95 lines
7.4 KiB
Markdown
95 lines
7.4 KiB
Markdown
# Cryptography Cheatsheet
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| Category | Algorithm/Method | Description | Example Usage | Security Level |
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|----------|------------------|-------------|--------------|----------------|
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| **Symmetric Encryption** |||||
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| Block Ciphers | AES-256 | Advanced Encryption Standard | `openssl enc -aes-256-cbc -in plain.txt -out encrypted.bin` | Strong (Recommended) |
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| | AES-128 | AES with 128-bit key | `openssl enc -aes-128-cbc -in plain.txt -out encrypted.bin` | Adequate |
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| | 3DES | Triple Data Encryption Standard | `openssl enc -des3 -in plain.txt -out encrypted.bin` | Legacy (Avoid) |
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| Stream Ciphers | ChaCha20 | Modern stream cipher | `openssl enc -chacha20 -in plain.txt -out encrypted.bin` | Strong |
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| | RC4 | Rivest Cipher 4 | `openssl enc -rc4 -in plain.txt -out encrypted.bin` | Broken (Avoid) |
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| Operation Modes | GCM | Galois/Counter Mode (authenticated) | `openssl enc -aes-256-gcm -in plain.txt -out encrypted.bin` | Strong (Recommended) |
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| | CBC | Cipher Block Chaining | `openssl enc -aes-256-cbc -in plain.txt -out encrypted.bin` | Adequate with proper IV |
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| | ECB | Electronic Codebook | `openssl enc -aes-256-ecb -in plain.txt -out encrypted.bin` | Weak (Avoid) |
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| | CTR | Counter Mode | `openssl enc -aes-256-ctr -in plain.txt -out encrypted.bin` | Strong with unique nonce |
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| **Asymmetric Encryption** |||||
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| Key Exchange | RSA-2048+ | Rivest-Shamir-Adleman | `openssl genrsa -out private.pem 4096` | Strong (≥2048 bits) |
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| | ECC (P-256) | Elliptic Curve Cryptography | `openssl ecparam -genkey -name prime256v1 -out ecc.key` | Strong (≥256 bits) |
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| | DH | Diffie-Hellman | `openssl dhparam -out dhparams.pem 2048` | Strong (≥2048 bits) |
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| | ECDH | Elliptic Curve Diffie-Hellman | Used in TLS handshakes | Strong |
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| Modern Standards | X25519 | Curve25519 for key exchange | Used in Signal Protocol | Very Strong |
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| | Ed25519 | Edwards-curve for signatures | `ssh-keygen -t ed25519` | Very Strong |
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| **Hashing Algorithms** |||||
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| Modern | SHA-256 | Secure Hash Algorithm 256-bit | `openssl dgst -sha256 file.txt` | Strong |
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| | SHA-3 | Secure Hash Algorithm 3 | `openssl dgst -sha3-256 file.txt` | Very Strong |
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| | BLAKE2 | Fast secure hash function | `b2sum file.txt` | Very Strong |
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| Legacy | SHA-1 | Secure Hash Algorithm 1 | `openssl dgst -sha1 file.txt` | Broken (Avoid) |
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| | MD5 | Message Digest 5 | `openssl dgst -md5 file.txt` | Broken (Avoid) |
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| Password Hashing | Argon2id | Memory-hard function | `argon2 password -id -t 3 -m 16 -p 4` | Strongest (Recommended) |
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| | bcrypt | Blowfish-based hash | `htpasswd -B -C 12 passfile user` | Strong |
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| | PBKDF2 | Key derivation function | `openssl pkeyutl -kdf PBKDF2` | Adequate (high iterations) |
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| | Scrypt | Memory-hard function | `scrypt password salt 16384 8 1 32` | Strong |
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| **Message Authentication** |||||
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| HMAC | HMAC-SHA256 | Hash-based Message Authentication | `openssl dgst -sha256 -hmac "key" file.txt` | Strong |
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| Authenticated Encryption | AES-GCM | Encryption with built-in auth | `openssl enc -aes-256-gcm -in file.txt` | Strong (Recommended) |
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| | ChaCha20-Poly1305 | Authenticated stream cipher | Used in TLS 1.3 | Strong (Recommended) |
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| **Digital Signatures** |||||
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| RSA-based | RSA-PSS | Probabilistic Signature Scheme | `openssl dgst -sha256 -sign key.pem -sigopt rsa_padding_mode:pss file` | Strong |
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| | PKCS#1 v1.5 | Traditional RSA signature | `openssl dgst -sha256 -sign key.pem file` | Adequate |
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| EC-based | ECDSA | Elliptic Curve Digital Signature | `openssl dgst -sha256 -sign ec.key file` | Strong |
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| | Ed25519 | Edwards-curve Digital Signature | `openssl dgst -sign ed.key file` | Very Strong (Recommended) |
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| **Key Derivation** |||||
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| Password-based | PBKDF2 | Password-Based Key Derivation | `openssl pkeyutl -kdf PBKDF2 -kdflen 32` | Adequate (≥10k iterations) |
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| | Argon2 | Memory-hard KDF | `argon2 password -id -t 3 -m 16 -p 4` | Strong (Recommended) |
