Web Crypto

Background

The Web Crypto API provides a set of low-level functions for common cryptographic tasks. The Workers runtime implements the full surface of this API, but with some differences in the supported algorithms compared to those implemented in most browsers.

Performing cryptographic operations using the Web Crypto API is significantly faster than performing them purely in JavaScript. If you want to perform CPU-intensive cryptographic operations, you should consider using the Web Crypto API.

The Web Crypto API is implemented through the SubtleCrypto interface, accessible via the global crypto.subtle binding. A simple example of calculating a digest (also known as a hash) is:

const myText = new TextEncoder().encode('Hello world!');

const myDigest = await crypto.subtle.digest(
  {
    name: 'SHA-256',
  },
  myText // The data you want to hash as an ArrayBuffer
);

console.log(new Uint8Array(myDigest));

Some common uses include signing requests.

Warning

The Web Crypto API differs significantly from the Node.js Crypto API. If you are working with code that relies on the Node.js Crypto API, you can use it by enabling the nodejs_compat compatibility flag.

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Constructors

• crypto.DigestStream(algorithm) DigestStream

  • A non-standard extension to the crypto API that supports generating a hash digest from streaming data. The DigestStream itself is a WritableStream that does not retain the data written into it. Instead, it generates a hash digest automatically when the flow of data has ended.

Parameters

• algorithmstring | object

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

Usage

JavaScript

export default {
  async fetch(req) {
    // Fetch from origin
    const res = await fetch(req);

    // We need to read the body twice so we `tee` it (get two instances)
    const [bodyOne, bodyTwo] = res.body.tee();
    // Make a new response so we can set the headers (responses from `fetch` are immutable)
    const newRes = new Response(bodyOne, res);
    // Create a SHA-256 digest stream and pipe the body into it
    const digestStream = new crypto.DigestStream("SHA-256");
    bodyTwo.pipeTo(digestStream);
    // Get the final result
    const digest = await digestStream.digest;
    // Turn it into a hex string
    const hexString = [...new Uint8Array(digest)]
      .map(b => b.toString(16).padStart(2, '0'))
      .join('')
    // Set a header with the SHA-256 hash and return the response
    newRes.headers.set("x-content-digest", `SHA-256=${hexString}`);
    return newRes;
  }
}

TypeScript

export default {
  async fetch(req): Promise<Response> {
    // Fetch from origin
    const res = await fetch(req);

    // We need to read the body twice so we `tee` it (get two instances)
    const [bodyOne, bodyTwo] = res.body.tee();
    // Make a new response so we can set the headers (responses from `fetch` are immutable)
    const newRes = new Response(bodyOne, res);
    // Create a SHA-256 digest stream and pipe the body into it
    const digestStream = new crypto.DigestStream("SHA-256");
    bodyTwo.pipeTo(digestStream);
    // Get the final result
    const digest = await digestStream.digest;
    // Turn it into a hex string
    const hexString = [...new Uint8Array(digest)]
      .map(b => b.toString(16).padStart(2, '0'))
      .join('')
    // Set a header with the SHA-256 hash and return the response
    newRes.headers.set("x-content-digest", `SHA-256=${hexString}`);
    return newRes;
  }
} satisfies ExportedHandler;

Methods

• crypto.randomUUID() : string

  • Generates a new random (version 4) UUID as defined in RFC 4122 ↗.

• crypto.getRandomValues(bufferArrayBufferView) : ArrayBufferView

  • Fills the passed ArrayBufferView with cryptographically sound random values and returns the buffer.

Parameters

• bufferArrayBufferView

  • Must be an Int8Array | Uint8Array | Uint8ClampedArray | Int16Array | Uint16Array | Int32Array | Uint32Array | BigInt64Array | BigUint64Array.

SubtleCrypto Methods

These methods are all accessed via crypto.subtle ↗, which is also documented in detail on MDN.

encrypt

• encrypt(algorithm, key, data) : Promise<ArrayBuffer>

  • Returns a Promise that fulfills with the encrypted data corresponding to the clear text, algorithm, and key given as parameters.

