Master Hill Cipher Encryption & Decryption: Step-by-Step Guide with Examples 🔐

Algorithm for Hill Cipher
1. Key Preparation:
- Choose a square key matrix (n × n) with non-zero determinant and ensure it's invertible mod 26.

2. Plaintext Processing:
- Convert plaintext into numerical equivalents (A=0, B=1, ..., Z=25).
- Divide plaintext into blocks of size n (matching the matrix size).

3. Encryption:
- Multiply each plaintext block vector with the key matrix.
- Apply modulo 26 to the result to get the ciphertext.

4. Decryption:
- Compute the inverse of the key matrix modulo 26.
- Multiply ciphertext blocks with the inverse key matrix and apply modulo 26.

Advantages of Hill Cipher
1. Security via Mathematics: Uses matrix multiplication and modular arithmetic, making it mathematically robust.
2. Resistance to Frequency Analysis: Unlike substitution ciphers, Hill Cipher encrypts blocks of text, not individual letters, reducing susceptibility to letter frequency analysis.
3. Efficiency: Fast encryption and decryption due to the use of simple mathematical operations.
4. Flexible Block Size: Can work with block sizes greater than two, providing better security for longer plaintexts.

Disadvantages of Hill Cipher
1. Key Sensitivity: Requires an invertible matrix as the key. If the determinant is 0 or not coprime with the modulus, decryption is impossible.
2. Vulnerability to Known-Plaintext Attacks: If enough plaintext-ciphertext pairs are known, the key can be deduced.
3. Computational Complexity: Decryption involves matrix inversion, which can be complex without computational tools.
4. Not Suitable for Modern Applications:Lacks robustness against advanced cryptanalytic methods and large-scale computation.