Opa galera, tudo bem ? Este código está passando pelo tabela ASCII inteira, gostaria de mudar para as letras de A à Z. 
  
#include <stdio.h>
#include <string.h>
 
 
 
int main(void)
{
 
 
char mess[100];
printf("Enter encoded message: ");
scanf("%s", mess);
 
 
char key[100];
printf("Enter the key: ");
scanf("%s", key);
 
 
 
//converting string into ascii values and storing into array mess
 
int i;
int messlen = strlen(mess);
for (i = 0; i < messlen; i++)
{
mess = mess - 64;
//printf("%d\n", mess);
}
 
 
 
//converting key into ascii values
 
int keylen = strlen(key);
for (i = 0; i < keylen; i++)
{
key = key - 64;
}
 
 
 
 
//decoding
 
char ans[100];
int count = 0;
i = 0;
 
while (i < messlen)
{
 
//if (mess == 1)
//{
// ans += 1;
//}
 
if (mess < key[count])
{
 
ans = (mess - key[count] + 26) + 64;
printf("%c", ans);
 
}
 
else if (mess > key[count])
{
 
ans = (mess - key[count]) + 64;
printf("%c", ans);
 
}
 
 
 
 
 
if (count == keylen -1)
{
count = -1;
}
 
count += 1;
i += 1;
}
 
 
printf("\n");
 
 
return(0);
}