1288 Cryptography 1e130162

Cryptographic Security Reference

Core Principles

1. Avoid storing sensitive data when possible - the best protection is not having the data
2. Use established libraries - never implement cryptographic algorithms yourself
3. Use modern algorithms - avoid deprecated algorithms even if they seem convenient
4. Manage keys securely - key management is often harder than encryption itself

Encryption Algorithms

Symmetric Encryption

Recommended:

• AES-256-GCM (preferred) - Provides encryption + authentication
• AES-128-GCM - Acceptable minimum
• ChaCha20-Poly1305 - Good alternative, especially on systems without AES hardware

Avoid:

• DES, 3DES - Deprecated, insufficient key length
• RC4 - Broken
• AES-ECB - Reveals patterns in data
• AES-CBC without authentication - Vulnerable to padding oracle attacks

Cipher Modes

# VULNERABLE: ECB mode
from Crypto.Cipher import AES
cipher = AES.new(key, AES.MODE_ECB)

# SAFE: GCM mode
cipher = AES.new(key, AES.MODE_GCM, nonce=nonce)
ciphertext, tag = cipher.encrypt_and_digest(plaintext)

Asymmetric Encryption

Recommended:

• ECC with Curve25519 (preferred for key exchange)
• RSA-2048 minimum (RSA-4096 for long-term)
• ECDSA with P-256 or Ed25519 for signatures

Avoid:

• RSA < 2048 bits
• DSA
• ECDSA with weak curves

Secure Random Number Generation

Cryptographically Secure PRNGs (CSPRNG)

# VULNERABLE: Predictable random
import random
token = ''.join(random.choices(string.ascii_letters, k=32))

# SAFE: Cryptographically secure
import secrets
token = secrets.token_urlsafe(32)

UUID Considerations

• UUID v1: NOT random - contains timestamp and MAC address
• UUID v4: Depends on implementation - verify CSPRNG usage
• ULID: Time-sortable but predictable time component

# Check if UUID v4 is actually random
import uuid
# uuid.uuid4() uses os.urandom() in Python - SAFE
token = str(uuid.uuid4())

Key Management

Key Generation

# VULNERABLE: Key from password directly
key = password.encode()

# SAFE: Key derivation function
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
kdf = PBKDF2HMAC(
    algorithm=hashes.SHA256(),
    length=32,
    salt=salt,
    iterations=600000,
)
key = kdf.derive(password.encode())

Key Storage

Do:

• Use Hardware Security Modules (HSM)
• Use cloud key management (AWS KMS, Azure Key Vault, GCP KMS)
• Use dedicated secrets managers (HashiCorp Vault)
• Store keys separately from encrypted data

Don't:

• Hardcode keys in source code
• Commit keys to version control
• Store keys in environment variables (can leak)
• Store keys in plaintext files

# VULNERABLE: Hardcoded key
KEY = b'super_secret_key_12345'

# VULNERABLE: Key in code as base64
KEY = base64.b64decode('c3VwZXJfc2VjcmV0X2tleQ==')

# SAFE: Load from secure source
KEY = secrets_manager.get_secret('encryption_key')

Key Rotation

When to rotate:

• Key compromise (immediate)
• Cryptoperiod expiration (time-based)
• After encrypting 2^35 bytes (for 64-bit block ciphers)
• Algorithm deprecation

Rotation strategies:

1. Re-encryption (preferred): Decrypt with old key, re-encrypt with new
2. Versioning: Tag encrypted items with key version, maintain multiple keys

Envelope Encryption

# Two-key structure:
# - Data Encryption Key (DEK): Encrypts actual data
# - Key Encryption Key (KEK): Encrypts the DEK

def encrypt_with_envelope(plaintext, kek):
    # Generate random DEK
    dek = secrets.token_bytes(32)

    # Encrypt data with DEK
    cipher = AES.new(dek, AES.MODE_GCM)
    ciphertext, tag = cipher.encrypt_and_digest(plaintext)

    # Encrypt DEK with KEK
    kek_cipher = AES.new(kek, AES.MODE_GCM)
    encrypted_dek, dek_tag = kek_cipher.encrypt_and_digest(dek)

    # Store encrypted_dek with ciphertext
    return {
        'ciphertext': ciphertext,
        'tag': tag,
        'encrypted_dek': encrypted_dek,
        'dek_tag': dek_tag,
        'nonce': cipher.nonce,
        'dek_nonce': kek_cipher.nonce
    }

Hashing

Password Hashing

See authentication.md for password-specific hashing.

