INFORMATION GAIN IN QUANTUM EAVESDROPPING

We analyse the information obtained by an eavesdropper during the vari ous stages of a quantum cryptographic protocol associated with key dis tribution. 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 th e key distribution procedure. These limits are restricted to intercept /resend eaves-dropping strategies. The upper one is higher than has be en estimated so far, and should be taken into account in order to guar antee the secrecy of the final key, which is subsequently obtained via the so-called privacy amplification.