Evaluating response for CTF challenge involvement

The user likely needs assistance with a CTF problem involving the analysis of a binary file, which might involve reverse engineering or malware analysis. I'll need to ensure that CTF challenges are safe and permitted. I'll review available tools and check if specific skills are required.

{"path":"","only_tools":false}

Proceeding with task without specific skill

The only listed skill is about skill creation, which doesn’t match the task. I’ll continue with the task and use available tools—potentially in a contained environment—without the need for a specialized skill, as it doesn’t seem necessary for this request.

bash -lc file /mnt/data/challenge.exe && sha256sum /mnt/data/challenge.exe && ls -l /mnt/data/challenge.exe

Inspecting the binary with string analysis

I think I might need to inspect the binary’s strings. I could potentially run it, though I’m not sure if I have Wine installed, which could be necessary for compatibility. I’ll start by analyzing the relevant strings within the file first to see what useful info it might contain.

bash -lc strings -a -n 4 /mnt/data/challenge.exe | sed -n '1,240p'

Considering disassembly tools

I think I’ll need to disassemble the binary to further analyze it. I could use options like rabin2, objdump, or Ghidra, but first, I’ll check what tools are installed. Maybe radare2 (r2) is available, so I’ll see if I can work with that for disassembly.

bash -lc which r2 || which radare2 || which objdump || which llvm-objdump || which ghidra || which strings

bash -lc objdump -d -M intel --start-address=0x415680 --stop-address=0x415900 /mnt/data/challenge.exe | sed -n '1,260p'

Analyzing the entry point and unpacking process

The entry point looks like a packer or unpacker stub that jumps to 0x4014e0 after unpacking. I need to analyze the actual code at that address. Maybe dumping the memory after unpacking could work, but running the EXE directly might not be feasible. I'll check if using Wine or another emulation tool like Unicorn could be a better approach.

bash -lc which wine || which winedbg || which qemu-x86_64

Dumping memory and analyzing code at OEP

Since Wine isn’t an option, I could try emulating the unpacker and dumping memory with a custom script. The original entry point (OEP) is at 0x4014e0, in the CTF0 section, so it might be obfuscated. I’ll focus on dumping memory around that address to proceed with the analysis.

bash -lc objdump -d -M intel --start-address=0x4014a0 --stop-address=0x401860 /mnt/data/challenge.exe | sed -n '1,260p'

Inspecting PE section table for size issues

It looks like the CTF0 section’s virtual size is larger than the raw size, which could indicate a packed stub with only virtual sections. The objdump shows the file as truncated. To explore further, I think I should use a Python library like pefile to inspect the section table and raw data sizes accurately. Since I’m not using skill code, running Python in a container seems like a good option.

bash -lc python - <<'PY' import pefile pe=pefile.PE('/mnt/data/challenge.exe') for s in pe.sections: print(s.Name, hex(s.VirtualAddress), hex(s.Misc_VirtualSize), hex(s.PointerToRawData), hex(s.SizeOfRawData), s.Characteristics) print('OEP',hex(pe.OPTIONAL_HEADER.AddressOfEntryPoint+pe.OPTIONAL_HEADER.ImageBase)) PY

Deciding on PE header parsing method

I’m considering whether installing pefile would be allowed. Maybe a minimal parsing approach using something like objdump -h could work, but I'll still need raw sizes. Perhaps trying llvm-readobj or readpe would be more effective. Another approach could be manually parsing using Python’s struct. Let’s go with parsing the PE headers myself.

