Quantum Cryptography
Here is an explanation of a quantum cryptography. Quantum cryptography is exploiting quantum mechanical properties to perform a cryptographic task.
The advantage of quantum cryptographic is it’s impossible to copy data encoded in a quantum state, one attempt to read encoded data, the quantum state will be changed. It’s detect eavesdropping in quantum key distribution.
In this series, we will focus on the BB84 protocol. Let’s start to understand the BB84 protocol step by step.
History
Brief history of BB84 protocol, a cryptography protocol developed in 1984 by IBMers,Charles Bennet with his friend Gilles Brassard.This first demonstration realized five years later of quantum key distribution by Bennett and colleague John Smolin at IBM [C. H. Bennett, F. Bessette, G. Brassard, L. Salvail, and J. Smolin, J. of Cryptography 5, 3 (1992) ]. Both Charles and John are still members of the IBM Quantum team.
Understanding BB84 protocol
Here you can see 4 photons which a horizontal and diagonal sign. Each photon a measure by 0 and 1 by observing a horizontal and diagonal state make sure this is a base of BB84 protocol to be remembered.
The goal of the BB84 protocol is to make a secret key between two users. Mistry and Musk, So both parties can use to encrypt and decrypt the message by key. Mistry chooses a random bit K and B, each consists of N bits. His bit string K is an original which he want to encode and B is a base that should be used to encode the original bit.For 𝑏𝑖=0 (i.e., if the 𝑖𝑡ℎ bit is zero),he encodes the 𝑖𝑡ℎ qubit in the standard {|0⟩,|1⟩} basis, while for 𝑏𝑖=1, heencodes it in the {|+⟩,|−⟩} basis, where |+⟩:=1/√2(|0⟩+|1⟩), |−⟩:=1/√2(|0⟩−|1⟩).If you can’t understand Let I make it simple by below presentation.
After Encoding all n qubit like above, Mistry sends these to Musk. Musk also chooses a random bit string B` of n bit it’s for determining a right base to going perform a measurement. Result of outcomes of measurement K`i with a basis bit B`i in a table.
Next, Mistry and Musk compare their basis bit Bi and B`.Musk measure different basis than Mistry’s qubit was encoded in. so he gets each outcome with probability 1/2. if Bi= B`i then Musk will get the key bit that Mistry encoded, K`i=Ki . These outcomes then compose the key.
Let’s take an example to get it clear about Quantum Cryptography.
Suppose Mistry’s random bit strings are K=’0111001' and B=’ 1101000' and Musk’s random bit string is B`=’1001101’. Try to understand the other entries in the below table. Note that in the case where the basis bits are different, Musk has a 50% chance to get an outcome, so here one of them was chosen randomly.
In the above example, a key is 0110 which must be secret and correct if someone remeasures a qubit on that way state could be changed and the probability of 1/4. Mistry and Musks bit will be different. By checking M bits, the probability to not notice an eavesdropper decrease as (3/4)M. Thus, if they check enough bits and they are all the same, they can assume that no one eavesdropped and their key is secret. However, to keep things simple.
Security Of BB84
I’ll show you a BB84 is high security as compared to normal classical encryption. Here a piece of information from the sender and send it to receive. In the quantum cryptosystem, it’s hard to get the right message. Receiving base one supply is chance to 50% either its horizontal bases or a diagonal bases. Other side lets see diagonal bases 45 degrees or 135 degrees of photon it has 50% either its 45 degrees polarized photon or 135 degrees polarized photon so its a 25% Therefor 75% chance to get current information.
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