Packet Decryption

BYTE *DecryptPacket(BYTE *Packet, DWORD len) { 
BYTE KeyIndex, curKey, lastByte; 
DWORD i; 

KeyIndex = *(CryptInfo + 0x115); 

lastByte = 0; 

for(i = 0; i < len; i++) {   //return the key to the initial state 
 KeyIndex--; 
  if(KeyIndex == 0xFF)   //-1, bytes are unsigned 
   KeyIndex = 39; 
} 

for(i = 0; i < len; i++) { 
curKey  = *(*(BYTE **)(CryptInfo + 0x11C) + KeyIndex); 
tmp = *(Packet + i); 
*(Packet + i) = (*(Packet + i ) - lastByte)) ^ curKey; 
lastByte = tmp;
} 

return Packet; 
}

typedef struct WoWKeyIndex { 
  BYTE Index; 
  BYTE LastByte; 
  BYTE CryptLen;   
} WoWKeyIndex; 

typedef struct WoWCryptInfo { 
  BYTE IsCrypted;        
  WoWKeyIndex SendKey; 
  WoWKeyIndex RecvKey; 
  BYTE KeyLen; 
  char *Key; 
} WoWCryptInfo; 

typedef struct WoWClientOP { 
  WORD Size; 
  DWORD Code; 
  BYTE Data[MAX_PACKET_LEN]; 
} WoWClientOP; 

typedef struct WoWServerOP { 
  WORD Size; 
  WORD Code; 
  BYTE Data[MAX_PACKET_LEN]; 
} WoWServerOP;