Mã hóa đơn giản

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef char chuoi[50];
void chieudai(chuoi);
void daonguoc(chuoi);
void dangvong(chuoi);
int main() {
    FILE *f;
    chuoi nguon;
    int chon;
    // Tao file moi
    f = fopen("/home/thieumao/Desktop/input.txt","w");
    fprintf(f,"abc ABC!xyz@XYZ");
    fclose(f);
    // Doc file
    f = fopen("/home/thieumao/Desktop/input.txt","r");
    fgets(nguon,50,f);
    fclose(f);

    printf("MA NGUON = %s\n",nguon);
    printf("\nChon so 1,2,3 tuong ung, hay Esc de thoat");
    printf("\n 1: Chieu dai");
    printf("\n 2: Dao nguoc");
    printf("\n 3: Dang vong");
    printf("\nchon = ");
    scanf("%d",&chon);
    system("clear");
    switch (chon){
        case 1: chieudai(nguon); break;
        case 2: daonguoc(nguon); break;
        case 3: dangvong(nguon); break;
    }
    return 0;
}
// ** Chieu dai
void chieudai(chuoi s){
    printf("\nMA NGUON = %s",s);
    int l = strlen(s);
    printf("\nCHIEU DAI: %d\n",l);
}
// ** Dao nguoc
void daonguoc(chuoi s){
    printf("\nMA NGUON = %s",s);
    printf("\nDAO NGUOC: ");
    int i;
    int l = strlen(s);
    for(i=l-1;i>=0;i--){
        printf("%c",s[i]);
    }
    printf("\n");
}
// ** Dang vong
void dangvong(chuoi s){
    printf("\nMA NGUON = %s",s);
    int i,v;
    int l = strlen(s);
    printf("\nNhap so vong ban muon dich chuyen: ");
    scanf("%d",&v);
    printf("DANG VONG: ");
    for(i=0;i<l;i++){
        if (65<=s[i] && s[i]<=90){
            if (s[i]+v>90){
                printf("%c",s[i]+v-26);
            } else {
                printf("%c",s[i]+v);
            }
        } else if (97<=s[i] && s[i]<=122){
            if (s[i]+v>122){
                printf("%c",s[i]+v-26);
            } else {
                printf("%c",s[i]+v);
            }
        } else {
            printf("%c",s[i]);
        }
    }
}