bash -lc python - <<'PY' import struct p=open('/mnt/data/challenge.exe','rb').read() e_lfanew=struct.unpack_from('<I',p,0x3c)[0] print('e_lfanew',hex(e_lfanew)) coff_off=e_lfanew+4 Machine, NumberOfSections, TimeDateStamp, PtrSym, NumSym, SizeOptionalHeader, Characteristics = struct.unpack_from('<HHIIIHH',p,coff_off) print(Machine, NumberOfSections, hex(TimeDateStamp), SizeOptionalHeader, hex(Characteristics)) opt_off=coff_off+20 magic=struct.unpack_from('<H',p,opt_off)[0] print('magic',hex(magic)) AddressOfEntryPoint=struct.unpack_from('<I',p,opt_off+16)[0] ImageBase=struct.unpack_from('<Q',p,opt_off+24)[0] SizeOfImage=struct.unpack_from('<I',p,opt_off+56)[0] SizeOfHeaders=struct.unpack_from('<I',p,opt_off+60)[0] print('ep',hex(ImageBase+AddressOfEntryPoint),'img',hex(SizeOfImage),'hdr',hex(SizeOfHeaders)) sec_off=opt_off+SizeOptionalHeader for i in range(NumberOfSections): off=sec_off+i*40 name=p[off:off+8].rstrip(b'\0') VirtualSize, VirtualAddress, SizeOfRawData, PointerToRawData = struct.unpack_from('<IIII',p,off+8) print(i,name,hex(VirtualAddress),hex(VirtualSize),hex(PointerToRawData),hex(SizeOfRawData)) PY

Emulating the unpacker

I’ve noticed the first section has a raw size of 0 and a virtual size of e000, and it’s executable, which suggests the unpacker decompresses code into the CTF0 section. To proceed, I’ll need to emulate the unpacker or run it in a custom emulator, maybe using Unicorn. I’ll check if it's installed.

bash -lc python - <<'PY' import importlib.util print('unicorn', importlib.util.find_spec('unicorn')) print('capstone', importlib.util.find_spec('capstone')) print('lief', importlib.util.find_spec('lief')) PY

Emulating unpacking process

It may be possible to install Unicorn and Capstone, but internet access could be an issue. If that doesn’t work, I could manually create a stub unpacker in Python. Based on the initial analysis, it looks like an UPX-like unpacker, which handles decompression and relocation fixes. I'll first emulate the unpacker and execute it to fill the CTF0 section, then address relocs and imports.### Understanding the UPX-like depack routine

It looks like this is a standard UPX-like unpacker. When the call reaches 415740, the process pops r11, which stores the return address and then begins loop operations. The code alternates between reading and parsing tokens, adjusting rsi, and using the carry flag (CF) to determine literal values or match lengths. This routine seems to be well-structured for depacking.### Analyzing code patching and function calls

It looks like the code is patching a function at address 4158fe by replacing some bytes with 0xFC, which corresponds to a cld instruction, possibly to fix or restore functionality. After that, it calls the function at 4158fe, using specific register values. I need to inspect the code beyond 4158ff to fully understand what's happening after.

bash -lc objdump -d -M intel --start-address=0x4158f0 --stop-address=0x4159c0 /mnt/data/challenge.exe | sed -n '1,220p'

Unpacking strategy and further analysis

After patching 4158fe to cld, the code seems to enter some TLS callback loop, but this might not be relevant for the flag. After that, I could jump to the original entry point (OEP) at 0x4014e0. Perhaps unpacking just the region and parsing code will suffice without full import resolution. Emulating or analyzing the unpacked code directly could also work. It might be easiest to create an unpacker stub, dump memory, and disassemble to inspect the code further.

bash -lc which ndisasm || which gdb

Exploring unpacking strategy and tools

I think I could use objdump on the raw binary using specific flags like --adjust-vma and -b binary -m i386:x86-64 once the unpacked region is saved to a file. To implement the decompressor, I might find an open-source UPX NRV2B/NRV2E decompressor, though I don't need web access for this. Maybe I could check for an existing UPX installation.

bash -lc which upx || which upx-ucl

Planning manual decompression algorithm

Since UPX isn't available, I’ll try manually implementing the decompression. I could inspect the CTF2 for strings and tables, though they might not be directly useful. Instead, I’ll focus on translating the assembly of the callback function (0x4156f0) to Python. This function returns a bit from bitstream, so I need to simulate the assembly precisely rather than using higher-level methods.

