RSA Cryptographic Text Processor: Securely Encrypt and Decrypt Your Messages

Building an RSA Cryptographic Text Processor — Design & Implementation Guide### Introduction

RSA remains one of the foundational public-key cryptosystems used for secure data exchange, digital signatures, and key encapsulation. A “Cryptographic Text Processor” in this context is an application (CLI, library, or web service) that accepts plaintext or ciphertext and performs RSA-based operations: encryption, decryption, signing, and verification. This guide walks through design decisions, security considerations, and a practical implementation plan for building a robust RSA cryptographic text processor suitable for educational, experimental, or production-adjacent use.

Goals and scope

Define clear goals before implementation:

Primary features: RSA encryption/decryption, signing/verification, key generation, key import/export (PEM), and basic key management.
Targets: CLI tool, with optional library/API bindings and a minimal GUI.
Security posture: Provide secure defaults (OAEP/PSS where applicable), defend against common attacks (padding oracle, side channels), and enable secure storage of private keys.
Usability: Human-readable input/output (base64, hex), file and stdin/stdout support, and clear error messages.
Extensibility: Pluggable backends (software vs. hardware security modules), algorithm agility (support other asymmetric schemes later).

Background: RSA basics

RSA relies on the mathematical properties of large primes. Key components:

Public key: (n, e) where n = p * q and e is the public exponent.
Private key: d (mod φ(n)) where φ(n) = (p−1)(q−1), and optionally CRT parameters (p, q, dP, dQ, qInv) for faster decryption.
Security depends on the difficulty of factoring n.

Cryptographic operations:

Encryption: c = m^e mod n
Decryption: m = c^d mod n
Signing: s = m^d mod n (with hashing and padding)
Verification: m = s^e mod n

Best practice: never encrypt raw messages with raw RSA; always use padding schemes like OAEP for encryption and PSS for signatures.

High-level architecture

A modular design reduces risk and improves maintainability:

CLI layer: parses arguments, validates input, routes operations.
Core crypto module: implements RSA operations using a vetted crypto library (OpenSSL, libsodium where applicable, or language-specific libraries like Rust’s ring).
Key management module: key generation, import/export (PEM/DER), secure storage with optional passphrase-based encryption (PBKDF2/Argon2 + AES-GCM).
I/O layer: supports stdin/stdout, files, base64/hex encodings, and streaming large inputs.
Policy & config: defines algorithm choices (hash, padding), key sizes, iteration counts for KDFs, and file paths.
Tests & CI: unit tests, integration tests, fuzzing, and reproducible builds.
Optional HSM/TPM backend: for production secret key storage and operations.

Choosing libraries and languages

Select languages and libraries based on security, performance, and ecosystem:

Rust: ring, rsa, or openssl crates — strong memory safety.
Go: crypto/rsa (standard library) — easy concurrency, deployment.
Python: cryptography library (bindings to OpenSSL) — rapid prototyping.
C/C++: OpenSSL or BoringSSL — highest control but greater risk of memory errors.

Recommendation: prototype in Python or Go, then implement production-grade components in Rust or Go for safety and performance.

Detailed design decisions

Key sizes and exponents

Minimum key size: 2048 bits for short-term security; 3072–4096 bits for long-term.
Public exponent: 65537 is the practical default for speed and security.

Padding and hashing

Encryption: use RSA-OAEP with SHA-256 (or stronger).
Signatures: use RSA-PSS with SHA-256 (or SHA-384/SHA-512 for larger keys).
For legacy interoperability, support PKCS#1 v1.5 only with clear warnings.

Randomness

Use a cryptographically secure RNG provided by the OS (e.g., getrandom, /dev/urandom). Avoid implementing custom RNGs.

Key storage

Private keys should be stored encrypted with a strong KDF: Argon2id (preferred) or PBKDF2-HMAC-SHA256 with high iteration counts. Encrypt with authenticated encryption (AES-GCM or ChaCha20-Poly1305).
Offer optional integration with OS key stores or HSMs.

Side-channel resistance

Use constant-time implementations from vetted libraries. Avoid exposing timing/acles via API. When performing decryption/signing, consider using blinding to mitigate timing attacks.

File formats & interoperability

Keys: support PEM (PKCS#1 and PKCS#8) and DER.
Encrypted output: base64-encoded ciphertext with metadata header (algorithm, key id, padding) in a simple JSON wrapper for ease of parsing.
Signatures: provide detached and attached signature formats; include signature metadata (hash algorithm, padding).

