Key Management - Everyday Cryptography - page 348

Tantalisingly, if a way could be found of making key establishment 'easy' then
perhaps a one-time pad could be used in practice. This is one motivation for
quantum key establishment , which is an attempt to make the establishment of
long, shared, randomly generated symmetric keys 'easy'.
Note that quantum key establishment is often inappropriately described as
'quantum cryptography'. The latter name suggests that it is something to do
with new cryptographic algorithms that are suitable for use to protect against
quantum computers (see Section 5.4.4). Quantum key establishment is in fact a
technique for establishing a conventional symmetric key, which can then be used
in any symmetric cryptosystem, including a one-time pad. Of course, it does have
some relevance to quantum computers, since a one-time pad still offers perfect
secrecy if an attacker is in the fortunate position of having a quantum computer,
whereas many modern encryption algorithms would no longer be secure (see
Section 5.4.4). Nonetheless, quantum key establishment is only what it claims to
be, a technique for key establishment.
THE BASIC IDEA
Quantum key establishment takes place over a quantum channel . This is typically
instantiated by an optical fibre network or free space. Alice and Bob must
have devices capable of sending and receiving information that is encoded as
quantum states, often termed qubits , which are the quantum equivalent of bits on
a conventional communication channel. These qubits are represented by photons .
In a conventional communication channel, one simple way of establishing a
symmetric key is for Alice to generate a key and then send it to Bob. The problem
with this approach is that an attacker could be listening in on the communication
and thus learn the key. Even worse, neither Alice nor Bob would be aware that
this has happened.
The basic idea behind quantum key establishment is to take advantage of
the fact that in a quantum channel such an attacker cannot 'listen in' without
changing the information in the channel. This is a very useful property, which
Alice and Bob can exploit to test whether an attacker has been listening to their
communication.
The most well known quantum key establishment protocol is the BB84
protocol . While the following conceptual overview of this protocol is simplified
and omits important background information, it should provide a flavour of the
basic idea. The BB84 protocol involves the following steps:
1. Alice randomly generates a stream of qubits, and sends these as a stream of
polarised photons to Bob.
2. Bob measures them using a polarisation detector , which will return either a 0
or a 1 for each photon.
3. Bob contacts Alice over a conventional authenticated channel (perhaps a secure
email, a telephone call, or a cryptographically authenticated channel), and Alice
then provides him with information that probably results in Bob discarding