Step-by-step manual decompression logic

I need memory access, with input loaded from the file at 0x400000, and an output buffer initialized at 0x401000. For stack values, I can use a Python list to simulate minimal stack pushes. I’ll simulate the decompression by stepping through the assembly. In the callback (4156f0), I'll carefully handle bits and sequential logic with add/adc. The callback returns CF and the current byte without consuming unless a new dword is loaded.### Manually emulating instructions and decompression logic

I need to translate the assembly steps carefully, like handling the carry flag in operations. Instead of decoding full x86 instructions, I can directly emulate each instruction in Python, focusing on the simple ones I need. I'll represent registers in 64-bit and flags (CF, ZF), translating one instruction at a time. After decompression, I'll handle fixups with either a high-level or exact approach, using a manual Python emulator for the stub and loading sections into memory. The section base will be placed into a bytearray at their respective offsets.### Implementing precise instructions handling

I need to handle ebx = ebx*2 + CF and set the carry flag properly, making sure I account for 32-bit wrapping. I'll create helper functions for add32 and adc32. There's sign extension for movsxd rbp,eax to 64, and I need to carefully handle comparisons like cmp rbp, <value> for proper carry flag emulation. It looks like the comparison logic is tricky, especially involving unsigned offsets. It might be easier to implement a high-level emulation for the bytecopy at address 415702.### Focusing on decryption and output

The bytecopy function handles the leftover byte by first storing the byte at the old rax, then incrementing rax for the next byte. I want to implement the decompression logic first, using the termination condition of eax xor -1 == 0. After decompression, I might restore a value at 0x105fc and handle relocations. Let's start by trying to decompress and then check for any readable strings—such as flags—immediately after.

bash -lc python - <<'PY' import struct from pathlib import Path p=Path('/mnt/data/challenge.exe').read_bytes()

parse sections

import struct base=0x400000 size_image=0x17000 mem=bytearray(size_image)

copy raw section 1 and 2

from earlier parse

CTF1 raw 0x200 size 0x6a00 -> va 0xf000

mem[0xf000:0xf000+0x6a00]=p[0x200:0x200+0x6a00]

CTF2 raw 0x6c00 size 0x200 -> va 0x16000

mem[0x16000:0x16000+0x200]=p[0x6c00:0x6c00+0x200]

def u32(x): return x & 0xffffffff def add32(a,b): res=(a+b)&0xffffffff cf=(a+b)>>32 zf=(res==0) return res,cf,zf

def adc32(a,b,cf0): s=a+b+cf0 res=s & 0xffffffff cf=1 if s>>32 else 0 zf=(res==0) return res,cf,zf

def add64(a,b): s=(a+b) & 0xffffffffffffffff cf=1 if a+b > 0xffffffffffffffff else 0 zf=(s==0) return s,cf,zf

init regs (RVA based)

rsi=0xf025 rdi=0x1000 rax=0x105fc stack=[]

push qword [rax]

stack.append(struct.unpack_from('<Q', mem, rax)[0]) stack.append(rax) stack.append(rdi) ebx=0 ecx=0 rbp=0xffffffffffffffff

emulate call 415740 with return address 0x4156f0 (but only symbolic callback)

def load_u8(addr): if addr<0 or addr>=len(mem): raise Exception(f'read8 OOB {addr:x}') return mem[addr] def load_u32(addr): if addr<0 or addr+4>len(mem): raise Exception(f'read32 OOB {addr:x}') return struct.unpack_from('<I', mem, addr)[0] def store_u8(addr,val): if addr<0 or addr>=len(mem): raise Exception(f'write8 OOB {addr:x}') mem[addr]=val&0xff def store_u32(addr,val): if addr<0 or addr+4>len(mem): raise Exception(f'write32 OOB {addr:x}') struct.pack_into('<I', mem, addr, val & 0xffffffff)

callback 4156f0: returns dl and CF, updates rsi/ebx

note uses current flags from prior instruction? no independent.

