Starting 2019/03/14, the ValidatePayment method of AuthorizeNetPaymentHandler will start failing because Authorize.Net is removing the MD5 Hash element from the SIM response.
Furthermore, as per the Upgrade Guide, the entire SIM method of API access is "deprecated". The suggested replacement is "Accept Hosted".
Related Authorize.Net support article: MD5 Hash End-of-Life & Signature Key Replacement
Starting 2019/03/14, the ValidatePayment method of AuthorizeNetPaymentHandler will start failing because Authorize.Net is removing the MD5 Hash element from the SIM response.
Furthermore, as per the Upgrade Guide, the entire SIM method of API access is "deprecated". The suggested replacement is "Accept Hosted".