Sign and verify data with ECDSA

Edit on Github

A digital signature proves that a message was produced by the holder of a private key and was not modified in transit. Anyone with the matching public key can check the signature, but only the private key can create it. This example uses ECDSA on the P-256 curve from the built-in Web Crypto API.

Generate an ECDSA key pair on the P-256 curve. The private key signs, the public key verifies.

const { privateKey, publicKey } = await crypto.subtle.generateKey(
  { name: "ECDSA", namedCurve: "P-256" },
  true,
  ["sign", "verify"],
);

Sign a message. ECDSA hashes the data first, here with SHA-256, and the signature changes on every run because ECDSA mixes in a random nonce.

const message = new TextEncoder().encode("release v2.0.0");
const signature = await crypto.subtle.sign(
  { name: "ECDSA", hash: "SHA-256" },
  privateKey,
  message,
);
console.log(new Uint8Array(signature).toHex()); // e.g. 989b58f4b154...

Verify the signature against the original message with the public key.

const valid = await crypto.subtle.verify(
  { name: "ECDSA", hash: "SHA-256" },
  publicKey,
  signature,
  message,
);
console.log(valid); // true

A tampered message fails verification, even for a one character change.

const tampered = new TextEncoder().encode("release v2.0.1");
console.log(
  await crypto.subtle.verify(
    { name: "ECDSA", hash: "SHA-256" },
    publicKey,
    signature,
    tampered,
  ),
); // false

Export the public key as JWK to share it with whoever needs to verify your signatures. The x and y fields hold the public curve point; a private key JWK would also carry the secret d field, so never share that.

const jwk = await crypto.subtle.exportKey("jwk", publicKey);
console.log(jwk.kty); // EC
console.log(jwk.crv); // P-256

The receiving side imports the JWK and gets a key usable for verify.

const imported = await crypto.subtle.importKey(
  "jwk",
  jwk,
  { name: "ECDSA", namedCurve: "P-256" },
  true,
  ["verify"],
);
console.log(
  await crypto.subtle.verify(
    { name: "ECDSA", hash: "SHA-256" },
    imported,
    signature,
    message,
  ),
); // true

Run this example locally using the Deno CLI:

deno run https://docs.deno.com/examples/scripts/ecdsa_signing.ts

Sign and verify data with ECDSA

Additional resources

Did you find what you needed?