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<?php

namespace Aws\Crypto\Polyfill;

use Aws\Exception\CryptoPolyfillException;

use InvalidArgumentException;

use RangeException;

/**

 * Class AesGcm

 *

 * This provides a polyfill for AES-GCM encryption/decryption, with caveats:

 *

 * 1. Only 96-bit nonces are supported.

 * 2. Only 128-bit authentication tags are supported. (i.e. non-truncated)

 *

 * Supports AES key sizes of 128-bit, 192-bit, and 256-bit.

 */

class AesGcm

{

    use NeedsTrait;

    /** @var Key $aesKey */

    private $aesKey;

    /** @var int $keySize */

    private $keySize;

    /** @var int $blockSize */

    protected $blockSize = 8192;

    /**

     * AesGcm constructor.

     *

     * @param Key $aesKey

     * @param int $keySize

     * @param int $blockSize

     *

     * @throws CryptoPolyfillException

     * @throws InvalidArgumentException

     * @throws RangeException

     */

    public function __construct(Key $aesKey, $keySize = 256, $blockSize = 8192)

    {

        /* Preconditions: */

        self::needs(

            \in_array($keySize, [128, 192, 256], true),

            "Key size must be 128, 192, or 256 bits; {$keySize} given",

            InvalidArgumentException::class

        );

        self::needs(

            \is_int($blockSize) && $blockSize > 0 && $blockSize <= PHP_INT_MAX,

            'Block size must be a positive integer.',

            RangeException::class

        );

        self::needs(

            $aesKey->length() << 3 === $keySize,

            'Incorrect key size; expected ' . $keySize . ' bits, got ' . ($aesKey->length() << 3) . ' bits.'

        );

        $this->aesKey = $aesKey;

        $this->keySize = $keySize;

    }

    /**

     * Encryption interface for AES-GCM

     *

     * @param string $plaintext  Message to be encrypted

     * @param string $nonce      Number to be used ONCE

     * @param Key $key           AES Key

     * @param string $aad        Additional authenticated data

     * @param string &$tag       Reference to variable to hold tag

     * @param int $keySize       Key size (bits)

     * @param int $blockSize     Block size (bytes) -- How much memory to buffer

     * @return string

     * @throws InvalidArgumentException

     */

    public static function encrypt(

        $plaintext,

        $nonce,

        Key $key,

        $aad,

        &$tag,

        $keySize = 256,

        $blockSize = 8192

    ) {

        self::needs(

            self::strlen($nonce) === 12,

            'Nonce must be exactly 12 bytes',

            InvalidArgumentException::class

        );

        $encryptor = new AesGcm($key, $keySize, $blockSize);

        list($aadLength, $gmac) = $encryptor->gmacInit($nonce, $aad);

        $ciphertext = \openssl_encrypt(

            $plaintext,

            "aes-{$encryptor->keySize}-ctr",

            $key->get(),

            OPENSSL_NO_PADDING | OPENSSL_RAW_DATA,

            $nonce . "\x00\x00\x00\x02"

        );

        /* Calculate auth tag in a streaming fashion to minimize memory usage: */

        $ciphertextLength = self::strlen($ciphertext);

        for ($i = 0; $i < $ciphertextLength; $i += $encryptor->blockSize) {

            $cBlock = new ByteArray(self::substr($ciphertext, $i, $encryptor->blockSize));

            $gmac->update($cBlock);

        }

        $tag = $gmac->finish($aadLength, $ciphertextLength)->toString();

        return $ciphertext;

    }

    /**

     * Decryption interface for AES-GCM

     *

     * @param string $ciphertext Ciphertext to decrypt

     * @param string $nonce      Number to be used ONCE

     * @param Key $key           AES key

     * @param string $aad        Additional authenticated data

     * @param string $tag        Authentication tag

     * @param int $keySize       Key size (bits)

     * @param int $blockSize     Block size (bytes) -- How much memory to buffer

     * @return string            Plaintext

     *

     * @throws CryptoPolyfillException

     * @throws InvalidArgumentException

     */

    public static function decrypt(

        $ciphertext,

        $nonce,

        Key $key,

        $aad,

        &$tag,

        $keySize = 256,

        $blockSize = 8192

    ) {

        /* Precondition: */

        self::needs(

            self::strlen($nonce) === 12,

            'Nonce must be exactly 12 bytes',

            InvalidArgumentException::class

        );

        $encryptor = new AesGcm($key, $keySize, $blockSize);

        list($aadLength, $gmac) = $encryptor->gmacInit($nonce, $aad);

        /* Calculate auth tag in a streaming fashion to minimize memory usage: */

        $ciphertextLength = self::strlen($ciphertext);

        for ($i = 0; $i < $ciphertextLength; $i += $encryptor->blockSize) {

            $cBlock = new ByteArray(self::substr($ciphertext, $i, $encryptor->blockSize));

            $gmac->update($cBlock);

        }

        /* Validate auth tag in constant-time: */

        $calc = $gmac->finish($aadLength, $ciphertextLength);

        $expected = new ByteArray($tag);

        self::needs($calc->equals($expected), 'Invalid authentication tag');

        /* Return plaintext if auth tag check succeeded: */

        return \openssl_decrypt(

            $ciphertext,

            "aes-{$encryptor->keySize}-ctr",

            $key->get(),

            OPENSSL_NO_PADDING | OPENSSL_RAW_DATA,

            $nonce . "\x00\x00\x00\x02"

        );

    }

    /**

     * Initialize a Gmac object with the nonce and this object's key.

     *

     * @param string $nonce     Must be exactly 12 bytes long.

     * @param string|null $aad

     * @return array

     */

    protected function gmacInit($nonce, $aad = null)

    {

        $gmac = new Gmac(

            $this->aesKey,

            $nonce . "\x00\x00\x00\x01",

            $this->keySize

        );

        $aadBlock = new ByteArray($aad);

        $aadLength = $aadBlock->count();

        $gmac->update($aadBlock);

        $gmac->flush();

        return [$aadLength, $gmac];

    }

    /**

     * Calculate the length of a string.

     *

     * Uses the appropriate PHP function without being brittle to

     * mbstring.func_overload.

     *

     * @param string $string

     * @return int

     */

    protected static function strlen($string)

    {

        if (\is_callable('\\mb_strlen')) {

            return (int) \mb_strlen($string, '8bit');

        }

        return (int) \strlen($string);

    }

    /**

     * Return a substring of the provided string.

     *

     * Uses the appropriate PHP function without being brittle to

     * mbstring.func_overload.

     *

     * @param string $string

     * @param int $offset

     * @param int|null $length

     * @return string

     */

    protected static function substr($string, $offset = 0, $length = null)

    {

        if (\is_callable('\\mb_substr')) {

            return \mb_substr($string, $offset, $length, '8bit');

        } elseif (!\is_null($length)) {

            return \substr($string, $offset, $length);

        }

        return \substr($string, $offset);

    }

}