Simpler: include in the contract a clause that says "the version of this text that counts is in a PDF with SHA1 23afb87c...".
You still need a way to prove that the other side really signed the document you hold, and willingly. Notaries work.
But this system is not forced - so it protects the side holding a real contract form the other side denying they signed, but does not protect you from a fraud turning up with a contract that you didn't really sign and claiming that you did, you just didn't bother to go to a notary with him.
If there were a way to publicly claim "from now on, only contracts signed by my private key count as really signed by me... But it's not easy to implement such a measure. Every one you sign contracts with needs a really easy way to check if you declared this in the past, otherwise you can fraud them by signing a contract only with a pen. And they need to be aware of this new law.
You still need a way to prove that the other side really signed the document you hold, and willingly. Notaries work.
But this system is not forced - so it protects the side holding a real contract form the other side denying they signed, but does not protect you from a fraud turning up with a contract that you didn't really sign and claiming that you did, you just didn't bother to go to a notary with him.
If there were a way to publicly claim "from now on, only contracts signed by my private key count as really signed by me... But it's not easy to implement such a measure. Every one you sign contracts with needs a really easy way to check if you declared this in the past, otherwise you can fraud them by signing a contract only with a pen. And they need to be aware of this new law.