How do you develop multiple features that rely on one another at the same time?

Suppose there are 3 new features X, Y and Z that need to be developed over the course of the next 2 months. Feature A relies on feature B. Feature B relies on feature C, but feature C also relies on feature A. So we have a situation where we have A -> B -> C -> A and the cycle can't be broken by simply doing B -> C -> A. How do you coordinate this work? <br><br>For example, suppose we are implementing a new form of digital signature. <br><br>Feature A is the digital signature itself. Feature B is the server which verifies that the signature is stored with the proper user, and feature C is the user interface which presents the signature to the user when they make the digital signature request, so that they can copy it and use it. <br><br>In order to develop A, we would need to make the server that will verify that the signature is stored with the proper user (B), but that server relies on the client side software that lets the user login (C), and in order to develop (C), we need to test the process of sending the signature to the server, but we can't test that until it's developed (B). <br><br>Obviously this is a very simplified example, but I'm sure it's a very common problem in development and I'm not sure how it's approached.