Common Crypto Mistakes

A cryptographic algorithm that survived public attack for years is boring on purpose. AES-GCM, ChaCha20-Poly1305, SHA-256, Argon2, TLS 1.3 libraries in your language’s standard or well-maintained packages — those are the default. Homebrew is: XOR with a repeating key, “encrypt” by reversing strings, a custom substitution table, a secret algorithm whose only defense is that nobody has seen the code. Secrecy of the algorithm is not a substitute for a key (Kerckhoffs’s principle: assume the method is public). This course will not print a broken cipher for you to point at other systems. If you need to see why XOR-with-a-password fails, read a textbook later — do not use this app as an attack workbook. Rolling your own protocol is the cousin: inventing a handshake in JSON because TLS “looked heavy,” inventing a MAC by appending MD5 of the password, inventing a token format that is base64(userid) and calling it signed. Use TLS for transport. Use vetted token libraries if you need tokens. Use vetted password hashes (slow KDF, unique salt) if you store passwords. The moment you type encrypt() as a 20-line function in a product, you are the incident.

ECB mode encrypts each block independently. Patterns leak. The classic warning is an image that still shows its outline after “encryption.” Do not use ECB. Modern systems want authenticated encryption (AES-GCM, ChaCha20-Poly1305) so ciphertext cannot be silently flipped. GCM and similar AEAD modes require a nonce (IV) that must not be reused with the same key. Reuse can destroy confidentiality — that is a famous footgun, not a puzzle this course will help you exploit against a live app. Operational rule: let a high-level library generate nonces; never hardcode IV=0; never copy a nonce between messages “to keep it simple.” verify=False (or equivalent) in an HTTP client turns TLS into encryption to whoever answered. An active attacker presents any certificate. You already know from Module 3 that authentication is the other half of the padlock. Disabling it in production because a lab CA was annoying is how you undo three lessons. Fix the trust store properly on machines you own, or fix the server certificate. Do not globally ignore hostname checks. Do not ship a client that skips verification “just for debugging” and then forget.

The file is a checklist you can reread before you merge code. It contains forbidden patterns and the replacement (vetted library, unique nonce, verify on). It does not contain a cipher implementation. It does not contain a live key. chmod 600 because Module 4’s lab folder will also hold a lab passphrase. Empty notes fail. Notes that include an attack script fail ethics even if you titled them “defensive.”

mkdir -p "$HOME/cyberlium-lab"
NOTES="$HOME/cyberlium-lab/crypto-mistakes-notes.txt"

{
  echo "=== COMMON CRYPTO MISTAKES (AVOID) ==="
  echo "1 Homebrew: no XOR-password 'ciphers', no secret algorithms. Use vetted libs (AES-GCM / TLS 1.3 stacks)."
  echo "2 Do not roll your own handshake or MAC. Use TLS for transport. Use slow salted KDFs for passwords."
  echo "3 ECB: do not use. Prefer authenticated encryption (AES-GCM, ChaCha20-Poly1305)."
  echo "4 Nonce/IV: never reuse with the same key. Never hardcode IV=0. Let the library generate."
  echo "5 verify=False / skip hostname checks: never production. Fix certs; do not disable TLS auth."
  echo "6 Keys in git/chat: never live secrets. Toy keys only. Next lesson: generate/store/rotate/revoke."
  echo ""
  echo "replacement: boring high-level crypto APIs with safe defaults"
  echo "ethics: this list is a STOP sign — not a broken-cipher kit to attack others"
  echo "no live API keys, no production PEMs, no classmate targets"
} > "$NOTES"

chmod 600 "$NOTES"

You now treat most crypto failures as wiring, not as “AES got solved.” Avoidance is the skill: boring libraries, unique nonces, verification on, no homebrew weapons. Next — Key Management Basics — generate, store, rotate, and revoke keys without leaving secrets in chat or git. Toy keys only. Then a lab that hashes and encrypts a sample file YOU created with a lab passphrase you will not reuse.