Game Genie converter

char *NESGenie(const char *Input, char *Output) {
  char Key[] = "APZLGITYEOXUKSVN", Upper[512];
  int i, Addr=0, Data=0, Compare=0;
  strcpy(Upper, Input);
  for(i=0;Upper[i];i++)
    Upper[i] = toupper(Upper[i]);

  if(strlen(Input) == 6 || strlen(Input) == 8) {
    char n[strlen(Input)];
    for(i=0;Input[i];i++) {
      char *Find = strchr(Key, Upper[i]);
      if(!Find) return "Unrecognized character in code";
      n[i] = Find - Key;
    }
    Addr = 0x8000 + (((n[3] & 7) << 12) | ((n[5] & 7) << 8) | ((n[4] & 8) << 8) \
        | ((n[2] & 7) << 4) | ((n[1] & 8) << 4) | (n[4] & 7) | (n[3] & 8));
    if(strlen(Input) == 6) {
      Data = ((n[1] & 7) << 4) | ((n[0] & 8) << 4) | (n[0] & 7) | (n[5] & 8);
      sprintf(Output, "%.4x %.2x", Addr, Data);
      return Output;
     } else {
      Data = ((n[1] & 7) << 4) | ((n[0] & 8) << 4) | (n[0] & 7) | (n[7] & 8);
      Compare = ((n[7] & 7) << 4) | ((n[6] & 8) << 4) | (n[6] & 7) | (n[5] & 8);
      sprintf(Output, "%.4x %.2x %.2x", Addr, Data, Compare);
      return Output;
    }
  } else if(strlen(Input) == 7 || strlen(Input) == 10) {
    int UseCompare = (strlen(Input)==10);
    Addr = strtol(Input+0, NULL, 16);
    Data = strtol(Input+5, NULL, 16);
    if(!UseCompare) {
      Output[0] = Key[(Data>>4 & 8) | (Data & 7)];
      Output[1] = Key[(Addr>>4 & 8) | (Data>>4 & 7)];
      Output[2] = Key[8 | (Addr>>4 & 7)];
      Output[3] = Key[(Addr & 8) | (Addr>>12 & 7)];
      Output[4] = Key[(Addr>>8 & 8) | (Addr & 7)];
      Output[5] = Key[(Data & 8) | (Addr>>8 & 7)];
      Output[6] = 0;
      return Output;
    } else {
      Compare = strtol(Input+8, NULL, 16);
      Output[0] = Key[(Data>>4 & 8) | (Data & 7)];
      Output[1] = Key[(Addr>>4 & 8) | (Data>>4 & 7)];
      Output[2] = Key[8 | (Addr>>4 & 7)];
      Output[3] = Key[(Addr & 8) | (Addr>>12 & 7)];
      Output[4] = Key[(Addr>>8 & 8) | (Addr & 7)];
      Output[5] = Key[(Compare & 8) | (Addr>>8 & 7)];
      Output[6] = Key[(Compare>>4 & 8) | (Compare & 7)];
      Output[7] = Key[(Data & 8) | (Compare>>4 & 7)];
      Output[8] = 0;
      return Output;
    }
  } else {
    return "Invalid syntax?";
  }
}