# Investigative Reversing 4 ## Problem > We have recovered a binary and 5 images: image01, image02, image03, image04, image05. See what you can make of it. There should be a flag somewhere. Its also found in /problems/investigative-reversing-4\_6\_f5c1435d5f45ad042614888d32091beb on the shell server. * [Binary](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Forensics/Investigative%20Reversing%204/mystery/README.md) * [Image 1](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Forensics/Investigative%20Reversing%204/Item01_cp.bmp) * [Image 2](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Forensics/Investigative%20Reversing%204/Item02_cp.bmp) * [Image 3](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Forensics/Investigative%20Reversing%204/Item03_cp.bmp) * [Image 4](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Forensics/Investigative%20Reversing%204/Item04_cp.bmp) * [Image 5](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Forensics/Investigative%20Reversing%204/Item05_cp.bmp) ## Solution 1. Reverse the binary file using [Ghidra](https://ghidra-sre.org/) ([cheat sheet](https://ghidra-sre.org/CheatSheet.html)). Open it and in the symbol tree click on main. The decompiled main function will show on the right. \`\`\`c++ undefined8 main(void) { size\_t sVar1; undefined4 local\_4c; undefined local\_48 \[52]; int local\_14; FILE \*local\_10; flag = local\_48; local\_4c = 0; flag\_index = \&local\_4c; local\_10 = fopen("flag.txt","r"); if (local\_10 == (FILE *)0x0) { puts("No flag found, please make sure this is run on the server"); } sVar1 = fread(flag,0x32,1,local\_10); local\_14 = (int)sVar1; if (local\_14 < 1) { puts("Invalid Flag"); /* WARNING: Subroutine does not return \*/ exit(0); } fclose(local\_10); encodeAll(); return 0; } ```` /* WARNING: Could not reconcile some variable overlaps */ void encodeAll(void) { ulong local_48; undefined8 local_40; undefined4 local_38; ulong local_28; undefined8 local_20; undefined4 local_18; char local_9; local_28 = 0x635f31306d657449; local_20 = 0x706d622e70; local_18 = 0; local_48 = 0x622e31306d657449; local_40 = 0x706d; local_38 = 0; local_9 = '5'; while ('0' < local_9) { local_48._0_6_ = CONCAT15(local_9,(undefined5)local_48); local_48 = local_48 & 0xffff000000000000 | (ulong)(uint6)local_48; local_28._0_6_ = CONCAT15(local_9,(undefined5)local_28); local_28 = local_28 & 0xffff000000000000 | (ulong)(uint6)local_28; encodeDataInFile(&local_48,&local_28,&local_28); local_9 = local_9 + -1; } return; } void encodeDataInFile(char *param_1,char *param_2) { size_t sVar1; char local_2e; char local_2d; int local_2c; FILE *local_28; FILE *local_20; uint local_18; int local_14; int local_10; int local_c; local_20 = fopen(param_1,"r"); local_28 = fopen(param_2,"a"); if (local_20 != (FILE *)0x0) { sVar1 = fread(&local_2e,1,1,local_20); local_c = (int)sVar1; local_2c = 0x7e3; local_10 = 0; while (local_10 < local_2c) { fputc((int)local_2e,local_28); sVar1 = fread(&local_2e,1,1,local_20); local_c = (int)sVar1; local_10 = local_10 + 1; } local_14 = 0; while (local_14 < 0x32) { if (local_14 % 5 == 0) { local_18 = 0; while ((int)local_18 < 8) { local_2d = codedChar((ulong)local_18,(ulong)(uint)(int)*(char *)(*flag_index + flag), (ulong)(uint)(int)local_2e, (ulong)(uint)(int)*(char *)(*flag_index + flag)); fputc((int)local_2d,local_28); fread(&local_2e,1,1,local_20); local_18 = local_18 + 1; } *flag_index = *flag_index + 1; } else { fputc((int)local_2e,local_28); fread(&local_2e,1,1,local_20); } local_14 = local_14 + 1; } while (local_c == 1) { fputc((int)local_2e,local_28); sVar1 = fread(&local_2e,1,1,local_20); local_c = (int)sVar1; } fclose(local_28); fclose(local_20); return; } puts("No output found, please run this on the server"); /* WARNING: Subroutine does not return */ exit(0); } ulong codedChar(int param_1,byte param_2,byte param_3) { byte local_20; local_20 = param_2; if (param_1 != 0) { local_20 = (byte)((int)(char)param_2 >> ((byte)param_1 & 0x1f)); } return (ulong)(param_3 & 0xfe | local_20 & 1); } ``` ```` 1. This script spreads the flag amongst the five ".bmp" image files provided in the challenge. For each image the program: 1. Jumps to offset 2019 bytes and encodes a byte of the flag using LSB in 8 bytes of the original image file. 2. Skips 4 bytes by copying 4 bytes from the original image file 2. However, the above steps are made slightly more complicated in the actual encoding program. It performs a loop 50 times. If the interval tracking variable is divisible by 5, then it will loop through and write 8 bits of the flag. If the interval tracking variable is not divisible by 5, then the program writes writes a value from the original image. This effectively does the above steps. 3. Run the decoding [script.py](https://github.com/HHousen/PicoCTF-2019/tree/24b0981c72638c12f9a8572f81e1abbcf8de306d/Forensics/Investigative%20Reversing%204/script.py) to get the flag. ### Flag `picoCTF{N1c3_R3ver51ng_5k1115_000000000002eea28cd}` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://picoctf2019.haydenhousen.com/forensics/investigative-reversing-4.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.