Cryptography Reference
In-Depth Information
3. Bob tells Alice his settings over a non-tamperproof channel (but the
message must not be vulnerable to manipulation). With this, Bob doesn't
give away anything to an eavesdropper: the sentence 'I used a filter set
horizontally in the 5th pulse' does not reveal the polarization he actually
measured in the 5th pulse.
4. Alice tells Bob the numbers of the correct settings, for example: 'You
measured the 1st, 3rd, 4th, and 5th pulses correctly'. Again, this won't
allow the eavesdropper to understand
what
Bob actually measured. The
measuring results for these pulses are pieces of
information
that Alice
transmitted securely
to Bob. 50 % of the settings should be correct on
average. The intruder eavesdropping on the line would necessarily falsify
the information. Alice and Bob could find this out, for example, by
comparing hash values. They would also be able to notice forgeries as
they exchange filter settings.
The last point shows us that quantum cryptography does not provide for a secure
data channel, but it does allow the conversers to detect whether or not somebody
listened in on them. For this reason, quantum cryptography is meaningful only
for transmitting a random key (which does not contain important information),
but less suitable for transmitting sensitive information.
The protocol can be expanded such that information can be transmitted securely
despite a (necessarily active) eavesdropper. The only problem could arise in
steps 3 and 4: man-in-the-middle attacks where an intruder pretends to both
conversers to be the other converser must be prevented in these steps. Asym-
metric methods can be used to reliably exclude such attacks. Notice that there
is a difference to using asymmetric encryption in hybrid methods, where crack-
ing would reveal the private key, and consequently all session keys and with
them all secret messages. Conversely, in quantum cryptography, only the fil-
ter settings would be compromised, which are not secret anyway. For this
reason, it is actually sufficient to exchange digitally signed messages about
the filter settings. Mallory, in the middle of the line, would have to forge
these messages as fast as Alice and Bob exchange them. To this end, Mal-
lory would have to know their secret keys in advance, which can normally be
ruled out.
Quantum cryptography is theoretically a heady approach. There is finally a
provable
security rather than the usual assumptions and speculation.
The fact that the entire thing works in practice is still cooler. Employees of
British Telecom managed to transmit information over a fiberglass line 10 km
