Using timingSafeEqual
timingSafeEqual
.The crypto.subtle.timingSafeEqual
function compares two values using a constant-time algorithm. The time taken is independent of the contents of the values.
When strings are compared using the equality operator (==
or ===
), the comparison will end at the first mismatched character. By using timingSafeEqual
, an attacker would not be able to use timing to find where at which point in the two strings there is a difference.
The timingSafeEqual
function takes two ArrayBuffer
or TypedArray
values to compare. These buffers must be of equal length, otherwise an exception is thrown.
In order to compare two strings, you must use the TextEncoder
API. Since the time taken to encode the values may reveal the length of our secret value, you should check the length of the strings before encoding.
interface Environment { MY_SECRET_VALUE?: string;
}
export default { async fetch(req: Request, env: Environment) { if (!env.MY_SECRET_VALUE) return new Response("Missing secret binding", { status: 500 });
const authToken = req.headers.get("Authorization") || "";
if (authToken.length !== env.MY_SECRET_VALUE.length) { return new Response("Unauthorized", { status: 401 }); }
const encoder = new TextEncoder();
const a = encoder.encode(authToken); const b = encoder.encode(env.MY_SECRET_VALUE);
if (a.byteLength !== b.byteLength) { return new Response("Unauthorized", { status: 401 }); }
if (!crypto.subtle.timingSafeEqual(a, b)) { return new Response("Unauthorized", { status: 401 }); }
return new Response("Welcome!"); },
};