Conceptual Quantum Key Distribution (QKD) Demonstration

This repository contains a simple, interactive web-based demonstration of a core principle behind Quantum Key Distribution (QKD), specifically the "measurement problem" as applied in protocols like BB84. The demo illustrates how the act of observing a quantum state inherently disturbs it, a property that can be leveraged to detect eavesdropping during cryptographic key exchange.

Disclaimer: This is a highly simplified conceptual model for educational purposes only. It is NOT a secure cryptographic implementation and should NEVER be used for real-world security applications.

Table of Contents

• Core Concepts Represented

• How the Demo Works (Step-by-Step Logic)

  • 1. Alice Generates Bits & Bases

  • 2. Eve Intercepts (Optional)

  • 3. Bob Measures Photons

  • 4. Key Reconciliation & Eavesdropping Detection

• How to Use the Demo

• Technical Details

Core Concepts Represented

• Bits (0s and 1s): The secret information Alice aims to share securely with Bob.

• Bases (Rectilinear '+' or Diagonal 'x'): These represent conceptual polarization filters used to encode and measure the "photons."

  • Rectilinear (0): Measures horizontal (H) or vertical (V) polarization.

  • Diagonal (1): Measures diagonal (D1 or D2) polarization.

• Measurement Disturbance: A fundamental quantum mechanics principle where observing a quantum state (like a photon's polarization) changes it, especially if measured in a mismatched basis.

How the Demo Works (Step-by-Step Logic)

The JavaScript code simulates the interactions between Alice, Bob, and an optional eavesdropper (Eve) to demonstrate QKD.

Global Variables and DOM Elements

The script begins by initializing global JavaScript arrays (e.g., aliceBits, aliceBases, bobMeasuredBits) to maintain the state of the simulation. It also retrieves references to various HTML elements (buttons, input fields, and display areas) to interact with the user interface.

Helper Functions

• updateOutput(element, data): A utility function to update the text content of the display boxes in the HTML.

• showMessage(message, type): A helper function to display status or error messages to the user within the resultMessageDiv, applying different styling based on the message type (success, warning, error).

1. Alice Generates Bits & Bases (generateAliceKeysBtn event listener)

• Action: When the "1. Alice Generates Bits & Bases" button is clicked.

• Process:

  • The keyLength is read from the input field (validated to be between 4 and 64 bits).

  • All simulation state arrays are reset.

  • Alice generates a sequence of random keyLength bits (0 or 1) and stores them in aliceBits.

  • For each bit, she also randomly selects a "polarization basis" (0 for Rectilinear, 1 for Diagonal) and stores it in aliceBases.

  • Display: The alicePhotonsDiv shows a conceptual representation of Alice's "sent photons" (e.g., 'H(0)', 'V(1)', 'D1(0)', 'D2(1)' based on bit and basis), and aliceBasesDiv shows her chosen bases ('Rect' or 'Diag').

• Outcome: A success message confirms Alice has generated her data and conceptually sent the photons.

2. Eve Intercepts (if enabled) (eveInterceptBtn event listener)

• Action: When the "2. Eve Intercepts (if enabled)" button is clicked.

• Process:

  • The eavesdropChance (percentage) is read from the input field.

  • The code iterates through each "photon" Alice sent.

  • Eavesdropping Logic: Based on the eavesdropChance, Eve decides whether to intercept a photon:

    • If Eve intercepts:

      • Eve randomly chooses a basis to measure the photon.

      • If Eve's chosen basis matches Alice's original basis, Eve measures the correct bit.

      • Crucially: If Eve's chosen basis does not match Alice's original basis, Eve's measurement yields a random bit (50% chance of being incorrect). More importantly, this incorrect measurement modifies the aliceBits[i] value, simulating the quantum disturbance.

      • Eve's measured bit and chosen basis are recorded in eveMeasuredBits and eveBases.

    • If Eve does NOT intercept: The photon passes through undisturbed, and 'N/A' is recorded for Eve's measurements.

• Outcome: The eveMeasuredDiv and eveBasesDiv are updated. A message indicates how many photons Eve intercepted and if any disturbance occurred.

3. Bob Measures Photons (bobMeasureBtn event listener)

• Action: When the "3. Bob Measures Photons" button is clicked.

• Process:

  • Bob iterates through the photons (which might have been altered by Eve).

  • For each photon, Bob randomly chooses a basis to measure it (bobBases).

  • If Bob's chosen basis matches Alice's original sending basis (aliceBases[i]), he measures the correct bit (or the bit that Eve might have altered).

  • If Bob's chosen basis does not match Alice's original sending basis, his measurement yields a random bit (50% chance of being incorrect).

  • Bob's measured bits and chosen bases are recorded.

• Outcome: The bobMeasuredDiv and bobBasesDiv are updated, and a success message is displayed.

4. Key Reconciliation & Eavesdropping Detection (reconcileKeysBtn event listener)

• Action: When the "4. Key Reconciliation & Eavesdropping" button is clicked.

• Process:

  • Alice and Bob (conceptually) publicly compare their chosen bases (aliceBases and bobBases).

  • They only keep the bits where their bases matched. These form rawAliceKey and rawBobKey.

  • They then compare these rawAliceKey and rawBobKey bit by bit to count errorsDetected.

  • Eavesdropping Detection:

    • If errorsDetected is 0, all bits where their bases matched are identical. This implies no significant disturbance, and they successfully establish a finalSharedKey.

    • If errorsDetected is greater than 0, it means discrepancies exist. This is the tell-tale sign that Eve (or significant channel noise) intercepted and disturbed the photons. In a real QKD protocol, they would immediately discard this key and attempt to establish a new one.

• Outcome: The sharedKeyDiv is updated with the final key (or empty if discarded), and a message indicates whether the key was established or discarded due to detected eavesdropping.

How to Use the Demo

1. Open the HTML file: Save the provided HTML code as an .html file (e.g., qkd_demo.html) and open it in a web browser.

2. Adjust Settings:

   • Key Length (bits): Choose how many "photons" Alice will send (between 4 and 64).

   • Eve's Eavesdrop Chance (%): Set the probability (0-100) that Eve will attempt to intercept each photon.

3. Follow the Steps:

   • Click "1. Alice Generates Bits & Bases" to start the process.

   • Click "2. Eve Intercepts (if enabled)" to simulate Eve's actions.

   • Click "3. Bob Measures Photons" to simulate Bob's measurements.

   • Click "4. Reconcile Keys & Check for Eavesdropping" to see if a shared key was established and if eavesdropping was detected.

4. Observe: Pay attention to the output boxes and the final message to understand how Eve's interference (when her chance is > 0 and she chooses a different basis) leads to detectable errors.

Technical Details

• HTML5: Provides the structure of the web page.

• Tailwind CSS: Used for responsive and modern styling.

• JavaScript: Implements the core logic of the QKD simulation and handles user interactions.

• Simulated Randomness: Math.random() is used to simulate the quantum randomness in bit generation, basis choices, and Eve's interception.