Source code for https://victorfont.com/encrypt-decrypt-formidable-form-fields/

add_filter.php

<?php

add_filter( 'frm_include_credit_card', '__return_true' );

custom_functions.php

<?php

function vmf_encrypt( $data ) {

    $key = FORMIDABLE_SALT;
    $method = FORMIDABLE_METHOD;
    $options = FORMIDABLE_OPTIONS;
    $iv = FORMIDABLE_IV;

    return openssl_encrypt ( $data, $method, $key, $options, $iv);
}

function vmf_decrypt( $encrypted ) {

    $key = FORMIDABLE_SALT;
    $method = FORMIDABLE_METHOD;
    $options = FORMIDABLE_OPTIONS;
    $iv = FORMIDABLE_IV;

    return openssl_decrypt ( $encrypted, $method, $key, $options, $iv);
}

define_constants.php

<?php

define('FORMIDABLE_SALT', 'Si-+7^9Xtv5.vb`<T~F)wG?~,v76T [h%7N,h3/4=QVrW~&,QV|uXu63a(K$e.G^');
define('FORMIDABLE_METHOD', 'AES-256-CBC');
define('FORMIDABLE_OPTIONS', 0);
define('FORMIDABLE_IV', ';8qRaMk*jhFDYy3p');

email_decrypt.php

<?php

add_filter('frmpro_fields_replace_shortcodes', 'decrypt_my_field_for_view', 10, 4);
function decrypt_my_field_for_view($replace_with, $tag, $atts, $field){

    if(isset($atts['decrypt'])){
        $encrypted = $replace_with;
        $replace_with = vmf_decrypt($encrypted);
    }
    return $replace_with;
}

formidable_decryption.php

<?php

add_filter('frm_setup_edit_fields_vars', 'decrypt_my_field', 20, 3);
function decrypt_my_field($values, $field, $args){

    if($field->id == 125){//Replace with the ID of your field
        $encrypted = $values['value'];
        $values['value'] = vmf_decrypt($encrypted);
    }
    return $values;
}

formidable_encryption.php

<?php

add_filter('frm_pre_create_entry', 'encrypt_my_field');
add_filter('frm_pre_update_entry', 'encrypt_my_field');
function encrypt_my_field($values){
  if ( $values['form_id'] == 5 ) { //change 5 to your form id
    $current_value = $values['item_meta'][25]; // change 25 to to id of your field
    $encrypted_value = vmf_encrypt( $current_value ); 
    // store the encrypted value to the DB
    $values['item_meta'][25] = $encrypted_value;
  }
  return $values;
}

open_ssl_syntax.php

<?php

openssl_encrypt ( string $data , string $method , string $key [, int $options = 0 [, string $iv = "" [, string &$tag = NULL [, string $aad = "" [, int $tag_length = 16 ]]]]] )
openssl_decrypt ( string $data , string $method , string $key [, int $options = 0 [, string $iv = "" [, string $tag = "" [, string $aad = "" ]]]] )