c - Stack around the variable 's' was corrupted -

I found this error in my RC4 algorithm, it works well, but every time this message is too large I get this error, like 1000kB, here's the code:

  char * rc4 (const int * key, int key_size, char * buff, int buff_size) {int i, j , K; Int es [255], RK [255]; // rk = random_key char * encrypted = alloc_char_buffer (buff_size); For (i = 0; i  
 'encrypted' to be "non-empty Ended ", so if the string is left 10 I will only assign 10 bytes, 11 for '11 0 'should not be  
 If you need this, here is the code for alloc_char_buffer (unsigned int)  
  char * alloc_char_buffer (unsigned int size) {char * buff = NULL; Buff = (four *) coloc (size + 1, size (four)); If (! Buff) _error ("Program fails to allocate memory."); to return; }   
 SWITCH:  
  // inversà £ ÃÆ'à ¢ â,¬Å¡Ãƒâ € šÃ, © zero-SWITCH (int * a, Int * b) {* (a) = * (a) ^ * (b); // a random number * (b) = * (a) ^ * (b); // one * (a) = * (a) ^ * (b); // get b}    
 
 
  char * encrypted = alloc_char_buffer (buff_size); / * ... * / Encrypted [buff_size] = 10;   
 Here is the problem that you assign to  buff_size  elements. Thus, the last valid index is  buff_size-1 , not  Buff_size .  
 Another issue:  
  j = (js [ja] + rk [ii])% 256;   
 Thus  j  has the limit of [0, 255], but the  is the legal index of  only [0, 254]. You either either  s  as a 256-element array or review algorithm implementation.