curious what InSign would look like.

InSign, will be more of a website for contract signing, where the signature will be a transaction on the block chain. Just like a contract involves multiple parties, the contract sender will be able to upload the contract, which will then be emailed to the signer, who will review the contract and send a unique INT amount to a unique address instructed in the email. This transaction will indicate that the contract has been signed.
What's to stop someone else from finding out the INT amount and sending it to the specified address, which in effect will be like forging a signature?