2 files changed, 116 insertions, 0 deletions
diff --git a/cst120/week_4/BUILD b/cst120/week_4/BUILD
index 574513f..c60ffcc 100644
--- a/cst120/week_4/BUILD
+++ b/cst120/week_4/BUILD
@@ -1,3 +1,4 @@
 load("//:rules.bzl", "assignment")
 
 assignment("quiz_4_program_part")
+assignment("homework_4")
diff --git a/cst120/week_4/homework_4.c b/cst120/week_4/homework_4.c
new file mode 100644
index 0000000..be274d8
--- /dev/null
+++ b/cst120/week_4/homework_4.c
@@ -0,0 +1,115 @@
+/* Author: Zoltan Szabatin
+ * Professor: Professor Phong N.
+ * Project: Homework 4
+ * Course: CST 120 */
+
+/* Disable security warnings when using MSVC */
+/* #ifdef _MSC_VER
+#define _CRT_SECURE_NO_WARNINGS
+#endif */
+
+#include <ctype.h> /* islower */
+#include <linux/limits.h>
+#include <stdio.h>  /* printf, scanf */
+#include <stdlib.h> /* malloc, realloc */
+#include <string.h> /* strlen */
+
+/* Caesar cipher encryption */
+char *caesar_cipher_encrypt(const char *, const int *);
+
+/* Rail Fence cipher encryption */
+char *rail_fence_encrypt(const char *);
+
+int main(void) {
+  /* Mutable variables for user input */
+  int user_key = 0;
+  char user_go_again = 'z';
+  char user_message[PATH_MAX] = {0};
+
+  /* Keep running while the user requests so */
+  do {
+    char *caesar_cipher_encrypted_message = NULL;
+    char *rail_fence_encrypted_message = NULL;
+
+    /* Prompt and accept cipher message */
+    printf("Enter a message: ");
+    scanf(" %s", user_message);
+
+    /* Prompt and accept cipher key */
+    printf("Enter a key: ");
+    scanf(" %d", &user_key);
+
+    /* Encrypt user's message by the Caesar cipher, then the Rail Fence cipher
+     */
+    caesar_cipher_encrypted_message =
+        caesar_cipher_encrypt(user_message, &user_key);
+    rail_fence_encrypted_message =
+        rail_fence_encrypt(caesar_cipher_encrypted_message);
+
+    /* Output final double-cipher-ed result */
+    printf("%s\n", caesar_cipher_encrypted_message);
+
+    /* Prompt user to repeat */
+    printf("Would you like to go again? (y/n) ");
+    scanf(" %c", &user_go_again);
+
+    /* Free the encrypted messages */
+    free((void *)caesar_cipher_encrypted_message);
+    free((void *)rail_fence_encrypted_message);
+  } while (user_go_again == 'y' || user_go_again == 'Y');
+
+  return 0;
+}
+
+char *caesar_cipher_encrypt(const char *message, const int *key) {
+  size_t message_index = 0;
+  size_t message_length = (size_t)strlen(message);
+  char *encrypted_message = malloc((message_length + 1) * sizeof(char));
+
+  /* **Not** optimized Caesar cipher encryption implementation */
+  for (message_index = 0; message_index < message_length; ++message_index) {
+    if (isupper(message[message_index])) {
+      encrypted_message[message_index] =
+          (char)((message[message_index] + *key - 'A') % 26 + 'A');
+    } else {
+      encrypted_message[message_index] =
+          (char)((message[message_index] + *key - 'a') % 26 + 'a');
+    }
+  }
+
+  encrypted_message[message_length] = '\0';
+
+  return encrypted_message;
+}
+
+char *rail_fence_encrypt(const char *message) {
+  size_t message_index = 0, encrypted_message_index = 0;
+  size_t message_length = (size_t)strlen(message);
+  char *encrypted_message = malloc((message_length + 1) * sizeof(char));
+
+  /* Obtain and append message's first rail to the encrypted message */
+  for (message_index = 0; message_index < message_length; ++message_index) {
+    if (message_index % 2 == 0) {
+      encrypted_message[encrypted_message_index++] = message[message_index];
+    }
+  }
+
+  /* Obtain and append message's second rail to the encrypted message */
+  for (message_index = 0; message_index < message_length; ++message_index) {
+    if (message_index % 2 != 0) {
+      encrypted_message[encrypted_message_index++] = message[message_index];
+    }
+  }
+
+  /* If the message parity is uneven, append an 'x' to the end. */
+  if (message_length % 2 != 0) {
+    encrypted_message =
+        realloc(encrypted_message, (message_length + 2) * sizeof(char));
+    encrypted_message[message_length] = 'x';
+  }
+
+  /* Null terminator to signify C-string status */
+  encrypted_message[message_length + 1] = '\0';
+
+  return encrypted_message;
+}