What's the word or phrase that describes the conversation/exchange that spies use to identify each other?

Question by Maureen: What's the word or phrase that describes the conversation/exchange that spies use to identify each other?
When two spies meet & follow a little script (i.e. 'The crow flies at midnight') to verify that they are speaking to the right person - what is that exchange of words & phrases called? Is there a word for that?

The first thing that comes to mind for me is 'code word', but since this is more than just one word, I'm wondering if the exchange has a name that

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