security-cheatsheets/cryptography/crypto-reference.md

Cryptography Cheatsheet

Category	Algorithm/Method	Description	Example Usage	Security Level
Symmetric Encryption
Block Ciphers	AES-256	Advanced Encryption Standard	openssl enc -aes-256-cbc -in plain.txt -out encrypted.bin	Strong (Recommended)
	AES-128	AES with 128-bit key	openssl enc -aes-128-cbc -in plain.txt -out encrypted.bin	Adequate
	3DES	Triple Data Encryption Standard	openssl enc -des3 -in plain.txt -out encrypted.bin	Legacy (Avoid)
Stream Ciphers	ChaCha20	Modern stream cipher	openssl enc -chacha20 -in plain.txt -out encrypted.bin	Strong
	RC4	Rivest Cipher 4	openssl enc -rc4 -in plain.txt -out encrypted.bin	Broken (Avoid)
Operation Modes	GCM	Galois/Counter Mode (authenticated)	openssl enc -aes-256-gcm -in plain.txt -out encrypted.bin	Strong (Recommended)
	CBC	Cipher Block Chaining	openssl enc -aes-256-cbc -in plain.txt -out encrypted.bin	Adequate with proper IV
	ECB	Electronic Codebook	openssl enc -aes-256-ecb -in plain.txt -out encrypted.bin	Weak (Avoid)
	CTR	Counter Mode	openssl enc -aes-256-ctr -in plain.txt -out encrypted.bin	Strong with unique nonce
Asymmetric Encryption
Key Exchange	RSA-2048+	Rivest-Shamir-Adleman	openssl genrsa -out private.pem 4096	Strong (≥2048 bits)
	ECC (P-256)	Elliptic Curve Cryptography	openssl ecparam -genkey -name prime256v1 -out ecc.key	Strong (≥256 bits)
	DH	Diffie-Hellman	openssl dhparam -out dhparams.pem 2048	Strong (≥2048 bits)
	ECDH	Elliptic Curve Diffie-Hellman	Used in TLS handshakes	Strong
Modern Standards	X25519	Curve25519 for key exchange	Used in Signal Protocol	Very Strong
	Ed25519	Edwards-curve for signatures	ssh-keygen -t ed25519	Very Strong
Hashing Algorithms
Modern	SHA-256	Secure Hash Algorithm 256-bit	openssl dgst -sha256 file.txt	Strong
	SHA-3	Secure Hash Algorithm 3	openssl dgst -sha3-256 file.txt	Very Strong
	BLAKE2	Fast secure hash function	b2sum file.txt	Very Strong
Legacy	SHA-1	Secure Hash Algorithm 1	openssl dgst -sha1 file.txt	Broken (Avoid)
	MD5	Message Digest 5	openssl dgst -md5 file.txt	Broken (Avoid)
Password Hashing	Argon2id	Memory-hard function	argon2 password -id -t 3 -m 16 -p 4	Strongest (Recommended)
	bcrypt	Blowfish-based hash	htpasswd -B -C 12 passfile user	Strong
	PBKDF2	Key derivation function	openssl pkeyutl -kdf PBKDF2	Adequate (high iterations)
	Scrypt	Memory-hard function	scrypt password salt 16384 8 1 32	Strong
Message Authentication
HMAC	HMAC-SHA256	Hash-based Message Authentication	openssl dgst -sha256 -hmac "key" file.txt	Strong
Authenticated Encryption	AES-GCM	Encryption with built-in auth	openssl enc -aes-256-gcm -in file.txt	Strong (Recommended)
	ChaCha20-Poly1305	Authenticated stream cipher	Used in TLS 1.3	Strong (Recommended)
Digital Signatures
RSA-based	RSA-PSS	Probabilistic Signature Scheme	openssl dgst -sha256 -sign key.pem -sigopt rsa_padding_mode:pss file	Strong
	PKCS#1 v1.5	Traditional RSA signature	openssl dgst -sha256 -sign key.pem file	Adequate
EC-based	ECDSA	Elliptic Curve Digital Signature	openssl dgst -sha256 -sign ec.key file	Strong
	Ed25519	Edwards-curve Digital Signature	openssl dgst -sign ed.key file	Very Strong (Recommended)
Key Derivation
Password-based	PBKDF2	Password-Based Key Derivation	openssl pkeyutl -kdf PBKDF2 -kdflen 32	Adequate (≥10k iterations)
	Argon2	Memory-hard KDF	argon2 password -id -t 3 -m 16 -p 4	Strong (Recommended)
	scrypt	Memory-hard KDF	openssl kdf -kdf scrypt -password pass -key-length 32	Strong
Key-based	HKDF	HMAC-based Extract-and-Expand	openssl kdf -kdf hkdf -salt salt -key key -out output.key	Strong
Random Number Generation
Cryptographic PRNGs	/dev/urandom	OS random source (Unix)	dd if=/dev/urandom of=rand bs=32 count=1	Strong
	CryptGenRandom	Windows API	Used via programming languages	Strong
	RDRAND	CPU instruction	Used in newer CPUs	Strong when combined
Protocols & Standards
TLS	TLS 1.3	Transport Layer Security	openssl s_client -tls1_3 -connect example.com:443	Strong (Recommended)
	TLS 1.2	Transport Layer Security	openssl s_client -tls1_2 -connect example.com:443	Adequate
	SSL 3.0, TLS 1.0/1.1	Legacy protocols	Disable in configurations	Weak (Avoid)
SSH	SSH-2	Secure Shell v2	ssh -o "Protocol 2" user@host	Strong
	SSH-1	Legacy Secure Shell	Disable in configurations	Broken (Avoid)
PGP/GPG	GPG	GNU Privacy Guard	gpg --encrypt --recipient user@example.com file	Strong

