Encrypts your text with AES-256-GCM, an authenticated cipher. Your passphrase is stretched into a 256-bit key with PBKDF2 (SHA-256, 200,000 iterations) using a fresh random 16-byte salt, and each message uses a fresh random 12-byte IV. The output is Base64 that packs salt + IV + ciphertext together, so you only need the passphrase to decrypt.
Because GCM is authenticated, decryption fails loudly if the passphrase is wrong or the data was tampered with. Everything runs locally through the browser's native WebCrypto — no text, key or passphrase is ever uploaded. Use a long, unique passphrase; there is no way to recover data if you forget it.