<?php
/**
 * Encrypt / decrypt data using a trivial character substitution algorithm.
 *
 * This should never be used, really. However, if neither Open SSL or mcrypt are available, we don't really
 * have choice for a better double way encryption.
 * If should consider to require either Open SSL or mcrypt to avoid the usage of this class.
 *
 * Normally we should implement an interface, but at the moment with PHP 5.2 and the "poor" autoloader we have,
 * an interface would just be another file in "/inc" folder causing confusion. Also, I don't want to create an
 * interface in a file named `class-...php`. When we get rid of PHP 5.2, we setup a better autoloader and we get rid of
 * WP coding standard, we finally could consider to introduce an interface.
 */
class BackWPup_Encryption_Fallback {

	const PREFIX = 'ENC1$';

	/**
	 * @return bool
	 */
	public static function supported() {

		/** @TODO: Should we inform the user about the security risk? and how? */
		return true;
	}

	/**
	 * @param string $enc_key
	 * @param string $key_type
	 */
	public function __construct( $enc_key, $key_type ) {
		$this->key      = md5( (string) $enc_key );
		$this->key_type = (string) $key_type;
	}

	/**
	 *
	 * Encrypt a string (Passwords)
	 *
	 * @param string $string value to encrypt
	 *
	 * @return string encrypted string
	 */
	public function encrypt( $string ) {

		$result = '';
		for ( $i = 0; $i < strlen( $string ); $i ++ ) {
			$char     = substr( $string, $i, 1 );
			$key_char = substr( $this->key, ( $i % strlen( $this->key ) ) - 1, 1 );
			$char     = chr( ord( $char ) + ord( $key_char ) );
			$result .= $char;
		}

		return BackWPup_Encryption::PREFIX . self::PREFIX . $this->key_type . base64_encode( $result );

	}

	/**
	 *
	 * Decrypt a string (Passwords)
	 *
	 * @param string $string value to decrypt
	 *
	 * @return string decrypted string
	 */
	public function decrypt( $string ) {

		if (
			! is_string( $string )
			|| ! $string
			|| strpos( $string, BackWPup_Encryption::PREFIX . self::PREFIX . $this->key_type ) !== 0
		) {
			return '';
		}

		$no_prefix = substr( $string, strlen( BackWPup_Encryption::PREFIX . self::PREFIX . $this->key_type ) );

		$encrypted = base64_decode( $no_prefix, true );
		if ( $encrypted === false ) {
			return '';
		}

		$result = '';
		for ( $i = 0; $i < strlen( $encrypted ); $i ++ ) {
			$char     = substr( $encrypted, $i, 1 );
			$key_char = substr( $this->key, ( $i % strlen( $this->key ) ) - 1, 1 );
			$char     = chr( ord( $char ) - ord( $key_char ) );
			$result .= $char;
		}

		return $result;
	}
}