108 lines
3.0 KiB
JavaScript
108 lines
3.0 KiB
JavaScript
/**
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* @author p-leriche [philip.leriche@cantab.net]
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* @copyright Crown Copyright 2025
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* @license Apache-2.0
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*/
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import Operation from "../Operation.mjs";
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import OperationError from "../errors/OperationError.mjs";
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import { parseBigInt, egcd } from "../lib/BigIntUtils.mjs";
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/* ---------- operation class ---------- */
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/**
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* Modular Inverse operation
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*/
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class ModularInverse extends Operation {
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/**
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* ModularInverse constructor
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*/
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constructor() {
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super();
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this.name = "Modular Inverse";
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this.module = "Crypto";
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this.description =
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"Computes the modular multiplicative inverse of <i>a</i> modulo <i>m</i>.<br><br>" +
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"Finds <i>x</i> such that a*x = 1 (mod m).<br><br>" +
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"<b>Input handling:</b> If either <i>a</i> or <i>m</i> is left blank, " +
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"its value is taken from the Input field.";
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this.infoURL = "https://wikipedia.org/wiki/Modular_multiplicative_inverse";
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this.inputType = "string";
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this.outputType = "string";
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this.args = [
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{
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name: "Value (a)",
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type: "string",
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value: ""
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},
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{
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name: "Modulus (m)",
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type: "string",
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value: ""
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}
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];
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}
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/**
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* @param {string} input
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* @param {Object[]} args
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* @returns {string}
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*/
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run(input, args) {
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const [aStr, mStr] = args;
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// Trim everything so "" and " " count as empty
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const aParam = aStr?.trim();
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const mParam = mStr?.trim();
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const inputVal = input?.trim();
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let a, m;
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if (aParam && mParam) {
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// Case 1: value and modulus both given as parameters
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a = aParam;
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m = mParam;
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} else if (!aParam && mParam) {
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// Case 2: value missing - take from input
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a = inputVal;
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m = mParam;
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if (!a) throw new OperationError("Value (a) must be defined");
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} else if (aParam && !mParam) {
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// Case 3: modulus missing - take from input
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a = aParam;
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m = inputVal;
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if (!m) throw new OperationError("Modulus (m) must be defined");
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} else if (!aParam && !mParam) {
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// Case 4: value and modulus both missing
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throw new OperationError("Value (a) and Modulus (m) must be defined");
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}
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const aBI = parseBigInt(a, "Value (a)");
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const mBI = parseBigInt(m, "Modulus (m)");
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if (mBI <= 0n) {
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throw new OperationError("Modulus must be greater than zero");
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}
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const aNorm = ((aBI % mBI) + mBI) % mBI;
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const [g, x] = egcd(aNorm, mBI);
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if (g !== 1n && g !== -1n) {
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throw new OperationError("Inverse does not exist because gcd(a, m) ≠ 1");
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}
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let inv = x;
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if (g === -1n) inv = -inv;
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inv = ((inv % mBI) + mBI) % mBI;
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return inv.toString();
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}
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}
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export default ModularInverse;
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