It depends entirely on the jurisdiction. In Australia for example, any technology that is accepted by both parties can be used. I have signed contracts with Gnu

It depends entirely on the jurisdiction. In Australia for example, any technology that is accepted by both parties can be used. I have signed contracts with GnuPG (a long time ago) for example. I have also sent documents to my Remarkable tablet, signed them there and sent them back without it ever being on paper. Who is the "other party" to this contract? What forms of signature will they accept?