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| | scrypt | Memory-hard KDF | `openssl kdf -kdf scrypt -password pass -key-length 32` | Strong |
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| Key-based | HKDF | HMAC-based Extract-and-Expand | `openssl kdf -kdf hkdf -salt salt -key key -out output.key` | Strong |
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| **Random Number Generation** |||||
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| Cryptographic PRNGs | /dev/urandom | OS random source (Unix) | `dd if=/dev/urandom of=rand bs=32 count=1` | Strong |
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| | CryptGenRandom | Windows API | Used via programming languages | Strong |
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| | RDRAND | CPU instruction | Used in newer CPUs | Strong when combined |
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| **Protocols & Standards** |||||
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| TLS | TLS 1.3 | Transport Layer Security | `openssl s_client -tls1_3 -connect example.com:443` | Strong (Recommended) |
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| | TLS 1.2 | Transport Layer Security | `openssl s_client -tls1_2 -connect example.com:443` | Adequate |
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| | SSL 3.0, TLS 1.0/1.1 | Legacy protocols | Disable in configurations | Weak (Avoid) |
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| SSH | SSH-2 | Secure Shell v2 | `ssh -o "Protocol 2" user@host` | Strong |
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| | SSH-1 | Legacy Secure Shell | Disable in configurations | Broken (Avoid) |
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| PGP/GPG | GPG | GNU Privacy Guard | `gpg --encrypt --recipient user@example.com file` | Strong |
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## Common Cryptographic Operations
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| Operation | OpenSSL Command | Example |
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|-----------|-----------------|---------|
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| Generate RSA key pair | `openssl genrsa` | `openssl genrsa -out private.pem 4096` |
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| Extract public key | `openssl rsa` | `openssl rsa -in private.pem -pubout -out public.pem` |
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| Generate ECC key | `openssl ecparam` | `openssl ecparam -genkey -name prime256v1 -out ec.key` |
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| Create CSR | `openssl req` | `openssl req -new -key private.pem -out cert.csr` |
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| Sign file | `openssl dgst` | `openssl dgst -sha256 -sign private.pem -out sig.bin file.txt` |
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| Verify signature | `openssl dgst` | `openssl dgst -sha256 -verify public.pem -signature sig.bin file.txt` |
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| Encrypt file (symmetric) | `openssl enc` | `openssl enc -aes-256-gcm -salt -in file.txt -out file.enc` |
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| Decrypt file | `openssl enc` | `openssl enc -d -aes-256-gcm -in file.enc -out file.txt` |
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| Generate random bytes | `openssl rand` | `openssl rand -base64 32` |
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## Key Length Recommendations (2023+)
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| Algorithm Type | Minimum Secure Length | Recommended Length | Notes |
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|----------------|------------------------|-------------------|-------|
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| AES | 128 bits | 256 bits | No known practical attacks |
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| RSA | 2048 bits | 4096 bits | Increases computational cost |
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| ECC | 256 bits | 384 bits | NIST P-256 or Curve25519 |
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| Hash functions | 256 bits | 384+ bits | SHA-256 or stronger |
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| HMAC | 256 bits | 384+ bits | Based on the underlying hash |
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| Symmetric key | 128 bits | 256 bits | For long-term security |
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## Common Vulnerabilities & Mitigations
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| Vulnerability | Description | Mitigation |
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|---------------|-------------|------------|
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| Padding Oracle | Leaks info about padding validity | Use authenticated encryption (GCM, ChaCha20-Poly1305) |
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| Key Reuse | Same key for multiple messages | Use unique keys/IVs for each encryption |
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| Weak RNG | Predictable random numbers | Use cryptographically secure RNGs (/dev/urandom, CryptGenRandom) |
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| Side-Channel Attacks | Timing, power analysis | Use constant-time implementations |
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| Downgrade Attacks | Force use of weaker protocols | Disable legacy protocols, use strict configurations |
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| Known Plaintext | Predictable plaintext locations | Add randomization where possible |
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| Insufficient Key Size | Too small keys are brute-forceable | Follow key length recommendations above |
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| Certificate Issues | Invalid/expired certificates | Automate certificate management, use HSTS |
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| Hash Collisions | Finding two inputs with same hash | Use collision-resistant algorithms (SHA-256+) |
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