diff --git a/ciphers/hill_cipher_2.cpp b/ciphers/hill_cipher_2.cpp
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+++ b/ciphers/hill_cipher_2.cpp
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+/**
+ * @file
+ * @brief [Hill cipher](https://en.wikipedia.org/wiki/Hill_cipher) encryption
+ * algorithm implementation.
+ * @details
+ * This implementation uses modular matrix multiplication to encrypt plaintext
+ * using a square key matrix. It includes data cleaning, key matrix usage,
+ * encryption block processing, and formatted output. Only lowercase
+ * alphabetical characters are supported. Non-letter characters are removed and
+ * padding with 'x' is used to fit the block size.
+ *
+ * @author
+ * Mohamed Khaled (https://github.com/mmohamedkhaled)
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <cassert>  /// for assert
+#include <cctype>
+#include <iostream>
+#include <string>
+#include <vector>
+#include <string>
+using namespace std;
+/**
+ * @namespace ciphers
+ * @brief Contains all cipher algorithm implementations.
+ */
+namespace ciphers {
+
+/**
+ * @brief Converts a lowercase character to its corresponding integer (a=0, ...,
+ * z=25).
+ * @param c Lowercase character.
+ * @returns Integer value of the character.
+ */
+int convert_char_to_int(char c) {
+    if (c >= 'a' && c <= 'z')
+        return c - 'a';
+    return -1;
+}
+
+/**
+ * @brief Converts an integer to its corresponding lowercase character.
+ * @param i Integer value (0-25).
+ * @returns Corresponding lowercase character.
+ */
+char convert_int_to_char(int i) { return 'a' + (i % 26); }
+
+/**
+ * @brief Cleans a string by removing non-letter characters and padding with
+ * 'x'.
+ * @param s Input string.
+ * @param matrix_dimension Key matrix size used for padding.
+ * @returns Cleaned and padded string.
+ */
+string clean_and_pad(string s, int matrix_dimension) {
+    string result = "";
+    for (int i = 0; i < s.size(); i++) {
+        if (isalpha(s[i])) {
+            result += tolower(s[i]);
+        }
+    }
+    while (result.size() % matrix_dimension != 0) {
+        result += 'x';
+    }
+    return result;
+}
+
+/**
+ * @brief Multiplies two matrices.
+ * @param a First matrix.
+ * @param b Second matrix.
+ * @returns Resultant matrix after multiplication.
+ */
+vector<vector<int>> matrix_multiplication(vector<vector<int>> a,
+                                          vector<vector<int>> b) {
+    vector<vector<int>> result(a.size(), vector<int>(b[0].size(), 0));
+
+    if (a[0].size() != b.size()) {
+        cout << "Invalid matrices" << endl;
+        return {};
+    }
+
+    for (int i = 0; i < a.size(); i++) {
+        for (int j = 0; j < b[0].size(); j++) {
+            for (int k = 0; k < b.size(); k++) {
+                result[i][j] += a[i][k] * b[k][j];
+            }
+        }
+    }
+
+    return result;
+}
+
+/**
+ * @brief Converts a string to a vector of integers.
+ * @param s Input string.
+ * @returns Vector of integers representing characters.
+ */
+vector<int> convert_string_to_vector(string s) {
+    vector<int> result(s.length(), 0);
+    for (int i = 0; i < s.size(); i++) {
+        result[i] = convert_char_to_int(s[i]);
+    }
+    return result;
+}
+
+/**
+ * @brief Converts a flat vector into a square matrix.
+ * @param v Input vector.
+ * @param size Dimension of the square matrix.
+ * @returns 2D square matrix.
+ */
+vector<vector<int>> convert_vector_to_matrix(const vector<int> &v, int size) {
+    if (size < 2 || size > 9) {
+        cout << "Invalid size" << endl;
+    }
+
+    vector<vector<int>> matrix(size, vector<int>(size, 0));
+    int k = 0;
+
+    for (int i = 0; i < size; i++) {
+        for (int j = 0; j < size; j++) {
+            if (k < v.size()) {
+                matrix[i][j] = v[k++];
+            }
+        }
+    }
+
+    return matrix;
+}
+
+/**
+ * @brief Prints a key matrix to the console.
+ * @param matrix Key matrix.
