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# Advanced Reversing Mechanics 1
[Original writeup here](https://ohaithe.re/post/624092576653426688/rgbctf-2020-writeups)
As the name ("ARM1") suggests, this is an ARM binary. It's also 32-bit, so make sure to open it in 32-bit IDA or you won't be able to decompile. The problem statement gives some bytes,
```
71, 66, 61, 42, 53, 45, 7A, 40, 51, 4C, 5E, 30, 79, 5E, 31, 5E, 64, 59, 5E, 38, 61, 36, 65, 37, 63, 7C,
```
This function is pretty simple: main passes the input to `encrypt_flag(char*)`, then prints out the result as series fo hex values. So what does `encrypt_flag` do?
```
char *__fastcall encryptFlag(char *result)
{
char v1; // r3
int v2; // t1
v1 = *result;
if ( *result )
{
do
{
*result = v1 - 1;
v2 = (unsigned __int8)(result++)[1];
v1 = v2;
}
while ( v2 );
}
return result;
}
```
It loops through the bytes and adds one to each. Great. So take the given array, look each character up in [http://asciitable.com], look one previous, and write that down. Honestly it was faster that way than automating it. And you get the flag!
