Information Gain in Quantum Eavesdropping

We analyse the information obtained by an eavesdropper during the various stages of a quantum cryptographic protocol associated with key distribution. We provide both an upper and a lower limit on the amount of information that may have leaked to the eavesdropper at the end of the key distribution procedure. These limits are restricted to intercept/resend eaves-dropping strategies. The upper one is higher than has been estimated so far, and should be taken into account in order to guarantee the secrecy of the final key, which is subsequently obtained via the so-called privacy amplification.