+ */
+void print_key_matrix(vector<vector<int>> matrix) {
+    cout << "\nKey matrix:\n";
+    for (int i = 0; i < matrix.size(); i++) {
+        for (int j = 0; j < matrix[0].size(); j++) {
+            printf("%4d", matrix[i][j]);
+        }
+        cout << endl;
+    }
+}
+
+/**
+ * @brief Prints ciphertext as characters.
+ * @param ciphertext Encrypted vector of integers.
+ */
+void print_ciphertext(const vector<int> &ciphertext) {
+    cout << "\nCiphertext:\n";
+    for (int i = 0; i < ciphertext.size(); i++) {
+        cout << convert_int_to_char(ciphertext[i]);
+        if ((i + 1) % 80 == 0)
+            cout << endl;
+    }
+    cout << endl;
+}
+
+/**
+ * @brief Prints plaintext as characters.
+ * @param plaintext Plaintext vector of integers.
+ */
+void print_plain_text(const vector<int> &plaintext) {
+    cout << "\nPlaintext:\n";
+    for (int i = 0; i < plaintext.size(); i++) {
+        cout << convert_int_to_char(plaintext[i]);
+        if ((i + 1) % 80 == 0)
+            cout << endl;
+    }
+    cout << endl;
+}
+
+/**
+ * @brief Splits input vector into a list of column matrices of given size.
+ * @param input_vector Input 1D vector.
+ * @param size Matrix size.
+ * @returns Vector of column matrices.
+ */
+vector<vector<vector<int>>> split_into_column_matrices(
+    const vector<int> &input_vector, int size) {
+    vector<vector<vector<int>>> matrices;
+    int num_blocks = input_vector.size() / size;
+
+    for (int i = 0; i < num_blocks; i++) {
+        vector<vector<int>> col_matrix(size, vector<int>(1, 0));
+        for (int j = 0; j < size; j++) {
+            col_matrix[j][0] = input_vector[i * size + j];
+        }
+        matrices.push_back(col_matrix);
+    }
+
+    return matrices;
+}
+
+/**
+ * @brief Applies matrix multiplication for each block to perform encryption.
+ * @param key Key matrix.
+ * @param plaintext_vector Vector representation of the plaintext.
+ * @param size Size of key matrix.
+ * @returns Encrypted vector.
+ */
+vector<int> Controller(vector<vector<int>> key,
+                       const vector<int> &plaintext_vector, int size) {
+    vector<vector<vector<int>>> column_matrices =
+        split_into_column_matrices(plaintext_vector, size);
+    vector<int> encrypted_result;
+
+    for (int i = 0; i < column_matrices.size(); i++) {
+        vector<vector<int>> encrypted =
+            matrix_multiplication(key, column_matrices[i]);
+        for (int j = 0; j < encrypted.size(); j++) {
+            encrypted_result.push_back(encrypted[j][0]);
+        }
+    }
+
+    return encrypted_result;
+}
+
+/**
+ * @brief Self-test function for Hill cipher.
+ */
+static void test() {
+    int size = 2;
+    vector<vector<int>> key_matrix = {{2, 4}, {3, 5}};
+    string plaintext = "testmessage";
+
+    string cleaned = clean_and_pad(plaintext, size);
+    vector<int> vectorized = convert_string_to_vector(cleaned);
+    vector<int> encrypted = Controller(key_matrix, vectorized, size);
+
+    assert(encrypted.size() == vectorized.size());
+    for (int v : encrypted) {
+        assert(v >= 0);  // ensure valid output
+    }
+
+    std::cout << "All tests passed!\n";
+}
+
+}  // namespace ciphers
+
+/**
+ * @brief Main function
+ * @returns 0 on successful execution
+ */
+int main() {
+    ciphers::test();  // Run self-test
+
+    int size = 2;
+    vector<vector<int>> key_matrix = {{2, 4}, {3, 5}};
+    string plaintext =
+        "notonlyistheuniversestrangerthanwethinkitisstrangerthanwecanthinkwerne"
+        "rheisenbergx";
+
+    string clean_plaintext = ciphers::clean_and_pad(plaintext, size);
+    vector<int> v = ciphers::convert_string_to_vector(clean_plaintext);
+    vector<int> encrypted = ciphers::Controller(key_matrix, v, size);
+
+    ciphers::print_key_matrix(key_matrix);
+    ciphers::print_plain_text(v);
+    ciphers::print_ciphertext(encrypted);
+
+    return 0;
+}