Magento's implementation of the Authorize.Net Direct Post payment method uses MD5 based hash, and on March 14, 2019 Authorize.net will stop supporting MD5 based hash usage. This will result in Magento merchants not being able to process payments using Authorize.Net Direct Post. To avoid disruption and to continue processing payments, merchants need to apply a patch provided by Magento and add a Signature Key (SHA-512) in the Magento Admin configuration settings. For information on applying this patch please see the Magento Help Center.

https://support.magento.com/hc/en-us/articles/360024368392