How secure is MD5 for one way authentication?
Every 10 min S1 publish the hash value of PS+the current time which is used as public identifier (PI). Every one can see the value of PI in the internet. As S2 has the value of PS, he also use MD5 to get the hash value of PS+the current time and compare it to the value published by S1 and therefore identify that the value provide from S1.


Essentially, you would be using MD5(M + K) as a MAC. That is insecure.

The construction where you append the key to the message and then hash is not a secure MAC when the hash function lacks collision resistance, like MD5 does. (Technically, you need a collision attack on the compression function.)

Even though using a collision resistant hash (like SHA-256) would likely make what you are doing secure, I would recommend using a real MAC. For example, HMAC, for which you ought to find an implementation in almost any programming environment.