1//One Time Pad Cipher
2#include <stdio.h>
3#include <string.h>
4#include <ctype.h>
5main()
6{
7    //All the text which ever entered is converted to upper and without spaces
8    int i, j, len1, len2, numstr[100], numkey[100], numcipher[100];
9    char str[100], key[100], cipher[100];
10    printf("Enter a string text to encrypt\n");
11    gets(str);
12    for (i = 0, j = 0; i < strlen(str); i++)
13    {
14        if (str[i] != ' ')
15        {
16            str[j] = toupper(str[i]);
17            j++;
18        }
19    }
20    str[j] = '\0';
21    //obtaining numerical plain text ex A-0,B-1,C-2
22    for (i = 0; i < strlen(str); i++)
23    {
24        numstr[i] = str[i] - 'A';
25    }
26    printf("Enter key string of random text\n");
27    gets(key);
28    for (i = 0, j = 0; i < strlen(key); i++)
29    {
30        if (key[i] != ' ')
31        {
32            key[j] = toupper(key[i]);
33            j++;
34        }
35    }
36    key[j] = '\0';
37    //obtaining numerical one time pad(OTP) or key
38    for (i = 0; i < strlen(key); i++)
39    {
40        numkey[i] = key[i] - 'A';
41    }
42
43    for (i = 0; i < strlen(str); i++)
44    {
45        numcipher[i] = numstr[i] + numkey[i];
46    }
47    //To loop the number within 25 i.e if addition of numstr and numkey is 27 then numcipher should be 1
48    for (i = 0; i < strlen(str); i++)
49    {
50        if (numcipher[i] > 25)
51        {
52            numcipher[i] = numcipher[i] - 26;
53        }
54    }
55    printf("One Time Pad Cipher text is\n");
56    for (i = 0; i < strlen(str); i++)
57    {
58        printf("%c", (numcipher[i] + 'A'));
59    }
60    printf("\n");
61}