The RSA Encrypt/Decrypt Calculator performs client-side asymmetric public-key cryptography and educational mathematical demonstrations. In Standard Mode, it leverages the browser-native Web Cryptography API to generate real RSA-OAEP key pairs and perform cryptographically secure encryption and decryption.

In Educational Mode, it utilizes JavaScript BigInt arbitrary-precision arithmetic to calculate RSA primes p and q, compute totient phi(n), determine private exponent d via the Extended Euclidean Algorithm, and execute modular exponentiation (c = m^e mod n).

Textbook RSA computes c = m^e mod n directly on raw numbers. This is deterministic: encrypting the same message twice produces identical ciphertext, allowing an attacker to confirm guesses. Furthermore, textbook RSA is homomorphic: multiplying two ciphertexts produces the ciphertext of the multiplied plaintexts.

Optimal Asymmetric Encryption Padding (OAEP), standardized in RFC 8017, incorporates a cryptographic hash function (such as SHA-256) and a random salt to make the ciphertext non-deterministic and all-or-nothing plaintext aware. Furthermore, because RSA can only encrypt messages shorter than its modulus size, production systems use RSA to encrypt a symmetric AES-256-GCM key, which then encrypts the bulk payload.