How do you sign a message with your monero private key using the cli-wallet, gui-wallet, and last with pure js using what libraries?
With monero-wallet-cli, you can sign the content of a file (e.g. message_file) with the command
sign message_file, and verify the signature with
verify message_file address_of_the_signer signature.
With monero-wallet-gui, in the menu go to
Advanced -> Sign/Verify, then you can sign (or verify) a message or a file by filling the appropriate boxes.