<?php
namespace Aws\Crypto;

use Aws\Exception\CryptoException;
use Aws\Kms\KmsClient;

/**
 * Uses KMS to supply materials for encrypting and decrypting data. This
 * V2 implementation should be used with the V2 encryption clients (i.e.
 * S3EncryptionClientV2).
 */
class KmsMaterialsProviderV2 extends MaterialsProviderV2 implements MaterialsProviderInterfaceV2
{
    const WRAP_ALGORITHM_NAME = 'kms+context';

    private $kmsClient;
    private $kmsKeyId;

    /**
     * @param KmsClient $kmsClient A KMS Client for use encrypting and
     *                             decrypting keys.
     * @param string $kmsKeyId The private KMS key id to be used for encrypting
     *                         and decrypting keys.
     */
    public function __construct(
        KmsClient $kmsClient,
        $kmsKeyId = null
    ) {
        $this->kmsClient = $kmsClient;
        $this->kmsKeyId = $kmsKeyId;
    }

    /**
     * @inheritDoc
     */
    public function getWrapAlgorithmName()
    {
        return self::WRAP_ALGORITHM_NAME;
    }

    /**
     * @inheritDoc
     */
    public function decryptCek($encryptedCek, $materialDescription, $options)
    {
        $params = [
            'CiphertextBlob' => $encryptedCek,
            'EncryptionContext' => $materialDescription
        ];
        if (empty($options['@KmsAllowDecryptWithAnyCmk'])) {
            if (empty($this->kmsKeyId)) {
                throw new CryptoException('KMS CMK ID was not specified and the'
                    . ' operation is not opted-in to attempting to use any valid'
                    . ' CMK it discovers. Please specify a CMK ID, or explicitly'
                    . ' enable attempts to use any valid KMS CMK with the'
                    . ' @KmsAllowDecryptWithAnyCmk option.');
            }
            $params['KeyId'] = $this->kmsKeyId;
        }

        $result = $this->kmsClient->decrypt($params);
        return $result['Plaintext'];
    }

    /**
     * @inheritDoc
     */
    public function generateCek($keySize, $context, $options)
    {
        if (empty($this->kmsKeyId)) {
            throw new CryptoException('A KMS key id is required for encryption'
                . ' with KMS keywrap. Use a KmsMaterialsProviderV2 that has been'
                . ' instantiated with a KMS key id.');
        }
        $options = array_change_key_case($options);
        if (!isset($options['@kmsencryptioncontext'])
            || !is_array($options['@kmsencryptioncontext'])
        ) {
            throw new CryptoException("'@KmsEncryptionContext' is a"
                . " required argument when using KmsMaterialsProviderV2, and"
                . " must be an associative array (or empty array).");
        }
        if (isset($options['@kmsencryptioncontext']['aws:x-amz-cek-alg'])) {
            throw new CryptoException("Conflict in reserved @KmsEncryptionContext"
                . " key aws:x-amz-cek-alg. This value is reserved for the S3"
                . " Encryption Client and cannot be set by the user.");
        }
        $context = array_merge($options['@kmsencryptioncontext'], $context);
        $result = $this->kmsClient->generateDataKey([
            'KeyId' => $this->kmsKeyId,
            'KeySpec' => "AES_{$keySize}",
            'EncryptionContext' => $context
        ]);
        return [
            'Plaintext' => $result['Plaintext'],
            'Ciphertext' => base64_encode($result['CiphertextBlob']),
            'UpdatedContext' => $context
        ];
    }
}