diff --git a/src/utils/crypto.ts b/src/utils/crypto.ts
index 13e2e66372..1b98f2e7be 100644
--- a/src/utils/crypto.ts
+++ b/src/utils/crypto.ts
@@ -24,6 +24,7 @@ export async function verifyGitHubSignature(rawBody: string, signatureHeader: st
 export function timingSafeEqualHex(left: string, right: string): boolean {
   const leftBytes = hexToBytes(left);
   const rightBytes = hexToBytes(right);
+  if (leftBytes === null || rightBytes === null) return false;
   if (leftBytes.length !== rightBytes.length) return false;
   let result = 0;
   for (let index = 0; index < leftBytes.length; index += 1) {
@@ -32,11 +33,13 @@ export function timingSafeEqualHex(left: string, right: string): boolean {
   return result === 0;
 }
 
-function hexToBytes(hex: string): Uint8Array {
-  if (!/^[0-9a-f]+$/i.test(hex) || hex.length % 2 !== 0) return new Uint8Array();
+function hexToBytes(hex: string): Uint8Array | null {
+  if (hex.length === 0 || hex.length % 2 !== 0 || !/^[0-9a-f]+$/i.test(hex)) return null;
   const bytes = new Uint8Array(hex.length / 2);
   for (let index = 0; index < bytes.length; index += 1) {
-    bytes[index] = Number.parseInt(hex.slice(index * 2, index * 2 + 2), 16);
+    const byte = Number.parseInt(hex.slice(index * 2, index * 2 + 2), 16);
+    if (Number.isNaN(byte)) return null;
+    bytes[index] = byte;
   }
   return bytes;
 }
diff --git a/test/unit/crypto.test.ts b/test/unit/crypto.test.ts
index 76ea5d8129..1ea9b947c9 100644
--- a/test/unit/crypto.test.ts
+++ b/test/unit/crypto.test.ts
@@ -1,6 +1,35 @@
 import { describe, expect, it } from "vitest";
 import { createOpaqueToken, hashToken, timingSafeEqual } from "../../src/auth/security";
-import { verifyGitHubSignature } from "../../src/utils/crypto";
+import { base64UrlEncode, sha256Hex, timingSafeEqualHex, verifyGitHubSignature } from "../../src/utils/crypto";
+
+describe("crypto helpers", () => {
+  it("hashes input with sha256Hex", async () => {
+    const digest = await sha256Hex("gittensory");
+    expect(digest).toMatch(/^[0-9a-f]{64}$/);
+    expect(digest).toBe(await sha256Hex("gittensory"));
+    expect(digest).not.toBe(await sha256Hex("gittensory-x"));
+  });
+
+  it("compares hex strings in constant time and rejects malformed input", () => {
+    expect(timingSafeEqualHex("ab12", "ab12")).toBe(true);
+    expect(timingSafeEqualHex("AB12", "ab12")).toBe(true);
+    expect(timingSafeEqualHex("ab12", "ab13")).toBe(false);
+    expect(timingSafeEqualHex("ab12", "ab1234")).toBe(false);
+    expect(timingSafeEqualHex("zz", "00")).toBe(false);
+    expect(timingSafeEqualHex("gg", "gg")).toBe(false);
+    expect(timingSafeEqualHex("abc", "def")).toBe(false);
+    expect(timingSafeEqualHex("", "00")).toBe(false);
+    expect(timingSafeEqualHex("00", "")).toBe(false);
+    expect(timingSafeEqualHex("0x12", "0012")).toBe(false);
+    expect(timingSafeEqualHex("12gh", "12gh")).toBe(false);
+  });
+
+  it("base64url-encodes strings and byte arrays without padding", () => {
+    expect(base64UrlEncode("hello")).toBe("aGVsbG8");
+    expect(base64UrlEncode(new Uint8Array([255, 254]))).toBe("__4");
+    expect(base64UrlEncode("subjects?_d")).toBe(base64UrlEncode("subjects?_d"));
+  });
+});
 
 describe("webhook signature verification", () => {
   it("accepts valid GitHub HMAC signatures and rejects tampering", async () => {
@@ -17,6 +46,11 @@ describe("webhook signature verification", () => {
     await expect(verifyGitHubSignature(body, null, secret)).resolves.toBe(false);
     await expect(verifyGitHubSignature(body, "bad-prefix", secret)).resolves.toBe(false);
     await expect(verifyGitHubSignature(body, `sha256=${signature}`, "")).resolves.toBe(false);
+    await expect(verifyGitHubSignature(body, "sha256=abc", secret)).resolves.toBe(false);
+    await expect(verifyGitHubSignature(body, "sha256=zz", secret)).resolves.toBe(false);
+    await expect(
+      verifyGitHubSignature(body, `sha256=${"a".repeat(63)}`, secret),
+    ).resolves.toBe(false);
   });
 
   it("uses timing-safe token comparisons and one-way token hashes", async () => {