Encrypt / Decrypt WP Options on Save (API keys, etc)

Encrypt_Decrypt_WP_Options.php

<?php

//Registers the pre_update_option filter for elements to encrypt. Format pre_update_option_(option name)
//Only do this for ones you want encrypted... in this case, first and third options.
function yourplugin_init() {
	add_filter( 'pre_update_option_your_first_option', 'yourplugin_update_option', 10, 2 );
	add_filter( 'pre_update_option_your_third_option', 'yourplugin_update_option', 10, 2 );
}
add_action( 'init', 'yourplugin_init' );


//Converts option value to encrypted form, only if it has changed
function yourplugin_update_option( $new_value, $old_value ) {

	if ($new_value != $old_value) {
		$cypher = 'aes-256-cbc'; //Pick the cypher to use
		$key = md5(SECURE_AUTH_SALT); //SECURE_AUTH_SALT can be replaced with any unique string.
    $new_value = openssl_encrypt("$new_value","$cypher","$key");
	}
	return $new_value;
}

//A function to retrieve the decrypted value of the options, to use them in your plugin
function yourplugin_decrypt_option($option) {

	 $cypher = 'aes-256-cbc'; //Be sure to use the same cypher as above
	 $key = md5(SECURE_AUTH_SALT); //Be sure to use same $key as above
	 $decrypted_value = openssl_decrypt("$option","$cypher","$key");

	 return $decrypted_value;

}