[Internetwache CTF 2016] [Reverse 70 – ServerfARM] Write UpDescription
Internetwache CTF 2016 : ServerfARM
Category: Reversing Points: 70 Solves: 156 Description:Description: Someone handed me this and told me that to pass the exam, I have to extract a secret string. I know cheating is bad, but once does not count. So are you willing to help me?
Attachment: rev70.zip
Resolution
We start by launching a file on the binary:
1 2 | laxa:Challenges:00:28:45$ file serverfarm serverfarm: ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=0xa0f66f9d4bcb99d0102ce6b6bb838f84098b1023, not stripped |
Then we fire IDA to see that the program is executhing that method:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 | int __fastcall handle_task(int result, char *a2){ size_t v2; // ST0C_4@5 char *s; // [sp+0h] [bp-1Ch]@1 int v4; // [sp+4h] [bp-18h]@1 size_t i; // [sp+8h] [bp-14h]@2 unsigned int v6; // [sp+Ch] [bp-10h]@1 v4 = result; s = a2; v6 = 0; switch ( result ) { case 0: for ( i = 0; strlen(s) > i; ++i ) v6 += (unsigned __int8)s[i]; v2 = v6 / strlen(s); printf("%s", "Here's your 1. block:"); if ( (signed int)v2 <= 35 ) { printf("%s", "IW{"); putchar(83); result = printf("%c%c\n", 46, 69); } else { result = puts("I{WAQ3"); } break; case 1: printf("%s", "Here's your 2. block:"); if ( (unsigned __int8)*s % (signed int)(unsigned __int8)s[1] == 65 ) { printf("%s", ".R."); putchar(86); result = printf("%c%c\n", 46, 69); } else { result = puts("WI{QA3"); } break; case 2: printf("%s", "Here's your 3. block:"); if ( !strcmp(s, "1337") ) result = puts(".R>=F:"); else result = printf("%c%s%c\n", 46, "Q.D.Q", 33, s, v4); break; case 3: if ( *a2 ) result = printf("%c%s%c\n", 65, ":R:M", 125, a2, result); break; default: return result; } return result;} |
From there, it’s pretty straighforward, given that the flag is this format: IW{xxxxx} we only have one possibilty for the flag to be.
Flag is: IW{S.E.R.V.E.R>=F:A:R:M}