#include <ctype.h>
#include <stdio.h>
 
char code(char ch,int key) { return isalpha(ch)?(isupper(ch)?'A'+(ch-'A'+key)%26:'a'+(ch-'a'+key)%26):ch; }
 
char *encode(char *str,int key)
  {
   char *p;
   key%=26;
   if(key<0) key+=26;
   for(p=str;*p;++p) *p=code(*p,key);
   return str;
  }
 
char *decode(char *str,int key) { return encode(str,-key); }
 
int main()
  {
   int key;
   char bufor[1000];
   for(;;)
     {
      printf("podaj klucz i tekst: ");
      if(scanf("%d %999[^\n]s",&key,bufor)!=2) return 0;
      printf("input:  \"%s\"\n",bufor);
      printf("encode: \"%s\"\n",encode(bufor,key)); 
      printf("decode: \"%s\"\n",decode(bufor,key)); 
     }   
  }

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NSBBbGEgbWEga290YQotNSBBbGEgbWEga290YQo1IFZndiBodiBmam92Ci01IEZxZiByZiBwdHlmCg==

5 Ala ma kota
-5 Ala ma kota
5 Vgv hv fjov
-5 Fqf rf ptyf