How exactly do Genie Intellicode, and other "Rolling Code" garage door openers achieve their security?

A lengthy Google search didn't turn anything up, which in and of itself gives me cause for concern. It is my understanding that any security system that relies on secrecy of the algorithm itself is immediately suspect.

The main thing that prompts this question is the process for having a garage door opener receiver "learn" about a new transmitter. This involves nothing more or less than putting the receiver in a mode where it listens for the transmitter, and then having the transmitter send two successive codes.

If the receiver can learn the code sequence from two adjacent codes, what is to prevent an attacker eavesdropping on me opening my garage door on two successive occasions, and applying the same algorithm as the receiver?

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