Web Crypto API AES (CBC/GCM/CTR) Export Keys with JWK format

by Anish

Posted on Monday september 17, 2018

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This sample chapter extracted from the book, Cryptography for JavaScript Developers.

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The Crypto interface represents an interface to general purpose cryptographic functionality including a cryptographically strong pseudo-random number generator seeded with truly random values.

The SubtleCrypto.generateKey() method returns a Promise of a newly generated CryptoKey

The Syntax

var result = crypto.subtle.generateKey(algo, extractable, keyUsages);

• algo: Supported algo are: AES-CBC, AES-CTR, AES-GCM, RSA-OAEP, AES-KW, HMAC, RSASSA-PKCS1-v1_5, ECDSA, ECDH, and DH.

• extractable is a Boolean indicating if the key can be extracted from the CryptoKey object at a later stage

• keyUsages is an Array indicating what can be done with the newly generated key. Possible values of the array are for AES encryption:

  • encrypt, allowing the key to be used for encrypting messages.
  • decrypt, allowing the key to be used for decrypting messages.
  • wrapKey, allowing the key to wrap a symmetric key for usage (transfer, storage) in unsecure environments.
  • unwrapKey, allowing the key to unwrap a symmetric key for usage (transfer, storage) in unsecure environments.

The SubtleCrypto.exportKey() method returns a Promise of the key encrypted in the requested format

var result = crypto.subtle.exportKey(format, key);

The Demo

java script example AES-GCM generateKey and Export Keys with JWK format

 function AESGCM_EXPORT_KEYS() {

    var iv = crypto.getRandomValues(new Uint8Array(16));
    window.crypto.subtle.generateKey({
            name: "AES-GCM",
            length: 256, //can be  128, 192, or 256
        },
        true, // the key is extractable (i.e. can be used in exportKey)
        ["encrypt", "decrypt", "wrapKey", "unwrapKey"] //can "encrypt", "decrypt" or  "wrapKey", or "unwrapKey"
    ).then(function(aesgcmkey) {
        var secretKey = aesgcmkey;
        return crypto.subtle.exportKey("jwk", secretKey).then(function(result) {
        key = result;
        document.getElementById("kty").value = key.kty;
        document.getElementById("key_ops").value = key.key_ops;
        document.getElementById("key_algo").value = key.alg
        document.getElementById("key").value = btoa(key.k);
    }, failAndLog);
    })
    .catch(function(err) {
        console.error(err);
    });
}

The HTML

 <div>
    <button type="button" onclick="AESGCM_EXPORT_KEYS()">AES256-GCM-GENERATE KEY AND EXPORT IN JWK</button>
</div>
<div>
    KTY: <input type="text" id="kty" size="10">
    ALGO: <input type="text" id="key_algo" size="10">
    use: <input type="text" id="key_ops" size="20">
    key: <input type="text" id="key" size="32">
</div>

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Download the sample code here

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Next Reading Perform AES Encryption Using importKey Method

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