Encrypt short messages with an RSA public key and decrypt with the private key using OAEP padding.
RSA is asymmetric: the public key encrypts, the private key decrypts, and the two come as a matched pair. This tool applies OAEP padding from PKCS#1, the standard scheme for encrypting small payloads, and works on keys pasted into the page. The surprise most people hit first is the payload ceiling: a 2048-bit key carries at most 190 bytes of message with SHA-256 (214 with SHA-1), so a paragraph, let alone a file, will not fit - which is why the tool is short-message only. Expect the output to be a single base64 block, different on every run even for identical input.
Before the RSA exponentiation, OAEP mixes the message with a fresh random seed, so encrypting the same text twice never yields the same ciphertext - by design, not a fault. On decryption, OAEP verifies the padding structure; if the wrong key or an edited ciphertext is supplied, the operation fails outright rather than returning corrupted-looking text. You cannot tell a wrong key from a damaged message by the error alone.
OAEP ciphertext is always exactly as long as the key: 256 bytes for 2048 bits, 512 bytes for 4096. The message budget is the key length minus two hash outputs and two bytes of overhead - 190 bytes with a 2048-bit key and SHA-256, 214 with SHA-1, and 446 with a 4096-bit key and SHA-256. Longer input is rejected before encryption begins, so the tool is limited to short messages.
No - RSA-OAEP capacity is fixed by the key size, about 190 bytes for a 2048-bit key with SHA-256, and longer input is rejected. RSA is designed to carry small secrets; large data is normally encrypted with a symmetric cipher like AES using a fresh random key, and only that key is RSA-encrypted - so-called hybrid encryption.
The encryption and decryption run locally in the browser and nothing is uploaded - but anything the page itself runs can see what you paste, and so can extensions, screenshots, or people watching your screen. For tests and experiments, generate a throwaway key pair; reserve your production private key for software you trust, and never share it with anyone.
The behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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