run fetch script before test execution
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:
Valid signature: ...6a5c51eb6f946b30d
Invalid signature: ...6a5c51eb6f946b30d0000 (should be rejected)How to detect:
# Add signature length check
if len(sig) != 128: # EdDSA signatures must be exactly 64 bytes (128 hex chars)
return FalseImpact: 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 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
Identify supported curves: ed25519 for EdDSA
Find test vectors:
testvectors_v1/ed25519_test.jsonParse test vectors: Load JSON and extract tests
Write test harness: Create parameterized tests
Run tests: Identify failures
Analyze root causes: Examine implementation code
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
The official repository contains:
All test vectors in
testvectors/andtestvectors_v1/JSON schemas in
schemas/Reference implementations in Java and JavaScript
Documentation in
doc/
Real-World Examples
The pycryptodome library integrates Wycheproof test vectors in their test suite, demonstrating best practices for Python crypto implementations.
Community Resources
C2SP Community - 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:
Catch subtle encoding and validation bugs
Prevent signature malleability issues
Ensure consistent behavior across implementations
Benefit from community-contributed test vectors
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.
Files
- ATTRIBUTION.mdAdded by AutoGineer937 B
- LICENSELicence file added by AutoGineer19.7 KiB
- SKILL.md20.2 KiB
- agents/openai.yaml249 B
- assets/trail-of-bits-mark.svg3.0 KiB
Requirements and compatibility
Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta
“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.
Needs: Needs a command-line tool, Needs Python
Security
No issues found by our static scan· No issues found
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Attribution and licence
- Published by
- Trail of Bits
- Publisher
- Trail of Bits (Official)
- Source
- trailofbits/skills · plugins/testing-handbook-skills/skills/wycheproof · commit e4dd0bb
- Licence
- Creative Commons Attribution-ShareAlike 4.0 International (redistribution allowed)
- LICENSE (added by AutoGineer from LICENSE)
- ATTRIBUTION.md (written by AutoGineer)
- Synced
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