What is RSA?
RSA encryption uses a recipient's public key to encrypt data that only their matching private key can decrypt. Because RSA can only encrypt data smaller than the key size minus padding, it is typically used to encrypt a small secret — such as a symmetric AES key — rather than large messages (this is called hybrid encryption). Use OAEP padding for new applications. Encryption runs entirely in your browser.
Small data only: RSA encrypts data smaller than the key. For large data, encrypt an AES key with RSA and the data with AES (hybrid). Runs locally.
How RSA works
RSA encryption is modular exponentiation with the public key:
- The message is padded (OAEP is recommended) to the key length.
- The padded value is raised to the public exponent e, modulo n.
- The result is the ciphertext, which only the private key can reverse.
How to use
- Enter the text you want to encrypt.
- Provide a key (and IV/mode settings where required).
- The encrypted output is produced in your browser.
- Share the ciphertext; share the key only through a safe channel.
Options explained
- Public key — Paste the recipient's RSA public key (PEM).
- Padding — OAEP is recommended; PKCS#1 v1.5 exists for legacy compatibility.
- Key size — Larger keys allow slightly larger messages and more security.
Common uses
- Encrypting a symmetric key for hybrid encryption.
- Protecting small secrets sent to a known recipient.
- Secure key exchange.
Frequently asked questions
Why can't I encrypt large data?
RSA only handles data smaller than the key. Use hybrid encryption: RSA for an AES key, AES for the data.
Which padding should I use?
OAEP for new applications; PKCS#1 v1.5 only for legacy systems.
How do I decrypt it?
The recipient uses the RSA Decryption tool with their private key.