BB84 Quantum Key Distribution : Fundamentals

April 2, 2026

In an increasingly digital world where sensitive information is exchanged at lightining speed, the need for secure communcation methods have never been more critical. Traditional encryption methods rely on mathematical complexity to ensure security, but they are vulnerable to advances in computing power and algorithmic improvements. Quantum cryptography, on the other hand, leverages the principles of quantum mechanics to provide a fundamentally secure way to distribute cryptographic keys. The BB84 protocol, proposed by Charles Bennett and Gilles Brassard in 1984, was the first practical implementation of quantum key distribution and laid the groundwork for modern quantum communication systems.

Understanding Quantum Key distribution

What is QKD ?

Quantum Key Distribution (QKD) is a secure communication method that enables two parties, commonly referred to as Alice and Bob, to generate a shared, random secret key. but what is a key ? and where it is used ?

Let go back to the fundamentals, and understand what is an information

As its core, information is the reduction of uncertainty

If you already know the outcome of an event, receiving the message changes nothing. BUT if the outcome is unknown, the message reduces the set of possibilities. That reduction is information.

This idea was formalized in Information Theory by Claude Shannon. In this framework, information is not about meaning but about probability. A message that could have been many different things carries more information than one that was almost certain.

For example:

• A coin toss has 1 bit of information (means he store two possible outcomes 1 or 0)
• A number chosen between 1 and 8 carries 3 bits (log₂(8) = 3)
• A long message is simply a sequence of bits reducing uncertainty step by step.

In digital systems, information is encoded as binary symbols, called bits, taking values 0 or 1. Everything from text to images to cryptographic keys is ultimately represented as sequences of these symbols.

Now that we understood a little bit what is information, let’s see what is a key.

A key, or to be exact a cryptography key is a piece of secret information shared between communicating parties.

If two people want to communicate privately, the must share something that an adversary doesn’t know. This shared secret is the key.

The security of many cryptographic systems depends not on hiding the algorithm but on protecting the key. This principle is known as Kerckhoffs’s principle: the system should remain secure even if everything except the key is public.

In practice, a key is simply a random sequence of bits.

Key: 1001010011100101....

The randomness is essential. If an attacker can predict the key, the encryption collapses.

Yeah I know, this is a lot of information, and still we didn’t saw any quantum here. But before moving to the quantum, we still need to see a last thing. the core of this article : What is encryption ?

Encryption is the transformation of information into unintelligible form using a key. Too much of complicated world, but the idea is very simple.

CipherText = Encrypt(Message, Key)

A CipherText is simple the message after being encrypted by a key. Only someone who knows the correct key can reverse the transformation :

Message: Decrypt(CipherText, Key)

This principle appears in the famous One-Time Pad, the only encryption method proven to be perfectly secure when used correctly. The concept was studied by Claude Shannon, who showed that if the key is truly random, as long as the message, and used only once, the ciphertext reveals zero information about the original message.

The problem, however, is not encryption itself.

The real challenge is key distribution.

How can two parties share a perfectly random secret key if an eavesdropper might intercept it?

THIS is precisely the problem that Quantum Key Distribution.

If Alice sends the key to Bob through a classical communication channel, an eavesdropper, called Eve, could simply intercept the transmission and copy it without leaving an trace. Classical information can always be perfectly duplicated, making undetectable interception possible.

This is where quantum physics changes the rules hehehe

Quantum Key Distribution relies on the laws of Quantum Mechanics to create a communication process where any attempt to observe the transmitted information inevitably disturbs it.

This fundamental property allows Alice and Bob not only to exchange information, but also to detect whether someone has been listening.

To understand how this works, we first need to look at how information can be encoded in quantum systems.