Encrypt a private key with a password just using WebCrypto API

Question

Is it possible to encrypt a CryptoKey (private key) with a string as a encryption key and just using the WebCryptoAPI? I implemented a function but I got the following error:

Uncaught (in promise) DOMException: AES key data must be 128 or 256 bits

Also, my function.

function encryptPrivateKey() {
    var promise = new Promise(function (resolve, reject) {
        try {
            var key = new TextEncoder().encode(pwd);
            var iv = crypto.getRandomValues(new Uint8Array(12));
            var alg = {name: 'AES-CTR', iv: iv};
            window.crypto.subtle.importKey('raw', key, alg, false, ['encrypt']).then(function (key) {
                window.crypto.subtle.encrypt(alg, key, privateKeyPEM).then(function (key) {
                    privateKey = key;
                })
            });
            resolve(privateKey);
        } catch (err) {
            reject(Error(err));
        }

    });
    return promise.then(function (result) {
        return result;
    }, function (err) {
        console.log(err);
    });
}

Answer 1

It is, first of all please use an authenticated mode of encryption such as AES-GCM.

Secondly, you should consider using an existing format such as PKCS#8 for this task. You can see PKI.js for a library that can help you do this.

Another approach would be to encrypt the key into a CMS message, here is a sample that does just that: https://pkijs.org/examples/CMSEnvelopedPreDefineDataExample.html

You can look at it to see how to do the associated webcrypto even if you wanted to do a custom format.

Third, it is much better to not encrypt with a password, anything encrypted with a password will be grindable, it is always better to generate a solid symmetric key and use that instead of a derived password.

If you do use a password be sure to use PBDKF2 to stretch the password, https://github.com/diafygi/webcrypto-examples#pbkdf2. If you do this your above sample is going to end up working.

If you use one of the existing formats for keys they will send you down this path naturally.