Add repeating XOR encryption

basics.ts

@@ -103,3 +103,21 @@ export const detectSingleByteXor: DetectSingleByteXor = (buffs) => {
     candidates.sort((a, b) => b[0] - a[0]);
     return candidates[0][1];
 };
+
+type EncryptRepeatingXOR = (buff: Buffer, key: Buffer) => Buffer;
+export const encryptRepeatingXOR: EncryptRepeatingXOR = (buff, key) => {
+    const resBuff = Buffer.alloc(buff.length);
+    range(0, buff.length).forEach(i => {
+        resBuff[i] = buff[i] ^ key[i % key.length];
+    });
+    return resBuff;
+};
+
+type DecryptRepeatingXOR = (buff: Buffer, key: Buffer) => Buffer;
+export const decryptRepeatingXOR: DecryptRepeatingXOR = (buff, key) => {
+    const resBuff = Buffer.alloc(buff.length);
+    range(0, buff.length).forEach(i => {
+        resBuff[i] = buff[i] ^ key[i % key.length];
+    });
+    return resBuff;
+};

data/repeatingXOR.txt

@@ -0,0 +1,2 @@
+Burning 'em, if you ain't quick and nimble
+I go crazy when I hear a cymbal

tests/basics.test.ts

@@ -2,10 +2,10 @@ import test from 'node:test';
 import * as path from 'node:path';
 import * as assert from 'node:assert/strict';
 
-const DATA_PATH = path.join(__dirname, '..', 'data');
-
 import { range, fileToBuff } from '../utils';
-import { hexToBuff, buffTo64, xorBuffers, encryptSingleByte, decryptSingleByte, crackSingleByteKeyMsg, detectSingleByteXor } from '../basics';
+import { hexToBuff, buffTo64, xorBuffers, encryptSingleByte, decryptSingleByte, crackSingleByteKeyMsg, detectSingleByteXor, encryptRepeatingXOR, decryptRepeatingXOR } from '../basics';
+
+const DATA_PATH = path.join(__dirname, '..', 'data');
 
 test('range', () => {
     assert.deepEqual(range(0, 10), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
@@ -42,3 +42,18 @@ test('detect single byte xor', () => {
     const expected = 'Now that the party is jumping';
     assert.equal(detectSingleByteXor(input), expected);
 });
+
+test('encrypt/decrypt repeating xor is symmetric', () => {
+    const input = 'random input to test this stuff outeqw e991231293 129083 h1207018 9';
+    const key = 'also random';
+    assert.equal(
+        decryptRepeatingXOR(encryptRepeatingXOR(Buffer.from(input, 'utf-8'), Buffer.from(key, 'utf-8')), Buffer.from(key, 'utf-8')).toString('utf-8'), input);
+});
+
+test('encrypt repeating xor', () => {
+    const input = fileToBuff(path.join(DATA_PATH, 'repeatingXOR.txt'), 'utf-8');
+    const parsedInput = Buffer.from(input.map(buff => buff.toString('utf-8')).join('\n'));
+    const key = 'ICE';
+    const expected = '0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a26226324272765272a282b2f20430a652e2c652a3124333a653e2b2027630c692b20283165286326302e27282f';
+    assert.equal(encryptRepeatingXOR(parsedInput, Buffer.from(key, 'utf-8')).toString('hex'), expected);
+});