If a ten page contract is signed by hand on the tenth page, one cannot be sure that the first nine pages have not been altered. However, if the contract was signed with digital signatures,a third party can verify that not one byte of the contract has been altered. Currently, if two people want to digitally sign a series of contracts, they might first sign a paper contract in which they agree to be bound in the future by any contracts digitally signed by them with a given signature method and minimum key size. Several efforts are underway to legislate the legality and use of digital signatures. Utah has implemented laws qualifying digital signatures.
Similar legislation is under way in California and New York, with other states following.
A digital signature is superior to a handwritten signature in that it attests to the contents of a message as well as to the identity of the signer. As long as a secure hash function is used, there is no way to take someone's signature from one document and attach it to another, or to alter the signed message in any way. The slightest change in a signed document will cause the digital signature verification process to fail. Thus, authentication allows people to check the integrity of signed documents. Of course, if a signature verification fails, it may be unclear if there was an attempted forgery or simply a transmission error.