hd-auth-wallet/lib/core/cryptoUtil.js

const crypto = require('crypto');
const algorithm = 'aes256';
const bip32 = require('bip32');

// NOTE I'm not a cryptographer... so don't be reusing any code from below as I don't have no idea what I'm doing

module.exports.encrypt = (plainText, password) => {
    const iv = crypto.randomBytes(16).toString('hex').slice(0, 16); // TODO figure out this iv stuff...

    const key = crypto.createHash('sha256').update(password, 'utf8').digest();
    const cipher = crypto.createCipheriv(algorithm, key, iv);

    var cipherText = cipher.update(plainText, 'utf8', 'hex');
    cipherText += cipher.final('hex');
    return {
        cipherText: cipherText,
        iv: iv
    };
}

module.exports.decrypt = (cipherText, password, iv) => {
    const key = crypto.createHash('sha256').update(password, 'utf8').digest();
    const decipher = crypto.createDecipheriv(algorithm, key, iv);
        
    var plainText = decipher.update(cipherText, 'hex', 'utf8');
    plainText += decipher.final('utf8');

    return plainText;
}

module.exports.randomDerivationPath = function(hardenedDerivation) {
    // TODO harden derivation path
    var randomNumbers = [];

    for(var i = 0; i < 3; i++) {
        randomNumbers.push(parseInt(crypto.randomBytes(3).toString('hex'), 16))
    }

    return randomNumbers.join('/')
}

module.exports.verifyChallenge = function(serviceXpub, challenge) {
    const requestSignature = challenge.request.signature;
    const requestDerivationPath = challenge.request.signatureDerivationPath;
    return this.verifyMessage(
        serviceXpub, 
        requestDerivationPath,
        JSON.stringify(this.unsignedChallenge(challenge)),
        requestSignature
    );
}

module.exports.verifyMessage = function(base58Key, derivationPath, message, signature) {
    if(derivationPath.startsWith("a/")) {
        derivationPath = derivationPath.split("a/")[1];
    }

    if(derivationPath.startsWith("c/")) {
        derivationPath = derivationPath.split("c/")[1];
    }

    const verificationNode = bip32.fromBase58(base58Key).derivePath(derivationPath);
    const hash = crypto.createHash('sha256').update(message, 'utf8').digest();

    return verificationNode.verify(hash, Buffer.from(signature, 'hex'));
}

module.exports.unsignedChallenge = function(challenge) {
    return {
        // keys must be alphabetically ordered
        derivationPath: challenge.derivationPath,
        message: challenge.message,
        xpub: challenge.xpub
    }
}

module.exports.signMessage = function(base58Key, derivationPath, message) {
    let signingNode = bip32.fromBase58(base58Key).derivePath(derivationPath);

    var hash = crypto.createHash('sha256').update(message, 'utf8').digest();

    return signingNode.sign(hash);
}