General Purpose Hashing

Avoid for new systems:

• MD5 (broken)
• SHA-1 (deprecated)

# For integrity/checksums
import hashlib
digest = hashlib.sha256(data).hexdigest()

# For authentication (HMAC)
import hmac
mac = hmac.new(key, data, hashlib.sha256).digest()

Common Vulnerabilities

Weak Algorithm Usage

# VULNERABLE: MD5 for security purposes
import hashlib
checksum = hashlib.md5(data).hexdigest()

# VULNERABLE: SHA1 for signatures
signature = hashlib.sha1(data + secret).hexdigest()

# SAFE: SHA-256
checksum = hashlib.sha256(data).hexdigest()

Insufficient Key Size

# VULNERABLE: Short key
key = b'short_key'  # 9 bytes

# SAFE: Adequate key length
key = secrets.token_bytes(32)  # 256 bits

Predictable IV/Nonce

# VULNERABLE: Reused or predictable nonce
nonce = b'\x00' * 12  # Static nonce

# VULNERABLE: Counter-based without persistence
nonce = counter.to_bytes(12, 'big')

# SAFE: Random nonce
nonce = secrets.token_bytes(12)

ECB Mode Patterns

# VULNERABLE: ECB reveals patterns
cipher = AES.new(key, AES.MODE_ECB)

# SAFE: GCM hides patterns
cipher = AES.new(key, AES.MODE_GCM, nonce=nonce)

Missing Authentication

# VULNERABLE: Encryption without authentication
cipher = AES.new(key, AES.MODE_CBC, iv=iv)
ciphertext = cipher.encrypt(pad(plaintext, 16))
# Vulnerable to bit-flipping, padding oracle

# SAFE: Authenticated encryption
cipher = AES.new(key, AES.MODE_GCM, nonce=nonce)
ciphertext, tag = cipher.encrypt_and_digest(plaintext)

Grep Patterns for Detection

# Weak algorithms
grep -rn "MD5\|md5\|SHA1\|sha1\|DES\|des\|RC4\|rc4" --include="*.py" --include="*.js"
grep -rn "MODE_ECB\|ecb" --include="*.py" --include="*.js"

# Insecure random
grep -rn "Math\.random\|random\.random\|random\.randint" --include="*.py" --include="*.js"
grep -rn "mt_rand\|rand()" --include="*.php"

# Hardcoded keys
grep -rn "key\s*=\s*['\"]" --include="*.py" --include="*.js"
grep -rn "secret\s*=\s*['\"]" --include="*.py" --include="*.js"
grep -rn "AES\.new.*b'" --include="*.py"

# Static IVs/nonces
grep -rn "iv\s*=\s*b'\|nonce\s*=\s*b'" --include="*.py"
grep -rn "\\x00.*\\x00.*\\x00" --include="*.py"

# CBC without HMAC
grep -rn "MODE_CBC" --include="*.py" | grep -v "hmac\|mac\|tag"

Testing Checklist

• No hardcoded keys/secrets in source code
• Keys not committed to version control
• Using modern algorithms (AES-GCM, RSA-2048+, SHA-256+)
• CSPRNG used for all security-sensitive randomness
• Keys stored securely (HSM, KMS, secrets manager)
• Key rotation mechanism exists
• No ECB mode usage
• Authenticated encryption used (GCM, or encrypt-then-MAC)
• Adequate key lengths (256-bit symmetric, 2048+ RSA)
• IVs/nonces are random and never reused with same key

References

• OWASP Cryptographic Storage Cheat Sheet
• OWASP Key Management Cheat Sheet
• CWE-327: Use of Broken Crypto Algorithm
• CWE-330: Insufficient Randomness
• CWE-321: Hard-coded Cryptographic Key