The PROPID_M_HASH_ALG property identifies the hashing algorithm used when authenticating messages.
The MSMQ run-time DLL on the source computer uses the hashing algorithm when creating a digital signature for a message. The target Queue Manger then uses the same hashing algorithm to authenticate the message when it is received.
When passing PROPID_M_HASH_ALG to MQReceiveMessage, the corresponding VT field in the aPropVar array can be set to VT_NULL.
For information on what MSMQ does to authenticate messages, see How MSMQ Authenticates Messages.