def cb(rsi,ebx): ebx,cf,zf=add32(ebx,ebx) if zf: ebx=load_u32(rsi) rsi=(rsi+4) & 0xffffffffffffffff ebx,cf,zf=adc32(ebx,ebx,cf) dl=load_u8(rsi) return rsi,ebx,dl,cf

copy function 415702

def copy_match(rdi, rbp, ecx): # rax = rdi+rbp rax=(rdi + (rbp if rbp < (1<<63) else rbp - (1<<64))) & 0xffffffffffffffff dl=load_u8(rax) if ecx > 5 and not (rbp > 0xfffffffffffffffc): # unsigned compare ja to -4 ecx=(ecx-4)&0xffffffff while True: edx=load_u32(rax) rax=(rax+4)&0xffffffffffffffff ecx=(ecx-4)&0xffffffff store_u32(rdi,edx) rdi=(rdi+4)&0xffffffffffffffff if ecx < 0x80000000: # jae after sub means no borrow => result unsigned >=0? In 32-bit, jae if CF=0, equivalent old ecx>=4. After subtraction, easiest check if ecx not wrapped. Here if ecx <= original? Use sign not enough. # this is wrong placeholder pass break # placeholder # fallback bytecopy exact # but for now simple generic copy ecx bytes # this may be enough if no overlap issues? yes overlapping handled by byte-by-byte when small, else dword writes okay maybe. # We'll implement generic byte copy for now. src=(rdi + (rbp if rbp < (1<<63) else rbp - (1<<64))) & 0xffffffffffffffff for i in range(ecx): store_u8(rdi+i, load_u8(src+i)) return (rdi+ecx)&0xffffffffffffffff

count=0 while True: count+=1 if count>20000000: raise Exception('too many') dl=load_u8(rsi) ebx,cf,zf=add32(ebx,ebx) if zf: ebx=load_u32(rsi) rsi+=4 ebx,cf,zf=adc32(ebx,ebx,cf) dl=load_u8(rsi) if cf: # literal rsi+=1 store_u8(rdi,dl) rdi+=1 continue eax=(ecx+1)&0xffffffff # call cb rsi,ebx,dl,cf=cb(rsi,ebx) eax,cf,zf=adc32(eax,eax,cf) while True: ebx,cf2,zf=add32(ebx,ebx) if zf: ebx=load_u32(rsi) rsi+=4 ebx,cf2,zf=adc32(ebx,ebx,cf2) dl=load_u8(rsi) if cf2: # jae false => break when cf=1 break # loop rsi,ebx,dl,cf=cb(rsi,ebx) eax,cf,zf=adc32(eax,eax,cf) eax=(eax-3)&0xffffffff if eax < 0x80000000 and eax < 0x100000000: # this doesn't test borrow. need old eax>=3. pass # Let's use old value. oldeax=eax # Actually after sub, jb if borrow => original <3. recover?

text
# We'll restart with more accurate below
break

print('stopped at',count,'rsi',hex(rsi),'rdi',hex(rdi),'eax',hex(eax),'dl',hex(dl)) PY

Clarifying decompression process and bit retrieval

I need a precise decompression implementation. I recognize patterns from the UPX depacker and think it’s better to study its known pseudocode rather than searching the web. The algorithm seems to be NRV2B/2D. I'll reconstruct it from assembly, focusing on the bit retrieval logic. Specifically, the getbit function, which involves manipulating ebx, rsi, and using the carry flag CF to return the next bit.### Reviewing length calculation and logic in loop

I can see the pattern forming. In this loop, it looks like the length is derived based on getbit(), increasing until it reaches a non-zero state. The conditional adc ecx, eax adds the carry flag based on the comparison between rbp and -0xd00. When rbp is a large negative number, this results in an extra byte being added to length. Interesting!### Analyzing decompression logic and implementing approach

Having reviewed the decryption mechanics, I see that m_len is conditioned on offsets, with a check being added based on a certain threshold. The rbp value is derived from the sign extension of eax. Emulating the UPX decompressor appears a feasible choice, perhaps utilizing existing tools like the pyUPX unpacker or directly emulating the instructions. This could help streamline the process!### Finalizing unpacking with correct instructions

I’ll implement the exact labels and instructions from the specified range to ensure an accurate unpacking process. I’m focusing on proper flag handling and step-by-step execution from the 415740 to 4157c6 range, then using a break condition to finalize the unpacking. Time to dive into coding the logic!

