Make padWithZeroes implementation less offensive

src/index.test.ts

@@ -1454,13 +1454,22 @@ describe('padWithZeroes', function () {
   it('pads a string shorter than the target length with zeroes', function () {
     const input = 'abc';
     expect(sigUtil.padWithZeroes(input, 4)).toStrictEqual(`0${input}`);
-  })
+  });
 
   it('returns a string longer than or equal to the target length without modifying it', function () {
     const input = 'abc';
     expect(sigUtil.padWithZeroes(input, 3)).toStrictEqual(input);
-  })
-})
+  });
+
+  it('throws an error if passed an invalid hex string', function () {
+    const inputs = ['0xabc', 'xyz', '-'];
+    for (const input of inputs) {
+      expect(() => sigUtil.padWithZeroes(input, 3)).toThrow(
+        new Error(`Expected an unprefixed hex string. Received: ${input}`),
+      );
+    }
+  });
+});
 
 it('personalSign and recover', function () {
   const address = '0x29c76e6ad8f28bb1004902578fb108c507be341b';

src/index.ts

@@ -686,12 +686,32 @@ function getPublicKeyFor<T extends MessageTypes>(
   return recoverPublicKey(msgHash, msgParams.sig);
 }
 
-export function padWithZeroes(number: string, length: number): string {
-  let myString = `${number}`;
-  while (myString.length < length) {
-    myString = `0${myString}`;
+/**
+ * Pads the front of the given hex string with zeroes until it reaches the
+ * target length. If the input string is already longer than or equal to the
+ * target length, it is returned unmodified.
+ *
+ * If the input string is "0x"-prefixed or not a hex string, an error will be
+ * thrown.
+ *
+ * @param hexString The hexadecimal string to pad with zeroes.
+ * @param targetLength The target length of the hexadecimal string.
+ * @returns The input string front-padded with zeroes, or the original string
+ * if it was already greater than or equal to to the target length.
+ */
+export function padWithZeroes(hexString: string, targetLength: number): string {
+  if (!/^[a-f0-9]+$/iu.test(hexString)) {
+    throw new Error(
+      `Expected an unprefixed hex string. Received: ${hexString}`,
+    );
+  }
+
+  if (hexString.length < targetLength) {
+    return `${new Array(targetLength - hexString.length)
+      .fill(0)
+      .join()}${hexString}`;
   }
-  return myString;
+  return hexString;
 }
 
 // converts hex strings to the Uint8Array format used by nacl