Quote from: deuteragenie on June 28, 2017, 12:35:59 PM

what is unclear to me is how to generate the signed message on the airgapped computer, but submit it on an online computer.

Submit it? Submit it where? I assume you mean "email it", or "text it", or "post it to a discussion forum", or something like that?

While you can sign any binary data, typically people sign text based messages. The binary signature is also typically converted to text with base64 encoding.

Since it is text, you can transfer it from the offline computer to the online computer using any method you like. Some possible examples are:

Save the text message and signature to a file on a USB
Generate a QR code from the message and signature and scan it with the online computer
Type the message carefully into the online com[puter with a keyboard
Take a picture of the message and signature

Quote from: deuteragenie on June 28, 2017, 12:35:59 PM

For that, I would need to be able to export the message/signed message on a USB key for example.
Is this possible using existing tools?

Sure. You can use the clipboard on the offline computer to copy the message and signature and paste it into your favorite text editor or word processing program.

Then, once you've got the message and signature on an online computer you can email it, or post it to a forum, or text it, or any other method you like to use for sharing text with someone.