We were inspired by the quantum threat and the race to build new forms of encryption. I learned that the simple BB84 protocol isn't enough; the real challenge is defending against practical attacks.
We built the project by first simulating the basic BB84 and E91 protocols in Qiskit. Then, we iteratively layered on real-world features, like asymmetric basis choice for efficiency, real error correction, and most importantly, the Decoy State protocol to defeat the Photon-Number Splitting (PNS) attack. We also simulated a classical Man-in-the-Middle (MITM) attack to show that QKD must be paired with classical authentication to be truly secure.
The biggest challenge was debugging the physics! Our E91 simulation kept giving an S-value of 0 instead of the expected 2.828. Finding that subtle math bug and finally seeing the simulation match the theory was the most rewarding part.
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