Parameters

• algorithmobject

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• keyCryptoKey

• dataBufferSource

decrypt

• decrypt(algorithm, key, data) : Promise<ArrayBuffer>

  • Returns a Promise that fulfills with the clear data corresponding to the ciphertext, algorithm, and key given as parameters.

Parameters

• algorithmobject

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• keyCryptoKey

• dataBufferSource

sign

• sign(algorithm, key, data) : Promise<ArrayBuffer>

  • Returns a Promise that fulfills with the signature corresponding to the text, algorithm, and key given as parameters.

Parameters

• algorithmstring | object

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• keyCryptoKey

• dataArrayBuffer

verify

• verify(algorithm, key, signature, data) : Promise<boolean>

  • Returns a Promise that fulfills with a Boolean value indicating if the signature given as a parameter matches the text, algorithm, and key that are also given as parameters.

Parameters

• algorithmstring | object

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• keyCryptoKey

• signatureArrayBuffer

• dataArrayBuffer

digest

• digest(algorithm, data) : Promise<ArrayBuffer>

  • Returns a Promise that fulfills with a digest generated from the algorithm and text given as parameters.

Parameters

• algorithmstring | object

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• dataArrayBuffer

generateKey

• generateKey(algorithm, extractable, keyUsages) : Promise<CryptoKey> | Promise<CryptoKeyPair>

  • Returns a Promise that fulfills with a newly-generated CryptoKey, for symmetrical algorithms, or a CryptoKeyPair, containing two newly generated keys, for asymmetrical algorithms. For example, to generate a new AES-GCM key:

  let keyPair = await crypto.subtle.generateKey(
    {
      name: 'AES-GCM',
      length: 256,
    },
    true,
    ['encrypt', 'decrypt']
  );

Parameters

• algorithmobject

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• extractablebool

• keyUsagesArray

  • An Array of strings indicating the possible usages of the new key ↗.

deriveKey

• deriveKey(algorithm, baseKey, derivedKeyAlgorithm, extractable, keyUsages) : Promise<CryptoKey>

  • Returns a Promise that fulfills with a newly generated CryptoKey derived from the base key and specific algorithm given as parameters.

Parameters

• algorithmobject

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.
• baseKeyCryptoKey
• derivedKeyAlgorithmobject
  • Defines the algorithm the derived key will be used for in an algorithm-specific format ↗.
• extractablebool
• keyUsagesArray
  • An Array of strings indicating the possible usages of the new key ↗

deriveBits

• deriveBits(algorithm, baseKey, length) : Promise<ArrayBuffer>

  • Returns a Promise that fulfills with a newly generated buffer of pseudo-random bits derived from the base key and specific algorithm given as parameters. It returns a Promise which will be fulfilled with an ArrayBuffer containing the derived bits. This method is very similar to deriveKey(), except that deriveKey() returns a CryptoKey object rather than an ArrayBuffer. Essentially, deriveKey() is composed of deriveBits() followed by importKey().

Parameters

• algorithmobject

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• baseKeyCryptoKey

• lengthint

  • Length of the bit string to derive.

importKey

• importKey(format, keyData, algorithm, extractable, keyUsages) : Promise<CryptoKey>

  • Transform a key from some external, portable format into a CryptoKey for use with the Web Crypto API.

Parameters

• formatstring

  • Describes the format of the key to be imported ↗.

• keyDataArrayBuffer

• algorithmobject

  • Describes the algorithm to be used, including any required parameters, in an algorithm-specific format ↗.

• extractablebool

• keyUsagesArray

  • An Array of strings indicating the possible usages of the new key ↗

exportKey

• exportKey(formatstring, keyCryptoKey) : Promise<ArrayBuffer>

  • Transform a CryptoKey into a portable format, if the CryptoKey is extractable.

Parameters

• formatstring

  • Describes the format in which the key will be exported ↗.

