How does Keybase verify the ownership of the PGP keys?

I am new to keybase and I found the client codebase pretty huge to get the answer of my question, but how does keybase verify that the client-generated PGP keys are actually owned by the client? does the server sends some challenge encrypted by the claimed public key and waits for the unencrypted version or what exactly?

Start with the Keybase client codebase, tracing PGP key generation and the ownership-verification flow described in the question. Document whether verification uses a server challenge or another mechanism, and cite the relevant entry points so the explanation can be checked against the implementation.

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