Building an RSA Cryptographic Text Processor: A Practical Guide

Date: February 7, 2026

This guide walks through designing and implementing an RSA-based cryptographic text processor that can encrypt, decrypt, sign, and verify text data reliably and securely. It focuses on practical choices, secure defaults, and performance considerations suitable for developer use (command-line tools, libraries, or backend services).

1. Goals and scope

Primary functions: Encrypt, decrypt, sign, verify text messages.
Input types: Plaintext strings of arbitrary length (UTF-8).
Output formats: Base64 or hex-encoded ciphertext and signatures for safe storage/transmission.
Threat model (brief): Protect confidentiality and authenticity against passive eavesdroppers and active message forgers; assume private keys must remain confidential and attacker may control the transport channel.

2. Core cryptographic design choices

RSA key size: Use at least 3072-bit for long-term confidentiality; 2048-bit acceptable for short-lived use.
Padding schemes:
- Use OAEP (RSA-OAEP with SHA-256) for encryption/decryption to prevent chosen-ciphertext attacks.
- Use PSS (RSA-PSS with SHA-256) for signatures to provide probabilistic signing and mitigation against existential forgery.
Hybrid encryption: RSA cannot directly encrypt large plaintexts securely or efficiently. Use RSA only to encrypt a symmetric key (e.g., AES-256-GCM) and use the symmetric cipher for the message body.
Authenticated encryption: Use an AEAD cipher (AES-256-GCM or ChaCha20-Poly1305) to ensure confidentiality and integrity of message payloads.
Encoding: Use Base64 for transmittable strings; include metadata (scheme, key id, IV, salt) in a compact JSON envelope.

3. Message envelope format (recommended)

Use a JSON structure like: { “version”: “1.0”, “enc”: “RSA-OAEP-SHA256+AES-256-GCM”, “key_id”: “key-2026-01”, “rsa_encrypted_key”: “”, “iv”: “”, “ciphertext”: “”, “tag”: “” }

version: Protocol version for forward compatibility.
enc: Combined scheme.
key_id: Identifier for recipient public key.
rsa_encrypted_key: RSA-OAEP-encrypted symmetric key.
iv: Initialization vector for AES-GCM.
tag: Authentication tag for AES-GCM (if not appended to ciphertext).

4. Key management

Key generation: Generate RSA keys in secure hardware or OS-provided keystore where possible (HSM, TPM, Secure Enclave). Use secure random sources (e.g., /dev/urandom, cryptographic API).
Key rotation: Rotate keys regularly; support key versioning in the envelope via keyid. Keep old private keys available to decrypt past messages until expiry.

Private key protection: Store private keys encrypted with a strong passphrase (e.g., PBKDF2/scrypt/Argon2 with high iteration/work factor) when not in HSM. Limit access and audit usage.

Public keys distribution: Use signed key directories or key servers; verify key fingerprints to avoid man-in-the-middle key replacement.

5. Implementation sketch (pseudocode)

Encryption:

Code
# Generate random symmetric key and IV sym_key = random_bytes(32)# AES-256 iv = random_bytes(12) # AES-GCM standard 96-bit IV Encrypt plaintext with AES-256-GCM ciphertext, tag = aes_gcm_encrypt(sym_key, iv, plaintext, aad=None) Encrypt sym_key with recipient’s RSA public key using OAEP-SHA256 rsa_encrypted_key = rsa_oaep_encrypt(recipient_pubkey, sym_key, hash=SHA256) Build envelope (base64 encode binary fields)
envelope = { “version”: “1.0”, “enc”: “RSA-OAEP-SHA256+AES-256-GCM”, “key_id”: recipient_key_id, “rsa_encrypted_key”: base64(rsa_encryptedkey), “iv”: base64(iv), “ciphertext”: base64(ciphertext), “tag”: base64(tag) }

Decryption:

Code
# Parse envelope and base64-decode fields sym_key = rsa_oaep_decrypt(recipient_privkey, base64_decode(envelope[“rsa_encrypted_key”]), hash=SHA256) plaintext = aes_gcm_decrypt(sym_key, base64_decode(envelope[“iv”]), base64_decode(envelope[“ciphertext”]), base64_decode(envelope[“tag”]), aad=None)

Signing and verifying:

Sign the canonicalized plaintext or the JSON envelope using RSA-PSS with SHA-256; include signature and signer key_id in envelope.

Verify by checking signature against signer’s public key before decrypting (if signature covers encrypted fields) or after decryption (if signature covers plaintext).

6. Practical security considerations

Preventing padding oracle attacks: Use OAEP and constant-time RSA operations; do not leak detailed error messages on decryption failures.

Replay protection: Include timestamps, nonces, or message sequence numbers inside the authenticated plaintext or AAD. Reject messages outside acceptable time windows.

Error handling: On verification or decryption failure, return a generic error without revealing which check failed.

Side-channel defenses: Use constant-time primitives provided by vetted crypto libraries; avoid custom crypto implementations.

Library selection: Prefer well-maintained libraries: libsodium, BoringSSL/OpenSSL (modern versions), WebCrypto (browsers), or language-specific wrappers (cryptography in Python, Node’s crypto with secure defaults).

7. Performance optimizations

Use hybrid encryption to handle large messages efficiently.

Cache parsed public keys and key identifiers.

Offload expensive RSA ops to worker threads or native bindings.

For very high throughput, prefer elliptic-curve schemes (e.g., X25519 for key exchange + Ed25519 for signatures) instead of RSA.

8. Example CLI usage (conceptual)

encrypt –recipient key-2026-01 –in message.txt –out message.rsajson

decrypt –key private.pem –in message.rsajson –out message.txt

sign –key signer.pem –in message.txt –out message.sig

verify –key signer_pub.pem –in message.txt –sig message.sig

9. Testing and validation

Unit tests for each primitive (encryption/decryption, sign/verify).

Fuzz test malformed envelopes.

Interoperability tests between languages/implementations.

Threat-model-based code review and optional external security audit.

10. Migration and interoperability

Support multiple envelope versions and detect older formats by the version field.

Provide clear error messages for unsupported schemes and offer a migration path (e.g., re-encrypt with new key sizes/padding).

11. Appendix — Recommended parameters

RSA key size: 3072 bits (minimum 2048 for short-lived use)

RSA padding: OAEP with SHA-256

Signature: RSA-PSS with SHA-256, 32-byte salt length recommended

Symmetric cipher: AES-256-GCM (96-bit IV) or ChaCha20-Poly1305

KDF for passphrases: Argon2id (memory 64–128 MB, iterations 2–4, parallelism 1–2) or scrypt with high N,r,p

Building an RSA Cryptographic Text Processor: A Practical Guide