• keyCryptoKey

wrapKey

• wrapKey(format, key, wrappingKey, wrapAlgo) : Promise<ArrayBuffer>

  • Transform a CryptoKey into a portable format, and then encrypt it with another key. This renders the CryptoKey suitable for storage or transmission in untrusted environments.

Parameters

• formatstring

  • Describes the format in which the key will be exported ↗ before being encrypted.

• keyCryptoKey

• wrappingKeyCryptoKey

• wrapAlgoobject

  • Describes the algorithm to be used to encrypt the exported key, including any required parameters, in an algorithm-specific format ↗.

unwrapKey

• unwrapKey(format, key, unwrappingKey, unwrapAlgo, 
unwrappedKeyAlgo, extractable, keyUsages) : Promise<CryptoKey>

  • Transform a key that was wrapped by wrapKey() back into a CryptoKey.

Parameters

• formatstring

  • Described the data format of the key to be unwrapped ↗.

• keyCryptoKey

• unwrappingKeyCryptoKey

• unwrapAlgoobject

  • Describes the algorithm that was used to encrypt the wrapped key, in an algorithm-specific format ↗.

• unwrappedKeyAlgoobject

  • Describes the key to be unwrapped, in an algorithm-specific format ↗.

• extractablebool

• keyUsagesArray

  • An Array of strings indicating the possible usages of the new key ↗

timingSafeEqual

• timingSafeEqual(a, b) : bool

  • Compare two buffers in a way that is resistant to timing attacks. This is a non-standard extension to the Web Crypto API.

Parameters

• aArrayBuffer | TypedArray

• bArrayBuffer | TypedArray

Supported algorithms

Workers implements all operations of the WebCrypto standard ↗, as shown in the following table.

A checkmark (✓) indicates that this feature is believed to be fully supported according to the spec.
An x (✘) indicates that this feature is part of the specification but not implemented.
If a feature only implements the operation partially, details are listed.

Algorithm	sign() verify()	encrypt() decrypt()	digest()	deriveBits() deriveKey()	generateKey()	wrapKey() unwrapKey()	exportKey()	importKey()
RSASSA PKCS1 v1.5	✓				✓		✓	✓
RSA PSS	✓				✓		✓	✓
RSA OAEP		✓			✓	✓	✓	✓
ECDSA	✓				✓		✓	✓
ECDH				✓	✓		✓	✓
Ed255191	✓				✓		✓	✓
X255191				✓	✓		✓	✓
NODE ED255192	✓				✓		✓	✓
AES CTR		✓			✓	✓	✓	✓
AES CBC		✓			✓	✓	✓	✓
AES GCM		✓			✓	✓	✓	✓
AES KW					✓	✓	✓	✓
HMAC	✓				✓		✓	✓
SHA 1			✓
SHA 256			✓
SHA 384			✓
SHA 512			✓
MD53			✓
HKDF				✓				✓
PBKDF2				✓				✓

Footnotes:

1. Algorithms as specified in the Secure Curves API ↗.

2. Legacy non-standard EdDSA is supported for the Ed25519 curve in addition to the Secure Curves version. Since this algorithm is non-standard, note the following while using it:

   • Use NODE-ED25519 as the algorithm and namedCurve parameters.
   • Unlike NodeJS, Cloudflare will not support raw import of private keys.
   • The algorithm implementation may change over time. While Cloudflare cannot guarantee it at this time, Cloudflare will strive to maintain backward compatibility and compatibility with NodeJS’s behavior. Any notable compatibility notes will be communicated in release notes and via this developer documentation.

3. MD5 is not part of the WebCrypto standard but is supported in Cloudflare Workers for interacting with legacy systems that require MD5. MD5 is considered a weak algorithm. Do not rely upon MD5 for security.

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Related resources

• SubtleCrypto documentation on MDN ↗
• SubtleCrypto documentation as part of the W3C Web Crypto API specification ↗
• Example: signing requests