ITPEC FE Morning April 2019 Question 39

Certifying that the data existed at a given time and has not been modified since — that is what a timestamp service provides.

A trusted time-stamping authority (TSA) receives a hash of the electronic data, binds it to the current date and time and signs the pair, returning a timestamp token. Later anyone can recompute the hash and check the token: a match proves the data existed at that time (existence) and has not changed since (non-alteration). It complements a digital signature, which proves who made the data but not when.

Answer (b)

Why not others:

• •(a) — describes biometric authentication with a registration date added; the date plays no part in proving anything about data
• •(c) — describes delivering the date and time to set the clocks of PCs and servers without tampering on the way, as authenticated NTP does; it protects the clock, not the integrity of a document
• •(d) — describes a site that displays the official time over an encrypted connection; showing the time certifies nothing about any data

Key rule: A timestamp answers "did this data exist then, unchanged?"; a digital signature answers "who created it?". Long-term evidence of electronic records often needs both.

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