Monoalphabetic Cipher in C

This is a C Program to implement Monoalphabetic Cipher. This program performs encryption and decryption using a monoalphabetic cipher. Only alphabetic characters are encrypted and case is ignored. Input is from stdin; Output is to stdout; Error is to stderr. One command line parameter is required of the form -dkey or -ekey where -e specifies encryption mode and -d specifies decryption mode. The key is always 26 alphabetic characters which specify mapping from abcd… to key.

Here is source code of the C Program to Implement the Monoalphabetic Cypher. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include<stdio.h>
```
```
char monocipher_encr(char);
```

char alpha[27][3] = { { 'a', 'f' }, { 'b', 'a' }, { 'c', 'g' }, { 'd', 'u' }, {

        'e', 'n' }, { 'f', 'i' }, { 'g', 'j' }, { 'h', 'k' }, { 'i', 'l' }, {

        'j', 'm' }, { 'k', 'o' }, { 'l', 'p' }, { 'm', 'q' }, { 'n', 'r' }, {

        'o', 's' }, { 'p', 't' }, { 'q', 'v' }, { 'r', 'w' }, { 's', 'x' }, {

        't', 'y' }, { 'u', 'z' }, { 'v', 'b' }, { 'w', 'c' }, { 'x', 'd' }, {

```
        'y', 'e' }, { 'z', 'h' } };
```
```
char str[20];
```
```
int main() {
```
```
    char str[20], str2[20];
```
```
    int i;
```
```
    printf("\n Enter String..");
```
```
    gets(str);
```
```
    for (i = 0; str[i]; i++) {
```

        str2[i] = monocipher_encr(str[i]);

```
    }
```
```
    str2[i] = '\0';
```

    printf("\n Before Decryption..%s", str);

    printf("\n After Decryption..%s\n", str2);

```
}
```
```
char monocipher_encr(char a) {
```
```
    int i;
```
```
    for (i = 0; i < 27; i++) {
```
```
        if (a == alpha[i][0])
```
```
            break;
```
```
    }
```
```
    return alpha[i][1];
```
```
}
```

Output:

$ gcc MonoalphabeticCipher.c
$ ./a.out
 
 Enter String..dharmendrahingu
 Before Decryption..dharmendrahingu
 After Decryption..ukfwqnruwfklrjz

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