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.
When you receive digitally signed messages, you can verify the signer's Digital ID to determine that no forgery or false representation has occurred. When you send messages, you can sign the messages and enclose your Digital ID to assure the recipient of the message that the message was actually sent by you. At the end of a Digital ID hierarchy is a top-level Certification Authority, which is trusted without a Digital ID from any other Certification Authority. The public key of the top-level Certification Authority must be independently known, for example by being widely published. You can also use a Digital ID to identify yourself to secure servers such as membership-based web servers.