Outline● Quantum Cryptography● Theoretical Background● Quantum Key Distribution (QKD)○ BB84 Protocol● Vulnerabilities & Attacks○ Faked - state attack
Quantum Cryptography- How it came up● Cryptography => Secure Communication=> Secure Data Transmission● Two techniques○ Symmetric - key encryption (shared key)■ Key - distribution problem○ Asymmetric - key encryption (pair of public&secret keys)■ Success based on hardware limitations, absence ofgood algorithms and non-use of quantum computers.Quantum Cryptography!
Quantum Cryptography● Quantum Cryptography is○ the use of laws of quantum physics, to:■ perform cryptographic functionalities■ break cryptographic systems● Examples:○ Quantum Key Distribution (next section)○ Quantum Computers to break existing protocols
Theoretical BackgroundFilter to distinguish polarized photons.Correct FilterappliedWrong Filterapplied
Quantum Key Distribution - BB84● First quantum cryptography protocol● Goal: describe a scheme of two users who want tocommunicate and exchange data securely.● Idea: distribute a key securely, based on the laws ofphysics.● Security proofs:○ If someone reads the state of photon -> state changes○ Not possible to copy the photon in order to encode it withall possible ways (basis)
Quantum Key Distribution - BB84Step 1● Alice has two choices, key (a) & basis (b), chosenrandomly● Combine bits of a and b, 1-1,● Four different states of qubit (photon polarization)● Sent through public quantum channels:○ Optical Fiber○ Free SpacePhoton Source
Quantum Key Distribution - BB84Step 2● Bob receives qubit from Alice● Bob measures it by choosing random basis usingBeam Splitter (BS), practically it could be 50/50 mirror● PBS sends qubit to certain detector using some rules
Quantum Key Distribution - BB84Step 2How PBS of a specific basis works● Let photon that polarized on that basis to pass through tothe correct detector● Otherwise, the photon can head randomly to any of thewrong detectors
Quantum Key Distribution - BB84Step 2Example of how PBS combining with detector works!
Quantum Key Distribution - BB84Step 3● 1st communication between Alice and Bob in publicchannel● They compare the basis used to encode and measurethe qubit● If Bob.basis == Alice.basis○ Keep the bit!● Else○ Discard the bit● The length of the initial key is reduced to half of its lengthbecause the probability of Bob choosing the same basisas Alice is 50%
Quantum Key Distribution - BB84Step 4● Check if someone has intruded the communication or ifsome imperfection of the devices or channel hasintroduced noise that distort the outcome● If Eve has intruded the communication, she willDEFINITELY left some traces due to HeisenbergUncertainty Principle (HUP) and non cloning theorem
Quantum Key Distribution - BB84Step 4● Alice and Bob performs MANY parity-checks● In this way, they can find out whether Eve has intrudedthe communication● Very simple example:○ Calculate parity of blocks of 4-bits● Alice sends the parities of her blocks and Bob checks them
Quantum Key Distribution - BB84Step 5● Now Alice and Bob have the same keys, all the bits aresame● The problem is, in Step 4, Eve manages to find out someportions of their key● Privacy Amplification comes into the rescue!
Quantum Key Distribution - BB84Step 5● Alice and Bob apply Hash function to compress the keyinto the final one. And they should use the same Hashfunction.
Vulnerabilities - Photon number attack● Sending more than one photon for each bit leads to photonnumber attack.○ Eve can steal extra photons to extract thestolen photons information.● Ensure photon spitter only sends exactly ONE photon eachtime.● Single photon ensures quantum mechanic laws aresatisfied.
Vulnerabilities - Spectral attack● If photons are created by four DIFFERENT laser photodiodes, they have different spectral characteristics.● Eve performs spectral attack by measuring COLOR, and notpolarization.
Vulnerabilities - Random numbers● Are our random numbers really "Random"?● Bob side, randomness is determined by BS.● Alice side, randomness if a bit stream cannot be provenmathematically○ Algorithms generate "random" sequences by followingspecific patterns => NOT that random!○ Eve can use same algorithm to extract information.Entangled Photon Pairs comes to the rescue!
Faked-state attackPractical Implementation - Detector replica● Eve has replica of Bobs detector● To capture the photon and measure it like Bob always does
Faked-state attackPractical Implementation - Fake Stated Generator● Blind Bobs detector○ Insensitive to photon● Forces Bobs detectors to have same "click" as what Evehas measured○ Bob and Eve have same information
Faked-state attackPractical Implementation - Blind all Bobs detectors● QKD detectors use Single Photon Avalanche Diode (SPAD)
Faked-state attackSingle Photo Avalanche Diode● Has two modes○ Geiger Mode○ Linear ModeHence, SPAD in Linear Mode can be considered asblind-to-photon.
Faked-state attackSingle Photo Avalanche Diode● How to make SPAD behaves in Linear Mode?
Faked-state attackSingle Photo Avalanche Diode● SPAD in Linear Mode● Bright illumination causes the capacitor has not enough timeto recharge and re-balance the voltage value at point 2● SPADs bias voltage below VBreakdown -> Linear Mode
Faked-state attackSingle Photo Avalanche Diode● SPAD in Linear Mode●
Faked-state attackPractical Implementation - Force Bobs detector to click● Blinding Bobs detector is not enough● Eve needs to force specific Bobs detector to "click"according to the measurement result in Eves detector
Faked-state attackPractical Implementation - Force Bobs detector to click● SPAD in linear mode ("blind SPAD) -> easily forced tocreate a "click"● Sending pulse of light with intensity power "I0"
Faked-state attackPractical Implementation - Blind the detector● Correct light pulse intensity is important● (2*I0) is the answer!