SubtleCrypto.sign()

The sign() method of the SubtleCrypto interface generates a digital signature.

It takes as its arguments a key to sign with, some algorithm-specific parameters, and the data to sign. It returns a Promise which will be fulfilled with the signature.

You can use the corresponding SubtleCrypto.verify() method to verify the signature.

Syntax

const signature = crypto.subtle.sign(algorithm, key, data);

Parameters

• algorithm is a string or object that specifies the signature algorithm to use and its parameters:
  • To use RSASSA-PKCS1-v1_5, pass the string "RSASSA-PKCS1-v1_5" or an object of the form { "name": "RSASSA-PKCS1-v1_5" }.
  • To use RSA-PSS, pass an RsaPssParams object.
  • To use ECDSA, pass an EcdsaParams object.
  • To use HMAC, pass the string "HMAC" or an object of the form { "name": "HMAC" }.
• key is a CryptoKey object containing the key to be used for signing. If algorithm identifies a public-key cryptosystem, this is the private key.
• data is an ArrayBuffer or ArrayBufferView object containing the data to be signed.

Return value

• signature is a Promise that fulfills with an ArrayBuffer containing the signature.

Exceptions

The promise is rejected when the following exception is encountered:

InvalidAccessError

Raised when the signing key is not a key for the request signing algorithm or when trying to use an algorithm that is either unknown or isn't suitable for signing.

Supported algorithms

The Web Crypto API provides four algorithms that can be used for signing and signature verification.

Three of these algorithms — RSASSA-PKCS1-v1_5, RSA-PSS, and ECDSA — are public-key cryptosystems that use the private key for signing and the public key for verification. These systems all use a digest algorithm to hash the message to a short fixed size before signing. The choice of digest algorithm is passed into the generateKey() or importKey() functions.

The fourth algorithm — HMAC — uses the same algorithm and key for signing and for verification: this means that the verification key must be kept secret, which in turn means that this algorithm is not suitable for many signature use cases. It can be a good choice however when the signer and verifier are the same entity.

RSASSA-PKCS1-v1_5

The RSASSA-PKCS1-v1_5 algorithm is specified in RFC 3447.

RSA-PSS

The RSA-PSS algorithm is specified in RFC 3447.

It's different from RSASSA-PKCS1-v1_5 in that it incorporates a random salt in the signature operation, so the same message signed with the same key will not result in the same signature each time. An extra property, defining the salt length, is passed into the sign() and verify() functions when they are invoked.

ECDSA

ECDSA (Elliptic Curve Digital Signature Algorithm) is a variant of the Digital Signature Algorithm, specified in FIPS-186, that uses Elliptic Curve Cryptography (RFC 6090).

HMAC

The HMAC algorithm calculates and verifies hash-based message authentication codes according to the FIPS 198-1 standard.

The digest algorithm to use is specified in the HmacKeyGenParams object that you pass into generateKey(), or the HmacImportParams object that you pass into importKey().

Examples

RSASSA-PKCS1-v1_5

This code fetches the contents of a text box, encodes it for signing, and signs it with a private key. See the complete source code on GitHub.

/*
Fetch the contents of the "message" textbox, and encode it
in a form we can use for the sign operation.
*/
function getMessageEncoding() {
  const messageBox = document.querySelector(".rsassa-pkcs1 #message");
  let message = messageBox.value;
  let enc = new TextEncoder();
  return enc.encode(message);
}

let encoded = getMessageEncoding();
let signature = await window.crypto.subtle.sign(
  "RSASSA-PKCS1-v1_5",
  privateKey,
  encoded
);

RSA-PSS

This code fetches the contents of a text box, encodes it for signing, and signs it with a private key. See the complete source code on GitHub.

/*
Fetch the contents of the "message" textbox, and encode it
in a form we can use for the sign operation.
*/
function getMessageEncoding() {
  const messageBox = document.querySelector(".rsa-pss #message");
  let message = messageBox.value;
  let enc = new TextEncoder();
  return enc.encode(message);
}

let encoded = getMessageEncoding();
let signature = await window.crypto.subtle.sign(
  {
    name: "RSA-PSS",
    saltLength: 32,
  },
  privateKey,
  encoded
);

ECDSA

This code fetches the contents of a text box, encodes it for signing, and signs it with a private key. See the complete source code on GitHub.

/*
Fetch the contents of the "message" textbox, and encode it
in a form we can use for the sign operation.
*/
function getMessageEncoding() {
  const messageBox = document.querySelector(".ecdsa #message");
  let message = messageBox.value;
  let enc = new TextEncoder();
  return enc.encode(message);
}

let encoded = getMessageEncoding();
let signature = await window.crypto.subtle.sign(
  {
    name: "ECDSA",
    hash: {name: "SHA-384"},
  },
  privateKey,
  encoded
);

HMAC

This code fetches the contents of a text box, encodes it for signing, and signs it with a secret key. See the complete source code on GitHub.

/*
Fetch the contents of the "message" textbox, and encode it
in a form we can use for the sign operation.
*/
function getMessageEncoding() {
  const messageBox = document.querySelector(".hmac #message");
  let message = messageBox.value;
  let enc = new TextEncoder();
  return enc.encode(message);
}

let encoded = getMessageEncoding();
let signature = await window.crypto.subtle.sign(
  "HMAC",
  key,
  encoded
);

Specifications

Browser compatibility

See also

• SubtleCrypto.verify().
• RFC 3447 specifies RSASSA-PKCS1-v1_5.
• RFC 3447 specifies RSA-PSS.
• FIPS-186 specifies ECDSA.
• FIPS 198-1 specifies HMAC.