SubtleCrypto

The SubtleCrypto interface of the Web Crypto API provides a number of low-level cryptographic functions. Access to the features of SubtleCrypto is obtained through the subtle property of the Crypto object you get from Window.crypto.

Properties

This interface doesn't inherit any properties, as it has no parent interface.

Methods

This interface doesn't inherit any methods, as it has no parent interface.

SubtleCrypto.encrypt()

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

SubtleCrypto.decrypt()

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

SubtleCrypto.sign()

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

SubtleCrypto.verify()

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.

SubtleCrypto.digest()

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

SubtleCrypto.generateKey()

Returns a Promise that fulfills with a newly-generated CryptoKey, for symmetrical algorithms, or a CryptoKeyPair, containing two newly generated keys, for asymmetrical algorithms. These will match the algorithm, usages, and extractability given as parameters.

SubtleCrypto.deriveKey()

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

SubtleCrypto.deriveBits()

Returns a Promise that fulfills with a newly generated buffer of pseudo-random bits derived from the master key and specific algorithm given as parameters.

SubtleCrypto.importKey()

Returns a Promise that fulfills with a CryptoKey corresponding to the format, the algorithm, raw key data, usages, and extractability given as parameters.

SubtleCrypto.exportKey()

Returns a Promise that fulfills with a buffer containing the key in the requested format.

SubtleCrypto.wrapKey()

Returns a Promise that fulfills with a wrapped symmetric key for usage (transfer and storage) in insecure environments. The wrapped key matches the format specified in the given parameters, and wrapping is done by the given wrapping key, using the specified algorithm.

SubtleCrypto.unwrapKey()

Returns a Promise that fulfills with a CryptoKey corresponding to the wrapped key given in the parameter.

Using SubtleCrypto

We can split the functions implemented by this API into two groups: cryptography functions and key management functions.

Cryptography functions

These are the functions you can use to implement security features such as privacy and authentication in a system. The SubtleCrypto API provides the following cryptography functions:

* sign() and verify(): create and verify digital signatures.
* encrypt() and decrypt(): encrypt and decrypt data.
* digest(): create a fixed-length, collision-resistant digest of some data.

Key management functions

Except for digest(), all the cryptography functions in the API use cryptographic keys. In the SubtleCrypto API a cryptographic key is represented using a CryptoKey object. To perform operations like signing and encrypting, you pass a CryptoKey object into the sign() or encrypt() function.

Generating and deriving keys

The generateKey() and deriveKey() functions both create a new CryptoKey object.

The difference is that generateKey() will generate a new distinct key value each time you call it, while deriveKey() derives a key from some initial keying material. If you provide the same keying material to two separate calls to deriveKey(), you will get two CryptoKey objects that have the same underlying value. This is useful if, for example, you want to derive an encryption key from a password and later derive the same key from the same password to decrypt the data.

Importing and exporting keys

To make keys available outside your app, you need to export the key, and that's what exportKey() is for. You can choose one of a number of export formats.

The inverse of exportKey() is importKey(). You can import keys from other systems, and support for standard formats like PKCS #8 and JSON Web Key helps you do this. The exportKey() function exports the key in an unencrypted format.

If the key is sensitive you should use wrapKey(), which exports the key and then encrypts it using another key; the API calls a "key-wrapping key".

The inverse of wrapKey() is unwrapKey(), which decrypts then imports the key.

Storing keys

CryptoKey objects can be stored using the structured clone algorithm, meaning that you can store and retrieve them using standard web storage APIs. The specification expects that most developers will use the IndexedDB API to store CryptoKey objects.

Supported algorithms

The cryptographic functions provided by the Web Crypto API can be performed by one or more different cryptographic algorithms: the algorithm argument to the function indicates which algorithm to use. Some algorithms need extra parameters: in these cases the algorithm argument is a dictionary object that includes the extra parameters.

The table below summarises which algorithms are suitable for which cryptographic operations:

Specifications

Browser compatibility

See also

• Web Crypto API
• Web security
• Privacy, permissions, and information security
• Crypto and Crypto.subtle.
• Crypto 101: an introductory course on cryptography.