QCryptWatch
Student labs

Lab 01 · Cryptography · 35–45 minutes

Quantum Threat Playground

How could a quantum computer threaten encryption? Start with hidden numbers and a repeating pattern, then explore two very different attacks.

Educational models, not a quantum computer or a Q-Day forecast.

Watch first: Why a repeating pattern can break RSA43 sec

Start here · No cryptography background needed

Why would a repeating pattern unlock anything?

The pattern is not an RSA password. It is a mathematical clue that can help uncover two hidden numbers. Here is the connection.

  1. 1. RSA hides two prime numbers behind their product

    A prime number has only two positive divisors: 1 and itself. Multiply the primes 3 and 5 and you get 15. The numbers 3 and 5 are factors of 15 because each divides it with no remainder.

    3 × 5 = 15

    RSA makes the product public but keeps the primes secret. With real RSA, these numbers are enormous. Multiplying them is easy; finding the two primes from only the product is the hard part.

  2. 2. A repeating pattern gives a clue to the factors

    Start at 1. Keep multiplying by 2, but each time keep only the remainder after dividing by 15. For example, 16 divided by 15 leaves a remainder of 1.

    12481

    After four multiplications, we return to 1 and the sequence starts over. The cycle length, also called the period, is 4. A useful period lets us calculate candidates that share factors with 15. You will see those calculations in the experiment.

  3. 3. The factors expose RSA's secret-key mathematics

    Knowing both secret primes lets an attacker derive an RSA private key from the public key. That threatens RSA-encrypted secrets and RSA signatures. This lab shows the factoring step, not a complete decryption attack.

    Shor's algorithm uses a quantum computer to find the period efficiently. The remaining factor calculations are classical. This browser finds the tiny period by ordinary repeated multiplication, not by running Shor on quantum hardware.

Your mission

First, follow the cycle to its return to 1 and connect it to the factors. Then compare that threat with Grover's different approach: speeding up a search for a secret key. Neither toy experiment predicts a Q-Day date.