play fair cipher

#include <stdio.h>
#include <conio.h>
#include <string.h>
#include <ctype.h>
#define MX 5
int choice;
void playfair(char ch1, char ch2, char key[MX][MX]) {
    int i, j, w, x, y, z;
    //find row and col position of pair text
    for (i = 0; i < MX; i++) {
        for (j = 0; j < MX; j++) {
            if (ch1 == key[i][j]) {
                w = i;
                x = j;
            } else if (ch2 == key[i][j]) {
                y = i;
                z = j;
            }
        }
    }
    //wand y row
    //x and z col

    if (w == y) {
            //encryption
        if(choice==1){
            x = (x + 1) % 5;
            z = (z + 1) % 5;
        }
        //decryptionm
        else{
            x = ((x - 1) % 5+5)%5;
            z = ((z - 1) % 5+5)%5;
        }
        printf("%c%c", key[w][x], key[y][z]);
    } else if (x == z) {
        if(choice==1){
            w = (w + 1) % 5;
            y = (y + 1) % 5;
        }
        else{
            w = ((w - 1) % 5+5)%5;
            y = ((y - 1) % 5+5)%5;
        }
        printf("%c%c", key[w][x], key[y][z]);
    }
    else {
        printf("%c%c", key[w][z], key[y][x]);
    }
}
void removeDuplicates(char str[]){
    int hash[256]        =  {0};
    int currentIndex     = 0;
    int lastUniqueIndex  = 0;
    while(*(str+currentIndex)){
        char temp = *(str+currentIndex);
        if(0 == hash[temp]){
            hash[temp] = 1;
            *(str+lastUniqueIndex) = temp;
            lastUniqueIndex++;
        }
        currentIndex++;
    }
    *(str+lastUniqueIndex) = '\0';

}
void main() {
    int i, j, k = 0, l, m = 0, n;
    char key[MX][MX], keyminus[25], keystr[10], str[25] = {
        0
    };

    char alpa[26] = {
        'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'
    };
    printf("\n1.Encryption\n2.Decryption\n\nChoice(1 or 2):");
    scanf("%d",&choice);
    if(choice!=1 && choice!=2){ printf("Invalid Choice"); return;}
    fflush(stdin);
    printf("\nEnter key:");
    gets(keystr);
    printf("Enter the text:");
    gets(str);
    removeDuplicates(keystr);
    n = strlen(keystr);
    //convert the key character to uppertext
    for (i = 0; i < n; i++) {
        if (keystr[i] == 'j') keystr[i] = 'i';
        else if (keystr[i] == 'J') keystr[i] = 'I';
        keystr[i] = toupper(keystr[i]);
    }
    //convert all the characters of plaintext to uppertext
    for (i = 0; i < strlen(str); i++) {
        if (str[i] == 'j') str[i] = 'i';
        else if (str[i] == 'J') str[i] = 'I';
        str[i] = toupper(str[i]);
    }
    // store all characters except key
    j = 0;
    for (i = 0; i < 26; i++) {
        for (k = 0; k < n; k++) {
            if (keystr[k] == alpa[i]) break;
            else if (alpa[i] == 'J') break;
        }
        if (k == n) {
            keyminus[j] = alpa[i];
            j++;
        }
    }
    //construct key keymatrix
    k = 0;
    for (i = 0; i < MX; i++) {
        for (j = 0; j < MX; j++) {
            if (k < n) {
                key[i][j] = keystr[k];
                k++;
            } else {
                key[i][j] = keyminus[m];
                m++;
            }
            printf("%c ", key[i][j]);
        }
        printf("\n");
    }
    // construct diagram and convert to cipher text
    printf("\nEntered text :%s\nOutput Text :", str);
    for (i = 0; i < strlen(str); i++) {
        if (str[i] == 'J') str[i] = 'I';
        if (str[i + 1] == '\0') playfair(str[i], 'X', key);
        else {
            if (str[i + 1] == 'J') str[i + 1] = 'I';
            if (str[i] == str[i + 1]) playfair(str[i], 'X', key);
            else {
                playfair(str[i], str[i + 1], key);
                i++;
            }
        }
    }
    if(choice==2) printf(" (Remove unnecessary X)");
}