feat: implement constant time equality checks

package.json

@@ -79,9 +79,11 @@
 		"./rsa": {
 			"import": "./dist/rsa.mjs",
 			"require": "./dist/rsa.cjs"
+		},
+		"./equal": {
+			"import": "./dist/equal.mjs",
+			"require": "./dist/equal.cjs"
 		}
 	},
-	"files": [
-		"dist"
-	]
-}
+	"files": ["dist"]
+}

src/equal.test.ts

@@ -0,0 +1,25 @@
+import { describe, it, expect } from "vitest";
+import { safeEqual } from "./equal";
+
+describe("safeEqualFallback", () => {
+	it("returns true for identical strings", () => {
+		expect(safeEqual("hello", "hello")).toBe(true);
+	});
+
+	it("returns false for different strings", () => {
+		expect(safeEqual("hello", "world")).toBe(false);
+	});
+
+	it("returns false for strings of different lengths", () => {
+		expect(safeEqual("short", "longer")).toBe(false);
+	});
+
+	it("handles empty strings", () => {
+		expect(safeEqual("", "")).toBe(true);
+		expect(safeEqual("", "a")).toBe(false);
+	});
+
+	it("handles similar strings", () => {
+		expect(safeEqual("aaaaaaaa", "aaaaaaab")).toBe(false);
+	});
+});

src/equal.ts

@@ -0,0 +1,15 @@
+export function safeEqual(a: string, b: string): boolean {
+	const aBytes = new TextEncoder().encode(a);
+	const bBytes = new TextEncoder().encode(b);
+
+	let len = Math.max(aBytes.length, bBytes.length);
+	let result = BigInt(aBytes.length ^ bBytes.length);
+
+	for (let i = 0; i < len; i++) {
+		const aByte = i < aBytes.length ? BigInt(aBytes[i]) : 0n;
+		const bByte = i < bBytes.length ? BigInt(bBytes[i]) : 0n;
+		result |= aByte ^ bByte;
+	}
+
+	return result === 0n;
+}