Example JSON wrapper: { “alg”: “RSA-OAEP”, “hash”: “SHA-256”, “key_id”: “[email protected]|2025-08-29”, “ciphertext”: “base64…” }

CLI usage examples

Generate keypair: rsa-tp gen –size 3072 –out private.pem –pubout public.pem
Encrypt file: rsa-tp encrypt –pub public.pem –in message.txt –out message.enc –format json
Decrypt file: rsa-tp decrypt –priv private.pem –in message.enc –out message.txt
Sign: rsa-tp sign –priv private.pem –in message.txt –out message.sig
Verify: rsa-tp verify –pub public.pem –in message.txt –sig message.sig

Implementation: sample in Python (cryptography)

Below is a compact implementation illustrating key generation, OAEP encryption/decryption, and PSS signing/verification using the cryptography library.

from cryptography.hazmat.primitives.asymmetric import rsa, padding from cryptography.hazmat.primitives import hashes, serialization from cryptography.hazmat.primitives.kdf.scrypt import Scrypt from cryptography.hazmat.primitives.ciphers.aead import AESGCM import os, base64 # Key generation def gen_rsa(key_size=3072, public_exponent=65537):     priv = rsa.generate_private_key(public_exponent=public_exponent, key_size=key_size)     pub = priv.public_key()     return priv, pub # Export PEM (unencrypted private) def priv_to_pem(priv):     return priv.private_bytes(         encoding=serialization.Encoding.PEM,         format=serialization.PrivateFormat.PKCS8,         encryption_algorithm=serialization.NoEncryption()     ) def pub_to_pem(pub):     return pub.public_bytes(         encoding=serialization.Encoding.PEM,         format=serialization.PublicFormat.SubjectPublicKeyInfo     ) # OAEP encrypt/decrypt def rsa_oaep_encrypt(pub, plaintext: bytes) -> bytes:     return pub.encrypt(         plaintext,         padding.OAEP(mgf=padding.MGF1(algorithm=hashes.SHA256()), algorithm=hashes.SHA256(), label=None)     ) def rsa_oaep_decrypt(priv, ciphertext: bytes) -> bytes:     return priv.decrypt(         ciphertext,         padding.OAEP(mgf=padding.MGF1(algorithm=hashes.SHA256()), algorithm=hashes.SHA256(), label=None)     ) # PSS sign/verify def rsa_pss_sign(priv, message: bytes) -> bytes:     return priv.sign(         message,         padding.PSS(mgf=padding.MGF1(hashes.SHA256()), salt_length=padding.PSS.MAX_LENGTH),         hashes.SHA256()     ) def rsa_pss_verify(pub, message: bytes, signature: bytes) -> bool:     try:         pub.verify(             signature,             message,             padding.PSS(mgf=padding.MGF1(hashes.SHA256()), salt_length=padding.PSS.MAX_LENGTH),             hashes.SHA256()         )         return True     except Exception:         return False

Security considerations and attacks to mitigate

Padding oracle attacks: never return distinct errors for padding failures. Use constant-time error handling.
Timing and side channels: prefer libraries with constant-time ops, use blinding for private-key operations.
Key reuse: avoid using RSA to encrypt large messages directly; use RSA to encrypt symmetric keys (hybrid encryption).
Randomness failures: validate RNG availability and source.
Metadata leakage: avoid leaking key identifiers or operation details in error messages.

Testing strategy

Unit tests for each crypto primitive and CLI path.
Interoperability tests using OpenSSL command-line.
Fuzz inputs for parsers and file handling.
Property-based tests for serialization/deserialization.
Static analysis and dependency vulnerability scanning.

Deployment and operational notes

Use secure defaults in config files, do not ship with debug keys.
Rotate keys periodically and provide clear procedures for revocation.
Audit logging: log operations but never log private key material or plaintexts. Consider redaction.
Provide a minimal, well-documented API for other services.

Extending the processor

Add hybrid encryption (RSA + AES-GCM) for large messages.
Implement hardware-backed key storage (HSM, YubiKey, TPM).
Support elliptic-curve algorithms (ECDSA, Ed25519) for scenarios where RSA is not ideal.
Add a server mode with authenticated access and rate limiting for automated workflows.

Conclusion

A secure, usable RSA Cryptographic Text Processor requires careful attention to padding schemes, key management, secure defaults, and the use of vetted libraries. Follow best practices (OAEP/PSS, Argon2/AES-GCM for key storage, constant-time operations) and design modular components so the system can evolve (HSM support, additional algorithms). The provided code snippets and architecture outline form a strong starting point for a production-quality implementation.