Reviewing Cryptography

Reviewing Cryptography

Almost no real system is broken by cryptanalysis. They are broken by misuse: a reused nonce, unauthenticated ciphertext, a comparison that returns early, a key checked into git. Review for misuse, and leave primitive design to cryptographers.

When to Use

• Auditing code that encrypts, decrypts, signs, verifies, or hashes
• Reviewing key management, rotation, and storage
• Assessing TLS/mTLS configuration and certificate validation
• Reviewing JWT, session token, and API signature schemes
• Checking password and secret storage
• Evaluating randomness quality for security-relevant values

When NOT to Use

• Designing a new primitive or protocol — that needs a cryptographer and

formal review, not a code audit

• Breaking cryptography in a CTF — different discipline; use

solving-oriented offensive skills and known-attack tooling

• General code review — use auditing-code-for-vulnerabilities
• Password cracking — use cracking-passwords

The Misuse Checklist

Work through these in order. Each has caught real production breaks.

1. Is the ciphertext authenticated?

Encryption without authentication is the single most common serious finding. CBC or CTR without a MAC means an attacker can modify plaintext — and with a decryption oracle, recover it (padding oracle).

Good:  AES-GCM, ChaCha20-Poly1305, AES-CBC + HMAC (encrypt-then-MAC)
Bad:   AES-CBC alone, AES-ECB (ever), CTR without a MAC, MAC-then-encrypt

rg -n 'AES/ECB|AES\.MODE_ECB|CipherMode\.ECB|"AES"\)' -i
rg -n 'AES/CBC/PKCS5Padding|MODE_CBC|createCipheriv\(.*cbc' -i

If you see CBC, find the MAC. If there is no MAC, that is a finding regardless of how the ciphertext is transported.

2. Nonce and IV handling

| Mode | Rule | Failure | | --- | --- | --- | | GCM / ChaCha20-Poly1305 | Never reuse a (key, nonce) pair | Catastrophic: reveals the auth key, forgery becomes trivial | | CBC | IV must be unpredictable and random per message | Chosen-plaintext attacks (BEAST-class) | | CTR | Never reuse a counter with the same key | Keystream reuse; XOR of plaintexts |

# The classic bug: a fixed or zero IV
rg -n 'iv\s*=\s*(b?["\x27]0|new byte\[\d+\]|bytes\(\d+\)|\[0\]\s*\*)' -i
rg -n 'IvParameterSpec\(new byte\[16\]\)|createCipheriv\([^,]+,[^,]+,\s*["\x27]'

Random 96-bit nonces for GCM are safe up to roughly 2^32 messages per key. A counter-based nonce is safer, but only if the counter state genuinely survives restarts and is not duplicated across instances. Ask where the counter is persisted; "in memory" plus horizontal scaling means reuse.

3. Key management

• Where does the key come from? Hardcoded, env var, KMS/HSM, derived?
• Hardcoded keys, keys in source, keys in config committed to the repo, keys

in container images, keys in CI logs — check all of these.

• Is a key used for exactly one purpose? Key reuse across encryption and

signing, or across tenants, is a finding.

• Is there a rotation path at all? A system that cannot rotate has no response

to compromise.

• Derived keys: is a proper KDF used (HKDF for key material, Argon2id/scrypt/

PBKDF2 for passwords)? A raw SHA-256 of a password is not a KDF.

rg -n 'BEGIN (RSA |EC |OPENSSH )?PRIVATE KEY|-----BEGIN'
rg -n '(secret|api[_-]?key|password|token)\s*[:=]\s*["\x27][A-Za-z0-9/+=]{16,}' -i
gitleaks detect --source . --redact      # history matters more than the tree

4. Password storage

Correct:   Argon2id (preferred), scrypt, bcrypt, PBKDF2-HMAC-SHA256 with a
           high iteration count — each with a per-user random salt
Wrong:     MD5, SHA-1, SHA-256, SHA-512 (raw or salted), any unsalted hash,
           encryption instead of hashing, a "pepper" as the only defense

Check the work factor against current guidance, not the value that was adequate when the code was written. And check that the verification path uses the library's constant-time verify function rather than comparing strings.

5. Randomness

Security-relevant values — tokens, session IDs, nonces, salts, password reset codes, IVs — must come from a CSPRNG.

rg -n 'math/rand|Math\.random\(\)|random\.random\(|rand\(\)|mt_rand|Random\(\)' 
# Correct: crypto/rand, secrets.token_bytes, window.crypto.getRandomValues,
#          SecureRandom, os.urandom, RandomNumberGenerator

Also check: seeding with a timestamp or PID, UUIDv1/v4-from-a-weak-source used as a secret, and predictable sequential IDs used where unguessability is assumed.

6. Timing side channels

Any comparison of a secret must be constant time: MACs, tokens, API signatures, password hashes, OTPs.

rg -n 'hmac.*==|token\s*==|signature\s*==|\.equals\(.*(hmac|token|sig)' -i
# Correct: hmac.compare_digest, crypto.timingSafeEqual, subtle.ConstantTimeCompare,
#          MessageDigest.isEqual, hash_equals

Early-return string comparison of an HMAC is a practical remote attack, not a theoretical one.

7. Signature verification

• Is the signature actually verified, or merely parsed?
• Is the algorithm taken from the message? (JWT alg confusion: none, and

RS256→HS256 where the public key becomes the HMAC key.)

• Is the key selected by an identifier the attacker controls (kid, jku,

x5u)? Those fields are attacker input; treat them as such.

• Are claims validated after signature verification: exp, nbf, iss,

aud, and — critically — the subject's current authorization?

rg -n 'jwt\.decode\(|verify\s*[:=]\s*(False|false)|algorithms\s*=\s*\[?["\x27]?none' -i
rg -n 'InsecureSkipVerify|verify\s*=\s*False|CURLOPT_SSL_VERIFYPEER.*0|rejectUnauthorized:\s*false'

8. TLS configuration

# Server side
testssl.sh --severity MEDIUM https://target
sslyze --regular target:443
nmap --script ssl-enum-ciphers -p 443 target

# Look for: TLS "      # scheme in the path is optional

This strips page boilerplate — roughly 78% fewer tokens on a prose page — and returns the full text rather than a summary, so you can grep it and trust a negative result.

Three things it is not for. Fetch JSON and API responses raw, because readability extraction mangles structured data. Fetch authenticated or JavaScript-rendered pages directly, because it retrieves them anonymously. And never route adversary infrastructure (phishing links, C2, malware hosting), client-owned hosts, or engagement URLs through it — the request leaves your machine to a third party, and for live adversary infrastructure it also tips off the operator.

Some sites block the extractor and return an error blob rather than the page — {"error":"Failed to fetch: 418 I'm a teapot"} from freedesktop.org, for instance. That is the fetch being refused, not the source saying the thing does not exist. Re-fetch the URL directly before drawing any conclusion from it.

References

• auditing-code-for-vulnerabilities — the surrounding code review
• cracking-passwords — offensive side of weak password storage
• testing-apis — token and signature handling at the API layer
• Libraries to recommend: libsodium/NaCl, Google Tink, age, platform AEAD APIs
• testssl.sh, sslyze, cryptography (Python) audit APIs, cargo-crev

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: EvilFreelancer
• Source: EvilFreelancer/secs
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.