AES Encrypt / Decrypt

Encrypt text with a passphrase into shareable ciphertext and decrypt it back (AES-256).

AES-256 is a symmetric cipher, which means the same secret both locks and unlocks — there is no separate key to hand out. The 256 refers to the key, and a passphrase is not a key, so one has to be derived from the other. That derivation step is where the real security of a passphrase-based scheme lives, because it decides how expensive guessing is.

Symmetric encryption solves exactly one problem: someone who intercepts the ciphertext learns nothing. It does not solve getting the passphrase to the other person, and sending both down the same channel undoes the whole exercise.

This is the right tool for a note, a set of credentials, or a block of text you want to park somewhere you do not fully trust. It is the wrong tool for a conversation, where you want end-to-end encrypted messaging, and for your passwords, where you want a password manager.