Ceasar encryption-decryption-cipher-decipher code

#include <stdio.h>
#include <string.h>

void menu(void){
  printf("Please enter a number: \n"
         "1-Encrypt\n"
         "2-Decrypt\n"
         "3-Exit\n"
         "prompt > ");
  fflush( stdout );
}

int getchoice(void){
  char buff[BUFSIZ];
  int  choice = 0;
  do {
    menu();
    if(fgets( buff, sizeof buff, stdin ) != NULL){
      /* success reading a line, does it make sense? */
      if(sscanf( buff, "%d", &choice) != 1){
        printf("Enter a number\n");
      }
    }else{
      /* user EOF, just exit now */
      choice = 3;
    }
  }while(choice < 1 || choice > 3);
  return choice;
}

void encode(void){
  char buff[BUFSIZ];
  int i = 0;
  int shift_value;
  printf( "Doing encrypt\n" );
  printf("\nPlease enter the text you wish to encrypt CAPS LOCK ONLY: ");
  fgets(buff, sizeof(buff), stdin);
  printf("\nEnter your encryption shift value (anything from +1 to 25): "); 
  scanf("%i", &shift_value);
  while(buff[i] != '\0'){
    if(buff[i] >= 'A' && buff[i] <= 'Z'){
      buff[i] = 'A' + (buff[i] - 'A' + shift_value) % 26;
    }
    i++; 
  }
  printf("\n Your encrypted text is: %s \n", buff);
}

void decode(void){
  char buff[BUFSIZ];
  int i = 0;
  int shift_value;
  printf( "Doing decrypt\n" );
  printf("\nPlease enter the text you wish to decrypt CAPS LOCK ONLY: ");
  fgets(buff, sizeof(buff), stdin);
  printf("\nEnter your encryption shift value (anything from +1 to 25): "); 
  scanf ("%i", &shift_value);

  while(buff[i] != '\0'){
    if(buff[i] >= 'A' && buff[i] <= 'Z'){
      int c = buff[i] - 'A' - shift_value;
      if(c < 0)
        c += 26;
      buff[i] = 'A' + c % 26;
    }
    i++;
  }
  printf("\nYour decrypted text is: %s \n", buff);
}

int main(){
  int choice;
  while((choice=getchoice()) != 3 ){
    if(choice == 1){
      encode();
    }else{
      if(choice == 2){
        decode();
      }
    }
  }
  return 0;
}