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 (see Question 94) is used, there is no way to take someone's signature from one document and attach it to another, or to alter a signed message in any way. The slightest change in a signed document will cause the digital signature verification process to fail. Thus, public-key authentication allows people to check the integrity of signed documents. If a signature verification fails, however, it will generally difficult to determine whether there was an attempted forgery or simply a transmission error.