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3.3. DETAILED BB84 PROTOCOL
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Eve receives the signal, from Quantum Channel, of the bits written in
Quantum buffer and must at least acknowledge them before Bob is allowed to
view them. This is to resolve the problem of synchronism in reality. Once Alice
has transmitted all of the bits in her message, she waits for Bob’s response.
When receiving a qubit, Eve chooses which attacks to be taken to read its
info or let it proceed to Bob. She has different choices: beam-splitting, intercept/resend, both attacks or nothing. Eve can apply some attacks separately
on the same location in quantum channel buffer. If she attempts to split the
photon, she must specify both its position, and the capacity of her splitting
mirror. This strength of 0 means that Eve ignores this photon and pass it to
Bob while the value of 1 shows that Eve detects all the photon in the pulse and
consequently, this pulse is entirely changed. In the case of a success of Eve
beam-splitting, Eve knows the Alice’s polarization which is saved in the buffer.
Eve’s method BeamSplitting() on the quantum channel
BeamSplitting(float MirrorStrength, int BufferLocation)
She can also use intercept/resend attack independently. She can do this
with command IR in provide the position of pulse, the polarization to measure
photon and also value for resend strength. And the rest of the work is for the
quantum channel object. It is responsible for checking the polarization chosen
by Eve, to update the buffer location and to return result of the measure to
Eve.
Eve’s method IR() on the quantum channel:
IR(
int Polarization,
float ResendIntensity,
int BufferLocation)
Once Eve finishes her job, she must acknowledge the value and shift the
flag of Bob’s permission. This is accomplished by below method.
Eve’s method tag() on the quantum channel:
tag(int BufferLocation)
After shifted bit of Bob’s permission, he can read info of this photon with
the same method as Eve’s IR except without parameter ResendIntensity.
This method will return to Bob the value of this photon or an empty message.
Bob’s read() includes the effects of noise in channel, Bob’s detector efficiency
and dark counts as well. All these are only applied to Bob whose technology is
realistic while Eve is assumed to have a perfect technology to detect pulses.
When all bits received, Bob steps in phase Bases Announcement, sends ReceivingBase command to Alice with his sequence of pairs containing position of
qubit and its guessed polarization. Bob can send to Alice a message like this:
ChosenBases:pos1:polar1:pos2:polar2...
Then, the diagonal and rectilinear polarizations are represented by digits 1s
or 0s. When this message reaches Alice, she prepares a BaseConfirm message
to reply to Bob in the same form of his ChosenBases except her parameters
c Eurocontrol, June 2004.
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