Theory Question: Why isn't RSA a 0-knowledge Proof

[Joseph M. Reagle Jr.]

Simple question, why isn't the hard problem of proving that I know a secret
key (d) for a given (e,n) (public key and modulus) a zero-knowledge proof?
Is some amount of information leaked during challanges?
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Regards,               Is this true or only clever? -Augustine Birrell
Joseph Reagle          http://farnsworth.mit.edu/~reagle/home.html
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