Sounds like a job for cryptography. For example, you could have each machine sign it's file using a private key. Then the master server can verify the file by checking it with the appropriate public key. Unless the attacker steals the private key from a terminal he can't alter the data.
A non-public-key crypto approach would be to just encrypt the whole file using a shared-secret like a password entered by the operator or stored on disk.
If you're new to cruptography I suggest you pick up a copy of Applied Cryptography by Bruce Schneier. It's a very gentle introduction to the field and will help you make an informed decision about what approach to use.
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