chore: 新增 generateTOTP 函數

Author: c-c-epr

src/utils/generateTOTP.ts

@@ -0,0 +1,84 @@
+export async function generateTOTP(secret: string, digits = 6, period = 30): Promise<string> {
+  // 1. 計算 counter
+  const counter = Math.floor(Date.now() / 1000 / period);
+
+  // 2. secret(base32) -> bytes
+  const key = base32Decode(secret);
+
+  // 3. counter -> 8-byte big-endian
+  const counterBuffer = new ArrayBuffer(8);
+  const counterView = new DataView(counterBuffer);
+
+  // 高 32 bits
+  counterView.setUint32(0, Math.floor(counter / 0x100000000));
+
+  // 低 32 bits
+  counterView.setUint32(4, counter >>> 0);
+
+  // 4. 建立 HMAC key
+  const cryptoKey = await crypto.subtle.importKey(
+    "raw",
+    key,
+    {
+      name: "HMAC",
+      hash: "SHA-1", // Google Authenticator 標準
+    },
+    false,
+    ["sign"],
+  );
+
+  // 5. HMAC(counter)
+  const hmac = await crypto.subtle.sign("HMAC", cryptoKey, counterBuffer);
+
+  const hmacBytes = new Uint8Array(hmac);
+
+  // 6. Dynamic truncation
+  const offset = hmacBytes[hmacBytes.length - 1] & 0xf;
+
+  const binary =
+    ((hmacBytes[offset] & 0x7f) << 24) |
+    ((hmacBytes[offset + 1] & 0xff) << 16) |
+    ((hmacBytes[offset + 2] & 0xff) << 8) |
+    (hmacBytes[offset + 3] & 0xff);
+
+  // 7. 取 digits 位數
+  const otp = binary % 10 ** digits;
+
+  return otp.toString().padStart(digits, "0");
+}
+
+/**
+ * Base32 解碼
+ * RFC4648
+ */
+function base32Decode(base32: string): ArrayBuffer {
+  const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
+
+  let bits = "";
+  let result: number[] = [];
+
+  base32 = base32.replace(/=+$/, "").toUpperCase();
+
+  for (const char of base32) {
+    const val = alphabet.indexOf(char);
+
+    if (val === -1) {
+      throw new Error(`Invalid base32 char: ${char}`);
+    }
+
+    bits += val.toString(2).padStart(5, "0");
+  }
+
+  for (let i = 0; i + 8 <= bits.length; i += 8) {
+    result.push(parseInt(bits.slice(i, i + 8), 2));
+  }
+
+  const buffer = new ArrayBuffer(result.length);
+  const bytes = new Uint8Array(buffer);
+
+  for (let i = 0; i < result.length; i++) {
+    bytes[i] = result[i];
+  }
+
+  return buffer;
+}