Authorize.net doesn't support MD5 hashing at the end of the month
I just saw this on Stack Exchange. It seems rather urgent, since it will be a breaking change for users. I'm guessing that February 1st we'll see a lot of questions about this on Stack Exchange.
Authorize.Net is phasing out the MD5 based transHash element in favor of the SHA-256 based transHashSHA2. The setting in the Merchant Interface which controls the MD5 Hash option will be removed by the end of January 2019, and the transHash element will stop returning values at a later date to be determined.
Merchants utilizing this feature will need to work with their web developer or solutions provider to verify if they are still utilizing MD5 based hash and if still needed to move to SHA-256 hash via Signature Key.