That's pretty much what I was talking about when I said that it may be
  possible to clone an arbitrarily large proportion of photons - and that
  Quantum Cryptography may not actually be secure.

A key point is the probability that the measurement/cloning operation
has of disturbing the original state.  Errors at the receiver are
assumed to be the result of eavesdropping.  The current canoncial
paper on how to calculate the number of bits that must be hashed away
due to detected eavesdropping and the inferred amount of undetected
eavesdropping is "Defense frontier analysis of quantum cryptographic
systems" by Slutsky et al:

(I don't want to take a position on whether cloning is or isn't
possible - that's way out of my area of expertise!)

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