Common Cryptographic Operations

Operation	OpenSSL Command	Example
Generate RSA key pair	openssl genrsa	openssl genrsa -out private.pem 4096
Extract public key	openssl rsa	openssl rsa -in private.pem -pubout -out public.pem
Generate ECC key	openssl ecparam	openssl ecparam -genkey -name prime256v1 -out ec.key
Create CSR	openssl req	openssl req -new -key private.pem -out cert.csr
Sign file	openssl dgst	openssl dgst -sha256 -sign private.pem -out sig.bin file.txt
Verify signature	openssl dgst	openssl dgst -sha256 -verify public.pem -signature sig.bin file.txt
Encrypt file (symmetric)	openssl enc	openssl enc -aes-256-gcm -salt -in file.txt -out file.enc
Decrypt file	openssl enc	openssl enc -d -aes-256-gcm -in file.enc -out file.txt
Generate random bytes	openssl rand	openssl rand -base64 32

Key Length Recommendations (2023+)

Algorithm Type	Minimum Secure Length	Recommended Length	Notes
AES	128 bits	256 bits	No known practical attacks
RSA	2048 bits	4096 bits	Increases computational cost
ECC	256 bits	384 bits	NIST P-256 or Curve25519
Hash functions	256 bits	384+ bits	SHA-256 or stronger
HMAC	256 bits	384+ bits	Based on the underlying hash
Symmetric key	128 bits	256 bits	For long-term security

Common Vulnerabilities & Mitigations

Vulnerability	Description	Mitigation
Padding Oracle	Leaks info about padding validity	Use authenticated encryption (GCM, ChaCha20-Poly1305)
Key Reuse	Same key for multiple messages	Use unique keys/IVs for each encryption
Weak RNG	Predictable random numbers	Use cryptographically secure RNGs (/dev/urandom, CryptGenRandom)
Side-Channel Attacks	Timing, power analysis	Use constant-time implementations
Downgrade Attacks	Force use of weaker protocols	Disable legacy protocols, use strict configurations
Known Plaintext	Predictable plaintext locations	Add randomization where possible
Insufficient Key Size	Too small keys are brute-forceable	Follow key length recommendations above
Certificate Issues	Invalid/expired certificates	Automate certificate management, use HSTS
Hash Collisions	Finding two inputs with same hash	Use collision-resistant algorithms (SHA-256+)