C programming byte data type - c

Question

I am doing a ctf and I have reversed and elf file and found the function that encodes the flag and i have made a decoder but because it is in c i cant use the byte data type. Is there any library that i can add and if not how did this code use the byte data type. I alredy did some challanges by this author and i solved the by deocidng the in c, and i think that this is something called dynamic string traformation.
// the original encoder
undefined8 main(void)
{
int iVar1;
ssize_t sVar2;
long in_FS_OFFSET;
int local_40;
byte local_38 [40];
long local_10;
local_10 = *(long *)(in_FS_OFFSET + 0x28);
initialize_flag();
puts("Give me your password: ");
sVar2 = read(0,local_38,0x1f);
local_38[(int)sVar2 + -1] = 0;
local_40 = 0x28;
while (local_40 < (int)sVar2 + -1) {
local_38[local_40] = local_38[local_40] ^ (char)local_40 + 10U;
local_38[local_40] = local_38[local_40] - 2;
local_40 = local_40 + 1;
}
iVar1 = strcmp((char *)local_38,"lp`7a<qLw\x1ekHopt(f-f*,o}V\x0f\x15J");
if (iVar1 == 0) {
puts("Thats the right password!");
printf("Flag: %s",flagBuffer);
}
else {
puts("Thats not the password!");
}
if (local_10 != *(long *)(in_FS_OFFSET + 0x28)) {
/* WARNING: Subroutine does not return */
__stack_chk_fail();
}
return 0;
}
Here is my encoder:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main()
{
ssize_t sVar2;
int local_40;
byte local_38 [40];
sVar2 = read(0,local_38,0x1f);
local_38[(int)sVar2 + -1] = 0;
local_40 = 27;// the flag lenght is 27
while (local_40 > 0) {
//this is the reverse of the og encoding
local_40 = local_40 - 1;
local_38[local_40] = local_38[local_40] - 2;
local_38[local_40] = (local_38[local_40] ^ (char)local_40) - 10U;
}
puts(local_38);
return 0;
}
//lp`7a<qLw\x1ekHopt(f-f*,o}V\x0f\x15J this is the encoded flag
// s after the original encoding should be w

Answer 1

The comments already have gotten you two great answers(Using stdint.h or chars), but otherwise...
If you do not have access to the stdint header and do not want to use chars, libraries such as Boost can provide you the uint8_t datatype, too.
In C++, you have std::byte accessible(Not sure you, specifically, will be helped by that, but others maybe)
If you wish to make sure a char is 8-bits of lenght, you can check the CHAR_BIT value defined in .
So your options(Ranking from best to worse) are:
(If you can use cpp) Using std::byte
Using the uint8_t datatype defined in <stdint.h>
Using a char/unsigned char
Using an external library
Note it's probably overkill using an external library for such a trivial task unless you already have that said library.
Hope this helps.

Answer 2

your decoder seems to have some errors like: local_38[local_40] = local_38[local_40] - 2;
It should be like this: local_38[local_40] = local_38[local_40] + 2;
I have written a decoder for above question in python
key="lp`7a<qLw\x1ekHopt(f-f*,o}V\x0f\x15J"
check=list(key)
string=str()
string=''
for i in range(26,-1,-1):
j=i+10
k=(ord(check[i])+2)
string=(chr(k^j)+string)
print(string)
Hope this will help