open-source all the things!!!
We solved this task in a fun way. Running file command on the supplied binary informed us that it's an ELF for
RISC-V architecture. We couldn't run it directly on our computer and installing an emulator was annoying (it didn't work
for some reason). So, we solved it in a blackbox way!
Running strings -n 20 riskv_and_reward gave us only one string:
tjb3csFt0rrutrh_wiv5__fi}k_1ih`{xIcrhsoyBmyw1CyT3rvxStT_jq40_zrq(
Although it seems quite messed up, we quickly noticed it has some resemblance to the flag format: it has two curly braces,
and all the capital letters from BITSCTF. It seems it was permutated in some way. So, the only thing left to do was
reversing the permutation. Running hexdump on the binary, we found an interesting part right after the aforementioned
string:
00001080 28 00 00 00 21 00 00 00 2f 00 00 00 34 00 00 00 |(...!.../...4...|
00001090 2d 00 00 00 36 00 00 00 06 00 00 00 1f 00 00 00 |-...6...........|
000010a0 25 00 00 00 3b 00 00 00 29 00 00 00 03 00 00 00 |%...;...).......|
000010b0 37 00 00 00 3e 00 00 00 1b 00 00 00 05 00 00 00 |7...>...........|
000010c0 22 00 00 00 13 00 00 00 14 00 00 00 3a 00 00 00 |"...........:...|
000010d0 31 00 00 00 30 00 00 00 1a 00 00 00 10 00 00 00 |1...0...........|
000010e0 08 00 00 00 23 00 00 00 07 00 00 00 24 00 00 00 |....#.......$...|
000010f0 3c 00 00 00 2c 00 00 00 00 00 00 00 18 00 00 00 |<...,...........|
00001100 43 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |C...............|
We noticed every number appears just once, so this was probably a transposition table. From now on, we had to just write a simple script to apply the permutation to get the flag.