As your initial instincts told you, it is quite a simple challenge and you are close. Let's understand together the logic of the program. As you did, I used radare2.
Let's open the program in radare2:
$ r2 reverse2
Analyze the binary and print the main function:
[0x08048350]> pdf @ main
If you want to, you can use radare's great Visual Graph Mode to easily detect the function's flow:
[0x08048350]> VV @ main
First the program checks whether you pass at least one argument to the program, i.e
num_of_arguments > 1. Remember that the filename counts as 1 argument so you'll need another one.
0x08048456 837d0801 cmp dword [arg_8h], 1
0x0804845a 7f16 jg 0x804847
Then it checks if the file name is more than 15 characters long:
0x08048472 8b450c mov eax, dword [arg_ch] ; [0xc:4]=-1 ; 12
0x08048475 8b00 mov eax, dword [eax]
0x08048477 89442418 mov dword [local_18h], eax
0x0804847b 8b442418 mov eax, dword [local_18h] ; [0x18:4]=-1 ; 24
0x0804847f 890424 mov dword [esp], eax
0x08048482 e8a9feffff call sym.imp.strlen ; size_t strlen(const char *s)
0x08048487 8944241c mov dword [local_1ch], eax
0x0804848b 837c241c15 cmp dword [local_1ch], 0x15 ; [0x15:4]=-1 ; 21
So, you need to change the file name to be more than 15 characters long and pass it at least one argument.
Now for the interesting part. In the following conditions, the program is checking whether
filename[some_offset] == chr(0x??) and if yes, it goes to another offset in the file name and checks the value there.
So, for example, in the first check you can see:
0x080484a7 8b442418 mov eax, dword [local_18h]
0x080484ab 83c014 add eax, 0x14
0x080484ae 0fb600 movzx eax, byte [eax]
0x080484b1 3c73 cmp al, 0x73 ; 's' ; 115
In this block it checks whether
filename[0x14] == chr(0x73) which radare hints you that 0x73 equals the letter 's'.
If yes, it jumps to another block:
0x080484b9 8b442418 mov eax, dword [local_18h]
0x080484bd 83c008 add eax, 8
0x080484c0 0fb600 movzx eax, byte [eax]
0x080484c3 3c67 cmp al, 0x67 ; 'g' ; 103
In this block the program checks whether
And so on until it checks all the offsets.
I'll leave it to you to solve it and figure out what should be the name of the program.