X-Cart:Authorize.Net error "MD5 transaction signature is incorrect! (Reason Code 0 / Sub 0)"

Usually this error is caused by incorrect values submitted for Authorize.Net in the X-Cart admin zone and/or in your merchant account back-office.

Check the settings in your X-Cart: "Payment method" page -> scroll down to Authorize.Net and click 'Configure'. Make sure that there are correct values for Login, Transaction key and MD5 hash. For example, it might be that MD5 hash value is wrong or empty.

Login (API Login ID) and Transaction key are recorded in your merchant account back-office on the "Account -> API Login ID and Transaction Key" page.

MD5 hash value is generated in your Authorize.Net account as well:

1. Log in on to the account,
2. Select Settings from the Main Menu
3. Click on MD5 Hash in the Security section,
4. Enter a new value (it is not recommended to use such characters like {, }, *, !),
5. Confirm the value entered,
6. Click Submit.