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

#define ES_LETRA(X) ((X>=65)&(X<=90))||((X>=97)&(X<=122))?1:0

void desencriptar(char *reg);
char restLetra(char letra,int cant);

int main()
{
    FILE *pEncrip,*pDesencrip;
    char reg[600];

    pEncrip=fopen("encriptado.txt","rt");
    if(!pEncrip){
        printf("No se pudo abrir el archivo\n");
        return 10;
    }
    pDesencrip=fopen("desencriptado.txt","wt");
    if(!pDesencrip){
        printf("No se pudo crear el archivo\n");
        fclose(pEncrip);
        return 11;
    }

    while(fgets(reg,600,pEncrip)){
        desencriptar(reg);
        fprintf(pDesencrip,"%s",reg);
    }
    fclose(pEncrip);
    fclose(pDesencrip);

    return 0;
}

void desencriptar(char *reg){
    int cont=1;
    char *aux,*pFinal,letra;

    printf("%s\n",reg);

    aux=reg;
    pFinal=reg;
    while(*pFinal!='\n'){

        //while((*aux!=' ')& (*aux!='\n')){
        while(ES_LETRA(*aux)){
            //if(ES_LETRA(*aux))
            *aux=restLetra(*aux,cont);
            aux++;
            cont++;
        }
        pFinal=aux;
        aux--;
        while(reg<aux){
            letra=*aux;
            *aux=*reg;
            *reg=letra;
            reg++;
            aux--;
        }
        aux=pFinal+1;
        reg=pFinal+1;
        cont=1;

    }
}

char restLetra(char letra,int cant){
    static const char abc[53] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    char *aux,letFin;

    aux=strchr(abc,letra);
    aux -= cant;
    if(aux<abc)
        aux+=52;
    letFin=*aux;

    return letFin;
}