Consider two questions that may be asked by a computer user as he or she views a digital document or on-line record. (1) Who is the author of this record - who wrote it, approved it, or consented to it? (2) When was this record created or last modified?

In both cases, the question is one about exactly this record-exactly this sequence of bits - whether it was first stored on this computer or was created somewhere else and then copied and saved here. An answer to the first question tells who & what: who approved exactly what is in this record? An answer to the second question tells when & what: when exactly did the contents of this record first exist?

Both of the above questions have good solutions. A system for answering the first question is called a digital signature scheme. Such a system was first proposed in and there is a wide variety of accepted designs for an implementation of this kind of system.