Using timingSafeEqual

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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. Note that this function is not constant time with respect to the length of the parameters and also does not guarantee constant time for the surrounding code. Handling of secrets should be taken with care to not introduce timing side channels.

Warning

Do not return early when the input and secret have different lengths. An early return leaks the length of the secret through response timing. Instead, always perform a constant-time comparison as shown in the examples below — when lengths differ, compare the user input against itself and negate the result so the check still fails but takes the same amount of time.

In order to compare two strings, you must use the TextEncoder API.

TypeScript

TypeScript

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") || "";

    const encoder = new TextEncoder();

    const userValue = encoder.encode(authToken);
    const secretValue = encoder.encode(env.MY_SECRET_VALUE);

    // Do not return early when lengths differ — that leaks the secret's
    // length through timing.  Instead, always perform a constant-time
    // comparison: when the lengths match compare directly; otherwise
    // compare the user input against itself (always true) and negate.
    const lengthsMatch = userValue.byteLength === secretValue.byteLength;
    const isEqual = lengthsMatch
      ? crypto.subtle.timingSafeEqual(userValue, secretValue)
      : !crypto.subtle.timingSafeEqual(userValue, userValue);

    if (!isEqual) {
      return new Response("Unauthorized", { status: 401 });
    }

    return new Response("Welcome!");
  },
};

Python

Python

from workers import WorkerEntrypoint, Response
from js import TextEncoder, crypto

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        auth_token = request.headers["Authorization"] or ""
        secret = self.env.MY_SECRET_VALUE

        if secret is None:
            return Response("Missing secret binding", status=500)

        encoder = TextEncoder.new()
        user_value = encoder.encode(auth_token)
        secret_value = encoder.encode(secret)

        # Do not return early when lengths differ — that leaks the secret's
        # length through timing.  Always perform a constant-time comparison.
        if user_value.byteLength == secret_value.byteLength:
            is_equal = crypto.subtle.timingSafeEqual(user_value, secret_value)
        else:
            is_equal = not crypto.subtle.timingSafeEqual(user_value, user_value)

        if not is_equal:
            return Response("Unauthorized", status=401)

        return Response("Welcome!")

Hono

TypeScript

import { Hono } from 'hono';

interface Environment {
  Bindings: {
    MY_SECRET_VALUE?: string;
  }
}

const app = new Hono<Environment>();

// Middleware to handle authentication with timing-safe comparison
app.use('*', async (c, next) => {
  const secret = c.env.MY_SECRET_VALUE;

  if (!secret) {
    return c.text("Missing secret binding", 500);
  }

  const authToken = c.req.header("Authorization") || "";

  const encoder = new TextEncoder();

  const userValue = encoder.encode(authToken);
  const secretValue = encoder.encode(secret);

  // Do not return early when lengths differ — that leaks the secret's
  // length through timing.  Instead, always perform a constant-time
  // comparison: when the lengths match compare directly; otherwise
  // compare the user input against itself (always true) and negate.
  const lengthsMatch = userValue.byteLength === secretValue.byteLength;
  const isEqual = lengthsMatch
    ? crypto.subtle.timingSafeEqual(userValue, secretValue)
    : !crypto.subtle.timingSafeEqual(userValue, userValue);

  if (!isEqual) {
    return c.text("Unauthorized", 401);
  }

  // If we got here, the auth token is valid
  await next();
});

// Protected route
app.get('*', (c) => {
  return c.text("Welcome!");
});

export default app;