Okay, thinking further: The (open) (pad)locks you distribute also carry a seal impression. Then:

- to encrypt, the user takes a padlock of the recipient and adds it to the message.

- to decrypt, the recipient uses his key to open the lock.

- to sign something, you use your key as a seal, creating a unique impression on the message

- to check a signature, the recipient compares the impression on the message with the impression on the lock.

Does that make at least some sense?