<?php

namespace Defuse\Crypto;

use Defuse\Crypto\Exception as Ex;

final class KeyProtectedByPassword
{
    const PASSWORD_KEY_CURRENT_VERSION = "\xDE\xF1\x00\x00";

    /**
     * @var string
     */
    private $encrypted_key = '';

    /**
     * Creates a random key protected by the provided password.
     *
     * @param string $password
     *
     * @throws Ex\EnvironmentIsBrokenException
     *
     * @return KeyProtectedByPassword
     */
    public static function createRandomPasswordProtectedKey($password)
    {
        $inner_key = Key::createNewRandomKey();
        /* The password is hashed as a form of poor-man's domain separation
         * between this use of encryptWithPassword() and other uses of
         * encryptWithPassword() that the user may also be using as part of the
         * same protocol. */
        $encrypted_key = Crypto::encryptWithPassword(
            $inner_key->saveToAsciiSafeString(),
            \hash(Core::HASH_FUNCTION_NAME, $password, true),
            true
        );

        return new KeyProtectedByPassword($encrypted_key);
    }

    /**
     * Loads a KeyProtectedByPassword from its encoded form.
     *
     * @param string $saved_key_string
     *
     * @throws Ex\BadFormatException
     *
     * @return KeyProtectedByPassword
     */
    public static function loadFromAsciiSafeString($saved_key_string)
    {
        $encrypted_key = Encoding::loadBytesFromChecksummedAsciiSafeString(
            self::PASSWORD_KEY_CURRENT_VERSION,
            $saved_key_string
        );
        return new KeyProtectedByPassword($encrypted_key);
    }

    /**
     * Encodes the KeyProtectedByPassword into a string of printable ASCII
     * characters.
     *
     * @throws Ex\EnvironmentIsBrokenException
     *
     * @return string
     */
    public function saveToAsciiSafeString()
    {
        return Encoding::saveBytesToChecksummedAsciiSafeString(
            self::PASSWORD_KEY_CURRENT_VERSION,
            $this->encrypted_key
        );
    }

    /**
     * Decrypts the protected key, returning an unprotected Key object that can
     * be used for encryption and decryption.
     *
     * @throws Ex\EnvironmentIsBrokenException
     * @throws Ex\WrongKeyOrModifiedCiphertextException
     *
     * @param string $password
     * @return Key
     */
    public function unlockKey($password)
    {
        try {
            $inner_key_encoded = Crypto::decryptWithPassword(
                $this->encrypted_key,
                \hash(Core::HASH_FUNCTION_NAME, $password, true),
                true
            );
            return Key::loadFromAsciiSafeString($inner_key_encoded);
        } catch (Ex\BadFormatException $ex) {
            /* This should never happen unless an attacker replaced the
             * encrypted key ciphertext with some other ciphertext that was
             * encrypted with the same password. We transform the exception type
             * here in order to make the API simpler, avoiding the need to
             * document that this method might throw an Ex\BadFormatException. */
            throw new Ex\WrongKeyOrModifiedCiphertextException(
                "The decrypted key was found to be in an invalid format. " .
                "This very likely indicates it was modified by an attacker."
            );
        }
    }

    /**
     * Constructor for KeyProtectedByPassword.
     *
     * @param string $encrypted_key
     */
    private function __construct($encrypted_key)
    {
        $this->encrypted_key = $encrypted_key;
    }
}
