Read
https://support.magento.com/hc/en-us/articles/360024368392
Issue :
Magento implements the Authorize.Net Direct Post payment method, using Authorize.Net's AIM (Advanced Integration Method) and DPM (Direct Post method) APIs, which use MD5 based hash.
Authorize.net will stop supporting MD5 based hash usage on March 14, 2019. Starting from this date, Magento Open Source, Magento Commerce and Magento Cloud merchants will not be able to process payments using Authorize.Net Direct Post payment method. To be able to continue successfully process payments using these methods, merchants need to apply the patch provided by Magento and replace the existing MD5 hash with a Signature Key in the Magento Admin configuration settings.
Best Answer
Now Apply Patch directly in vendor folder
This patch is applicable on all Magento 2.x and 1.x versions.
Alternatively you can use patched files from following address and replace them in your vendor folder.
https://github.com/MeetanshiInc/Magento-Authorize-Net-Direct-Post-Patch-for-M1-M2
If Need Run
php bin/magento setup:upgrade; php bin/magento setup:static-content:deploy -f;After apply patch you will see Signature Key