Whats the cryptography term for this requirement?

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C#

Hi. I want to generate a unique key viz can be a sequence of only alphabets, or only numbers or both, such that the target application which receives the key can identify and regenerate inspite of the apparent randomness.

The scenario where I "plan" to use it: I want to dedicate certain web methods, services, remote methods, etc to certain applications. Only those applications can access the service. Only they know how to generate the key. No other application can access those services...