45 lines
1.6 KiB
TypeScript
45 lines
1.6 KiB
TypeScript
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import { randomInt } from "node:crypto";
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/**
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* The number-matching challenge.
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*
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* The laptop shows ONE number; the phone asks for it. This defeats blind
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* approval — an attacker who can trigger a prompt but cannot see the victim's
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* screen has to guess.
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*
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* How much it buys is arithmetic, and worth stating because "number matching"
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* sounds stronger than the weak variant is:
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*
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* mode "type" two digits, entered on a keypad log2(90) = 6.5 bits
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* mode "choice" one of three buttons log2(3) = 1.6 bits
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*
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* With a single attempt and a cap of 3 sessions per source, "choice" leaves an
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* attacker at 1 - (2/3)^3 = 70%, which is not a control. "type" leaves them at
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* 1 - (89/90)^3 = 3.3%. Hence `type` is the default; `choice` exists because it
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* is the friendlier UX and some deployments will want it with eyes open.
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*/
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export type NumberMode = "type" | "choice";
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/** Two digits. Never 0-9: a leading zero reads ambiguously across fonts. */
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export function pickNumber(): number {
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return randomInt(10, 100);
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}
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/**
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* Two decoys for `choice` mode, distinct from each other and from the answer.
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* Returned already shuffled, so the correct one is not positionally biased.
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*/
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export function pickDecoys(answer: number): number[] {
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const chosen = new Set<number>([answer]);
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while (chosen.size < 3) chosen.add(pickNumber());
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const all = [...chosen];
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// Fisher-Yates with a CSPRNG. Math.random would be fine for display order,
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// but using it here invites someone to reuse it where it is not.
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for (let i = all.length - 1; i > 0; i--) {
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const j = randomInt(0, i + 1);
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[all[i], all[j]] = [all[j], all[i]];
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}
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return all;
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}
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