This script, when executed under IDA, writes the correct input to an output file :
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| from idc import * | |
| from ctypes import c_uint32 | |
| def bruteforce_word(dword1,dword2): | |
| for i in range(0,256) : | |
| for j in range(0,256) : | |
| k = 0 | |
| result = c_uint32(0xffffffff) | |
| while k < 2 : | |
| if k == 0 : | |
| result.value ^= c_uint32(i).value | |
| else : | |
| result.value ^= c_uint32(j).value | |
| l = 7 | |
| while l >= 0 : | |
| result.value = ( result.value >> 1 ) ^ -(result.value & 1) & dword1 | |
| l -= 1 | |
| k += 1 | |
| result.value = ~result.value | |
| if result.value == dword2 : | |
| return [i,j] | |
| def prepare() : | |
| RunTo(BeginEA()) | |
| GetDebuggerEvent(WFNE_SUSP,-1) | |
| #Modify the necesarry registers | |
| RunTo(0x4007F1) | |
| GetDebuggerEvent(WFNE_SUSP,-1) | |
| SetRegValue(0x400000,"RDI") | |
| RunTo(0x40084F) | |
| GetDebuggerEvent(WFNE_SUSP,-1) | |
| SetRegValue(0x40089D,"RAX") | |
| #First stage | |
| RIP_access_list = [] | |
| prepare() | |
| ptr_input = DbgQword(GetRegValue("RDX")) | |
| AddBptEx(ptr_input,1,BPT_RDWR) | |
| while GetDebuggerEvent(WFNE_CONT|WFNE_ANY, -1) == BREAKPOINT : | |
| RIP_access_list.append(GetRegValue("RIP")) | |
| DelBpt(ptr_input) | |
| #Second stage | |
| last_access = RIP_access_list[-1] | |
| AddBptEx(last_access,1,BPT_EXEC) | |
| prepare() | |
| GetDebuggerEvent(WFNE_CONT|WFNE_ANY,-1) | |
| DelBpt(last_access) | |
| file = open("flag.txt","wb") | |
| while DbgByte(ptr_input) != 0 : | |
| dword1 = DbgDword(GetRegValue("RBP") - 0x3c) | |
| dword2 = DbgDword(GetRegValue("RIP") + 0x54) | |
| bytes = bruteforce_word(dword1,dword2) | |
| file.write(chr(bytes[0])+chr(bytes[1])) | |
| DelBpt(ptr_input) | |
| ptr_input += 2 | |
| AddBptEx(ptr_input,1,BPT_RDWR) | |
| if GetDebuggerEvent(WFNE_CONT|WFNE_ANY,-1) != BREAKPOINT : | |
| break | |
| DelBpt(ptr_input) | |
| file.close() | |
| print "[+] Done !" |
The flag is simply the md5sum of this file :
