PyCrypt Best Practices: Secure Key Management and Usage

Building a Simple Encryption Library with PyCrypt

Overview

A compact, opinionated Python library that wraps PyCrypt primitives to provide easy-to-use, secure encryption for strings and files: AES-GCM for symmetric encryption, RSA for asymmetric key exchange/signing, secure key derivation (HKDF/PBKDF2), and safe serialization (JSON + base64). Focus: correctness, minimal surface area, clear API.

Core Features

• Symmetric encryption: AES-256-GCM with random 96-bit nonce, returns ciphertext + tag + nonce.
• Key derivation: PBKDF2-HMAC-SHA256 for passphrase → key; HKDF-SHA256 for expanding master keys.
• Asymmetric: RSA 3072 for key exchange and signatures (or use Ed25519/Curve25519 if available).
• File support: Chunked streaming encryption for large files with associated data (filename/metadata).
• Serialization: Compact JSON containing base64-encoded fields and version tag for future upgrades.
• Secure defaults: Constant-time comparisons, zeroing sensitive memory where possible, strict nonce reuse prevention.

API Design (suggested)

• encrypt_bytes(key: bytes, plaintext: bytes, aad: bytes|None=None) -> dict
• decrypt_bytes(key: bytes, payload: dict, aad: bytes|None=None) -> bytes
• derive_key_from_password(password: str, salt: bytes=None, iterations: int=200_000) -> (key, salt)
• generate_rsa_keypair(bits: int=3072) -> (private_pem, public_pem)
• encrypt_file(…), decrypt_file(…)

Example Implementation Sketch (conceptual)

• Use PyCrypt’s AES-GCM primitive for encryption.
• Generate a secure random nonce per message and include it in the serialized payload.
• For password-based keys, use a strong iteration count and unique salt per derivation.
• For file streaming, encrypt per-chunk with incremental counters included in AAD to prevent reorder attacks.

Security Notes

• Never reuse nonces with the same key.
• Use authenticated encryption (AES-GCM or ChaCha20-Poly1305).
• Prefer modern curves (X25519/Ed25519) over RSA when interoperability allows.
• Rotate keys periodically and include versioning in serialized output.
• Validate all inputs and fail closed on any integrity/authentication failure.

Deployment & Testing

• Unit tests for edge cases (empty plaintext, max-size, corrupted ciphertext).
• Fuzz tests on serialization parsing.
• Static analysis (bandit) and dependency checks.
• Provide clear migration path for future algorithm changes via versioned payloads.

Minimal Example (pseudo-output)

• encrypt_bytes returns JSON: {“v”:1,“alg”:“AES-GCM”,“nonce”:“…”,“ct”:“…”,“tag”:“…”,“salt”:“…”}