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1/*
2
3CS50x: Implementation of Vigenere Cipher.
4
5Usage:
6    ./vigenere <secret-key>
7
8Args:
9    secret-key: A word used to encrypt the plaintext.
10
11Author: Adrian Arumugam
12Date: 23-Sep-2016
13
14*/
15
16#include <cs50.h>
17#include <ctype.h>
18#include <string.h>
19#include <stdio.h>
20#include <stdlib.h>
21
22int main(int argc, string argv[]){
23    
24    // Fail fast if argument count is not 2.
25    // This covers the actual command and first argument.
26    if (argc != 2)
27    {
28        printf("Please provide a single argument for the key.\n");
29        return 1;
30    }
31    
32    char* keyword = argv[1]; // Initialize keyword
33    int c = strlen(keyword);
34    
35    // Loop through the characters to identify if they are
36    // any disallowed characters.
37    for (int i = 0; i < c; i++)
38    {
39        if isalpha(keyword[i])
40        {
41            continue;
42        }
43        else
44        {
45            printf("Please enter a word for the secret key.\n");
46            return 1;
47        }
48    }
49    
50    // Lets get ready to rumble with the Vigenere cipher.
51    
52    char* plaintext = GetString(); // Prompt user string to encrypt
53    int n = strlen(plaintext); // Counter for for loop
54    char ciphertext[n]; // Initialize array the length of plaintext
55    int cipherval; // Initialize cipher value
56    int plainval; // Initialize plain value
57    int subkeyval; // Initialize subtraction key
58    int keyval; // Initialize to store value for key
59    int j = 0; // Counter for key char
60    const int numalpha = 26; // Number of characters in alphabet
61    const int asciinum = 65; // Used to determing correct key value
62    const int capitalend = 90; // End ascii code of A-Z
63    const int lowercaseend = 122; // End ascii code of a-z
64    
65    // Loop through the plaintext array which stores the string to encrypt.
66    //
67    // Iterating through each character of the string and performing the appropriate
68    // actions.
69
70    for (int i = 0; i <= n; i++)
71    {
72        j = j % strlen(keyword);
73        plainval = (int) plaintext[i]; // Convert the current char to an integer
74        keyval = (int) toupper(keyword[j]) - asciinum; // Uppercase and lower case treated the same. This determines value(0-25)
75        cipherval = plainval + keyval; // Default cipherval
76        subkeyval = numalpha - keyval; // Default subtraction key
77        
78        // Alphabet data validaton - A-Z and a-z.
79        //
80        // If True then we potentially need to execute further
81        // cipher value modification for correct encryption.
82        if isalpha(plaintext[i])
83        {
84            // If the character is an uppercase character the following happens:
85            //
86            // 1. If cipherval is > the end of the uppercase alphabet(90)
87            //    then we look at whether the keyval is equal to [Zz] if yes
88            //    then cipherval plainval - subkeyval(which would be 1 in this instance)
89            // 2. Otherwise the cipherval is plainval - keyval.
90            //
91            // e.g BARFOO is encrypted as CAQGON with a key of BAZ
92            //     66,65,82,79,79 > 67,65,81,79,78
93            if isupper(plaintext[i])
94            {
95                if (cipherval > capitalend)
96                {
97                    if (keyval != numalpha - 1)
98                    {
99                        cipherval = plainval - keyval;
100                    }
101                    else
102                    {
103                        cipherval = plainval - subkeyval;
104                    }
105                }
106            }
107            // If the character is a lowercase character the following happens:
108            //
109            // 1. If cipherval is > than the end of the lowercase alphabet(122)
110            //    then cipherval is equal to plainval - subkeyval.
111            // 2. Otherwise cipherval is equal to plaintext val - keyval.
112            //
113            // e.g barfoo encrypted as caqgon with a key of BAZ
114            //     98,97,114,102,111,111 -> 99,97,113,103,111,110
115            if islower(plaintext[i])
116            {
117                if (cipherval > lowercaseend)
118                {
119                    cipherval = plainval - subkeyval;
120                }
121                else
122                {
123                    cipherval = plainval + keyval;
124                }
125            }
126            j++;
127        }
128        // If the character is not A-Z or a-z then we don't encrypt
129        // just copy the actual plaintext integer value to cipherval.
130        else
131        {
132            cipherval = plainval;
133        }
134        // Store the cipher value in the array
135        ciphertext[i] = (char) cipherval;
136    }
137    // Once the loop exits we can print out the cipher string.
138    // This will be the encrypted string based on the secret key provided.
139    printf("%s\n", ciphertext);
140    return 0; // Return 0 to signal completion.
141}