The official NIST publication I saw says for federal applications 3DES with three keys should be used. I have such questions:
what does it mean federal? Does it mean for industrial applications that requirement doesn't hold? What security do I get with 2 keys? by two keys I mean when K1==K3, but K2 is different? Is it average security?
Three key approach with (globally) fixed second key which is still different from each K1 and K3 pair: Imagine I have server which communicates with many clients. Is it ok if with each client i it uses distinct (K1[i],K3[i]) pairs (but K3[i]==K1[i] for each pair), and K2 I take such that it is different from each (K1[i], K3[i]) key pair in my system, but it is fixed (globally) for all clients?
EDIT: The crypto will be done by dongle so that someone will decompile my soft and get 2nd key doesn't hold as mentioned in one of the answers.