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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.

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

Usage

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;
  }
}

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

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

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

  • 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

  • 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

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

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

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

  • 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

exportKey

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

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

Parameters

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

unwrapKey

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

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

Parameters

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.

Algorithmsign()
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.