Added classical implementation of Shor’s Algorithm

[adityakrmishra]

• Implemented a classical simulation of Shor’s Algorithm in algorithms/cryptography/shor_algorithm.py
• The algorithm factors a composite number N without using quantum computing libraries
• Uses modular exponentiation and order-finding to determine factors
• Includes an example usage for testing

Describe your change:

Summary

This PR adds a classical implementation of Shor’s Algorithm to the cryptography module. The algorithm factors a given composite number N by:

• Checking for trivial factors using GCD
• Finding the order of a random integer a modulo N
• Using the order to derive factors of N

Changes

• Added shor_algorithm.py in algorithms/cryptography/
• Implemented modular exponentiation and order-finding methods
• Included an example usage for verification

Issue Reference

Closes #12318

Testing

• Manually tested with N = 15, 21, 35, 55
• Outputs correct factor pairs in all cases

Possible Improvements

• Add unit tests in tests/cryptography/test_shor_algorithm.py
• Optimize order-finding using more efficient methods

Looking forward to feedback! 🚀