A high-security facility tunes its fingerprint reader so that an impostor is almost never admitted, accepting that genuine staff will occasionally be turned away. Which rate has it driven down, and at what cost?

Answer

FMR down, at the cost of a higher FNMR

Options - A. FMR down, at the cost of a higher FNMR - B. FNMR down, at the cost of a higher FMR - C. Both rates down, by raising the matching threshold - D. Neither rate changes; the threshold only affects processing time Why - A. Correct — the false MATCH rate is the rate of admitting an impostor, and driving it down means demanding a closer match, which turns more genuine users away. That is the high-security operating point. - B. Exactly inverted, and this is the confusion the question exists to catch. A low FNMR means valid users are rarely rejected — the FORENSIC operating point, where a miss is the unrecoverable error and false matches are filtered by a human. - C. Impossible on one curve. The two rates are the tails either side of a single cut through two overlapping score distributions; moving the cut trades one for the other. Only a better sensor or algorithm moves both. - D. The threshold is the decision boundary, and it determines which errors the system makes. Processing cost is set by verification versus identification, not by the threshold. Decode the names before answering. False MATCH = matched when it should not have = impostor admitted. False NONMATCH = failed to match when it should have = valid user refused.

Stallings & Brown, Computer Security 5e, ch3 §3.4–§3.7