encrypt_decrypt.php

<?php // from https://web.archive.org/web/20161021193734/https://andrewho.nl/encrypt-decrypt-strings-php-easy-way/
function dec_enc($action, $string) {
    $output = false;
 
    $encrypt_method = "AES-256-CBC";
    $secret_key = 'This is my secret key';
    $secret_iv = 'This is my secret iv';
 
    // hash
    $key = hash('sha256', $secret_key);
    
    // iv - encrypt method AES-256-CBC expects 16 bytes - else you will get a warning
    $iv = substr(hash('sha256', $secret_iv), 0, 16);
 
    if( $action == 'encrypt' ) {
        $output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
        $output = base64_encode($output);
    }
    else if( $action == 'decrypt' ){
        $output = openssl_decrypt(base64_decode($string), $encrypt_method, $key, 0, $iv);
    }
 
    return $output;
}

How to Encrypt and Decrypt Advanced Custom Fields

Informationfrom https://support.advancedcustomfields.com/forums/topic/encrypting-field-for-ssn/

We would use 2 filters
decrypt with acf/load_value/name={$field_name}
and encrypt with acf/update_value/name={$field_name}