Signatures have been used for centuries to prove the authenticity of a document. In the digital domain, tampering with a document is very easy and leaves no traces. This makes it essential to have a means to verify that an electronc document is authentic. Digital signatures are a way to achieve this. To be useful, a digital signature must be coupled to the electronic document to which it applies. This coupling is established using public-key cryptography and so-called cryptographic hash functions.
An important difference between paper signatures and digital signatures is that the latter are typically generated by a computer in response to a user's request. This makes it -at least in theory- possible to place valid digital signatures on completely different documents without the user knowing it.