Encrypt Messages Locally — Never Trust a Browser Checkbox
Some hidden services offer a convenience checkbox that promises to encrypt your message on the server after you hit submit. Never rely on it. If you type a plaintext shipping address or private note into a browser form, that plaintext travels across the network to the server before any encryption happens — meaning a man-in-the-middle proxy or a compromised server logs every word in the clear.
Instead, install an open-source client from openpgp.org or gnupg.org, import the recipient's public key (and the market's key from our PGP verification reference), and encrypt your text on your own machine first. The only thing you should ever paste into a browser text box is an already-encrypted -----BEGIN PGP MESSAGE----- block.