Custom Algorithm

<?php

declare(strict_types=1);

namespace Acme\Algorithm;

use Base64Url\Base64Url;
use Jose\Component\Core\JWK;
use const Sodium\CRYPTO_AEAD_CHACHA20POLY1305_IETF_NPUBBYTES;

/**
 * This algorithm is a custom algorithm that use the ChaCha20 + Poly 1305 with a 192 bits nonce (IETF variant).
 */
final class ChaCha20Poly1305IETF implements KeyEncryption //The algorithm acts as a Key Encryption algorithm
{
    /**
     * {@inheritdoc}
     */
    public function name(): string
    {
        return 'ChaCha20+Poly1305+IETF'; // The name of our algorithm. This name will be used in our JWE headers
    }

    /**
     * {@inheritdoc}
     */
    public function allowedKeyTypes(): array
    {
        return ['oct']; // Key types for this algorithm are octet keys
    }

    /**
     * {@inheritdoc}
     */
    public function encryptKey(JWK $key, string $cek, array $completeHeader, array &$additionalHeader): string
    {
        $this->checkKey($key); // We check the key
        $kek = Base64Url::decode($key->get('k')); // We retrieve the secret
        $nonce = random_bytes(CRYPTO_AEAD_CHACHA20POLY1305_IETF_NPUBBYTES); // We create a nonce
        $additionalHeader['nonce'] = Base64Url::encode($nonce); // We add the nonce to the header

        return sodium_crypto_aead_chacha20poly1305_ietf_encrypt($cek, '', $nonce, $kek); // We return the encrypted CEK
    }

    /**
     * {@inheritdoc}
     */
    public function decryptKey(JWK $key, string $encrypted_cek, array $header): string
    {
        $this->checkKey($key); // We check the key
        $this->checkAdditionalParameters($header); // We verify the nonce is in the headers
        $nonce = Base64Url::decode($header['nonce']); // We retrieve the nonce
        $kek = Base64Url::decode($key->get('k')); // an the secret

        $decrypted = sodium_crypto_aead_chacha20poly1305_ietf_decrypt($encrypted_cek, '', $nonce, $kek); // We try to decrypt the CEK
        if (false === $decrypted) { // If it fails we throw an exception
            throw new \RuntimeException('Unable to decrypt.');
        }

        return $decrypted; // Otherwise we return the decrypted CEK
    }

    /**
     * @param JWK $key
     */
    protected function checkKey(JWK $key)
    {
        if (!in_array($key->get('kty'), $this->allowedKeyTypes())) {
            throw new \InvalidArgumentException('Wrong key type.');
        }
        if (!$key->has('k')) {
            throw new \InvalidArgumentException('The key parameter "k" is missing.');
        }
    }

    /**
     * @param array $header
     */
    protected function checkAdditionalParameters(array $header)
    {
        foreach (['nonce'] as $k) {
            if (!array_key_exists($k, $header)) {
                throw new \InvalidArgumentException(sprintf('Parameter "%s" is missing.', $k));
            }
        }
    }

    /**
     * {@inheritdoc}
     */
    public function getKeyManagementMode(): string
    {
        return self::MODE_ENCRYPT; //Key Management Mode is 'enc'.
    }
}