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Require (a, 26) to be coprime in affine encode #1788

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merged 2 commits into from
Apr 13, 2024
Merged

Require (a, 26) to be coprime in affine encode #1788

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a09f845
Require (a, 26) to be coprime in affine encode
EvieHarv Apr 10, 2024
00f7914
Fix affine encode testcase
EvieHarv Apr 10, 2024
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6 changes: 6 additions & 0 deletions src/core/lib/Ciphers.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,13 +3,15 @@
*
* @author Matt C [matt@artemisbot.uk]
* @author n1474335 [n1474335@gmail.com]
* @author Evie H [evie@evie.sh]
*
* @copyright Crown Copyright 2018
* @license Apache-2.0
*
*/

import OperationError from "../errors/OperationError.mjs";
import Utils from "../Utils.mjs";
import CryptoJS from "crypto-js";

/**
Expand All @@ -30,6 +32,10 @@ export function affineEncode(input, args) {
throw new OperationError("The values of a and b can only be integers.");
}

if (Utils.gcd(a, 26) !== 1) {
throw new OperationError("The value of `a` must be coprime to 26.");
}

for (let i = 0; i < input.length; i++) {
if (alphabet.indexOf(input[i]) >= 0) {
// Uses the affine function ax+b % m = y (where m is length of the alphabet)
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2 changes: 1 addition & 1 deletion tests/browser/02_ops.js
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Expand Up @@ -37,7 +37,7 @@ module.exports = {
testOp(browser, ["From Hex", "Add Text To Image", "To Base64"], Images.PNG_HEX, Images.PNG_CHEF_B64, [[], ["Chef", "Center", "Middle", 0, 0, 16], []]);
testOp(browser, "Adler-32 Checksum", "test input", "16160411");
testOp(browser, "Affine Cipher Decode", "test input", "rcqr glnsr", [1, 2]);
testOp(browser, "Affine Cipher Encode", "test input", "njln rbfpn", [2, 1]);
testOp(browser, "Affine Cipher Encode", "test input", "gndg zoujg", [3, 1]);
testOp(browser, "AMF Decode", "\u000A\u0013\u0001\u0003a\u0006\u0009test", /"\$value": "test"/);
testOp(browser, "AMF Encode", '{"a": "test"}', "\u000A\u0013\u0001\u0003a\u0006\u0009test");
testOp(browser, "Analyse hash", "0123456789abcdef", /CRC-64/);
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