Constant-time compare

Compare two strings in constant time using node:crypto timingSafeEqual, preventing timing side-channel attacks. Returns whether the strings are equal and whether their lengths match. Different-length strings are always unequal but comparison does not leak which is longer via timing.

Input

Field	Type	Description
a *	string	First string to compare
b *	string	Second string to compare

Example output

{
  "equal": false,
  "lengthMatch": false
}

Try it — see the 402 challenge (free)

curl -i -X POST https://agent402.tools/api/constant-compare \
  -H "Content-Type: application/json" \
  -d '{"a":"expected-token-value","b":"actual-token-value"}'

The response is HTTP 402 Payment Required with exact payment requirements. Any x402 v2 client pays automatically and retries:

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/constant-compare", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    "a": "expected-token-value",
    "b": "actual-token-value"
  }),
});

No wallet? Pay with compute

This is a pure-CPU tool, so an agent without a wallet can pay with proof-of-work instead of USDC: fetch a challenge, solve the sha256 puzzle (16 leading zero bits — a fraction of a second of CPU, no money, no AI tokens), and resend with the X-Pow-Solution header.

import { createHash } from "node:crypto";
const lz = (b) => { let t = 0; for (const x of b) { if (!x) { t += 8; continue; } t += Math.clz32(x) - 24; break; } return t; };
const c = await (await fetch("https://agent402.tools/api/pow/challenge?slug=constant-compare")).json();
let n = 0;
while (lz(createHash("sha256").update(c.challenge + ":" + n).digest()) < c.difficulty) n++;
await fetch("https://agent402.tools/api/constant-compare", { method: "POST", headers: { "X-Pow-Solution": c.token + ":" + n, "Content-Type": "application/json" }, body: JSON.stringify({"a":"expected-token-value","b":"actual-token-value"}) });

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