Encrypt/Decrypt using WordPress constant SECURE_AUTH_KEY

Encrypt_Decrypt.php

<?php
// $this->_default_secure_auth_key defined in case SECURE_AUTH_KEY is not defined.

/**
 * Encrypt code using OpenSSL
 *
 * @param string $code The code to encrypt.
 */
public function encrypt_code( $code ) {
  if ( empty( $code ) ) {
    return '';
  }

	$key = defined( 'SECURE_AUTH_KEY' ) ? SECURE_AUTH_KEY : $this->_default_secure_auth_key;
	$encryption_key = base64_decode( $key );
	$iv = substr( openssl_random_pseudo_bytes( openssl_cipher_iv_length( 'aes-256-cbc' ) ), 0, 16 );
	$encrypted = openssl_encrypt( $code, 'aes-256-cbc', $encryption_key, OPENSSL_RAW_DATA, $iv );

	// Append the $iv variable to use for decrypting later.
	return base64_encode( $encrypted . '::' . $iv );
}

/**
 * Decrypt code using OpenSSL
 *
 * @param string $code The code to decrypt.
 */
public function decrypt_code( $code ) {
  $key = defined( 'SECURE_AUTH_KEY' ) ? SECURE_AUTH_KEY : $this->_default_secure_auth_key;
  $encryption_key = base64_decode( $key );

	// Grab the $iv from earlier, to decrypt.
	list( $encrypted_data, $iv ) = explode( '::', base64_decode( $code ), 2 );

	return openssl_decrypt( $encrypted_data, 'aes-256-cbc', $encryption_key, OPENSSL_RAW_DATA, $iv );
}