Crypto cheat sheet

Key size

128 / 192 / 256-bit key, 96-bit nonce

Speed

Very fast — GB/s with hardware acceleration

Use it when

Files, database fields, disk volumes, and everything after a TLS handshake. Default choice for 'encrypt this data'.

Watch out

Never reuse a nonce with the same key, and never share the key over an insecure channel.

Key size

256-bit key, 96-bit nonce

Speed

Very fast, even without AES instructions

Use it when

Mobile devices, embedded CPUs, or any platform lacking AES hardware support.

Watch out

Same nonce rule as AES-GCM — one nonce per message, always.

Key size

No key — 256-bit (32-byte) digest

Speed

Fast — hundreds of MB/s

Use it when

File integrity checks, digital signatures, blockchains, deduplication, commit IDs.

Watch out

Storing passwords directly — it is far too fast, so brute force is cheap.

Key size

Any length key (≥ 256-bit recommended)

Speed

Fast — about two SHA-256 passes

Use it when

Signing API requests, webhook verification, session tokens, cookie integrity.

Watch out

Comparing tags with ==; use a constant-time comparison to prevent timing leaks.

Key size

Salt + tuned work factor (e.g. 600k PBKDF2 iterations)

Speed

Intentionally slow — ~100 ms per hash

Use it when

Password storage and deriving encryption keys from passphrases. Argon2id is the modern pick.

Watch out

Plain SHA-256, and any scheme without a unique random salt per password.

Key size

2048–4096-bit modulus (2048 is today's minimum)

Speed

Slow — thousands of times slower than AES

Use it when

Certificates, signatures, and wrapping a small AES key. Widely supported by legacy systems.

Watch out

Encrypting large payloads directly — RSA-2048 tops out near 190 bytes with OAEP.

Key size

256-bit key ≈ RSA-3072 security

Speed

Fast signing and verification

Use it when

SSH keys, JWT signing, software updates, modern certificates. Prefer over RSA for new systems.

Watch out

ECDSA with a reused or predictable nonce — it leaks the private key.

Key size

256-bit ephemeral key pair per session

Speed

Fast — sub-millisecond per handshake

Use it when

Every TLS 1.3 handshake and any protocol wanting forward secrecy.

Watch out

Reusing the ephemeral key across sessions — that defeats forward secrecy.

Key size

No key — grows data by ~33%

Speed

Instant

Use it when

Email attachments, Data URIs, JSON payloads, PEM certificates, JWT parts.

Watch out

Treating it as a security measure — anyone can decode it in one line of code.

Key size

Any — only secure as a one-time pad (key as long as message)

Speed

Fastest possible — one CPU instruction

Use it when

Learning bit operations, checksums, and as a stage inside stream ciphers.

Watch out

Short repeating keys — trivially broken by frequency analysis.

Key size

1 shift value (Caesar) or a keyword (Vigenère)

Speed

Instant

Use it when

Teaching substitution, puzzles, and understanding why key length matters.

Watch out

Any real secret — both fall to pen-and-paper cryptanalysis.