feat: add RSA-OAEP padding support with backwards compatibility

[script3r]

Summary

This PR adds support for RSA-OAEP (Optimal Asymmetric Encryption Padding) while maintaining full backwards compatibility with existing code using PKCS#1 v1.5 padding.

Why OAEP?

PKCS#1 v1.5 (current default) has known vulnerabilities:

• Susceptible to Bleichenbacher's padding oracle attack (1998)
• Timing side-channel attacks can leak private key information
• Considered legacy by modern cryptographic standards

OAEP provides stronger security guarantees:

• Provably secure under the RSA assumption in the random oracle model
• Recommended by NIST (SP 800-56B) and PKCS#1 v2.x standards
• Used by default in modern cryptographic libraries (OpenSSL 3.x, WebCrypto API)

Backwards Compatibility

• All existing code continues to work without any changes
• The new padding parameter defaults to 'PKCS1'
• Users can opt-in to OAEP by explicitly passing 'OAEP'

API Changes

type RSAPadding = 'PKCS1' | 'OAEP';

// All RSA functions now accept an optional padding parameter
encryptRSA(data: string, publicKey: string, padding?: RSAPadding): string;
decryptRSA(data: string, privateKey: string, padding?: RSAPadding): string;
encryptAsyncRSA(data: string, publicKey: string, padding?: RSAPadding): Promise<string>;
decryptAsyncRSA(data: string, privateKey: string, padding?: RSAPadding): Promise<string>;

Usage

import { encryptRSA, decryptRSA } from 'rn-encryption';

// Existing code works unchanged (uses PKCS1 - backwards compatible)
const encrypted = encryptRSA(data, publicKey);
const decrypted = decryptRSA(encrypted, privateKey);

// Opt-in to more secure OAEP padding
const encryptedSecure = encryptRSA(data, publicKey, 'OAEP');
const decryptedSecure = decryptRSA(encryptedSecure, privateKey, 'OAEP');

Implementation Details

Platform	PKCS1	OAEP
Android	RSA/ECB/PKCS1Padding	RSA/ECB/OAEPWithSHA-256AndMGF1Padding
iOS	SecKeyAlgorithm.rsaEncryptionPKCS1	SecKeyAlgorithm.rsaEncryptionOAEPSHA256

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