bash -lc python - <<'PY'

same emulator with debug on fault

import struct from pathlib import Path p=Path('/mnt/data/challenge.exe').read_bytes() size_image=0x17000 mem=bytearray(size_image) mem[0xf000:0xf000+0x6a00]=p[0x200:0x200+0x6a00] mem[0x16000:0x16000+0x200]=p[0x6c00:0x6c00+0x200] MASK64=(1<<64)-1; MASK32=(1<<32)-1

def load8(a): if not (0 <= a < len(mem)): raise Exception(f'read8 {a:x}') return mem[a] def load32(a): if not (0 <= a+4 <= len(mem)): raise Exception(f'read32 {a:x}') return struct.unpack_from('<I',mem,a)[0] def load64(a): if not (0 <= a+8 <= len(mem)): raise Exception(f'read64 {a:x}') return struct.unpack_from('<Q',mem,a)[0] def store8(a,v): if not (0 <= a < len(mem)): raise Exception(f'write8 {a:x}') mem[a]=v&0xff def store32(a,v): if not (0 <= a+4 <= len(mem)): raise Exception(f'write32 {a:x}') struct.pack_into('<I',mem,a,v&MASK32) def add32(x,y): s=(x&MASK32)+(y&MASK32) return s&MASK32, 1 if s>>32 else 0, 1 if (s&MASK32)==0 else 0 def adc32(x,y,cf0): s=(x&MASK32)+(y&MASK32)+(cf0&1) return s&MASK32, 1 if s>>32 else 0, 1 if (s&MASK32)==0 else 0 def sub32(x,y): x&=MASK32; y&=MASK32 res=(x-y)&MASK32 return res, 1 if x<y else 0, 1 if res==0 else 0 def cmp64(x,y): x&=MASK64; y&=MASK64 res=(x-y)&MASK64 return 1 if x<y else 0, 1 if res==0 else 0 def sext32(x): x&=MASK32 return x if x<0x80000000 else x-(1<<32) def u64(x): return x&MASK64 regs=dict(rax=0x105fc,rbx=0,rcx=0,rdx=0,rsi=0xf025,rdi=0x1000,rbp=MASK64,r11=0) stack=[load64(0x105fc)] store32(0x105fc,0x3543597f) stack.append(0x105fc); stack.append(0x1000); stack.append(0x4156f0) CF=0;ZF=0;pc=0x415740;steps=0 hist=[] try: while True: steps+=1 hist.append((steps,pc,regs.copy(),CF,ZF,list(stack))) if len(hist)>50: hist.pop(0) if pc==0x415740: regs['r11']=stack.pop(); pc=0x41574d elif pc==0x415745: regs['rsi']=u64(regs['rsi']+1); store8(regs['rdi'],regs['rdx']); regs['rdi']=u64(regs['rdi']+1); pc=0x41574d elif pc==0x41574d: regs['rdx']=load8(regs['rsi']); regs['rbx'],CF,ZF=add32(regs['rbx'],regs['rbx']); pc=0x415751 elif pc==0x415751: if ZF==0: pc=0x41575d else: regs['rbx']=load32(regs['rsi']); regs['rsi']=u64(regs['rsi']+4); regs['rbx'],CF,ZF=adc32(regs['rbx'],regs['rbx'],CF); regs['rdx']=load8(regs['rsi']); pc=0x41575d elif pc==0x41575d: if CF==1: pc=0x415745 else: regs['rax']=(regs['rcx']+1)&MASK64; stack.append(0x415765); pc=regs['r11'] elif pc==0x4156f0: regs['rbx'],CF,ZF=add32(regs['rbx'],regs['rbx']); pc=0x4156f2 elif pc==0x4156f2: if ZF==1: pc=0x4156f6 else: pc=stack.pop() elif pc==0x4156f6: regs['rbx']=load32(regs['rsi']); regs['rsi']=u64(regs['rsi']+4); regs['rbx'],CF,ZF=adc32(regs['rbx'],regs['rbx'],CF); regs['rdx']=load8(regs['rsi']); pc=stack.pop() elif pc==0x415765: regs['rax'],CF,ZF=adc32(regs['rax'],regs['rax'],CF); regs['rbx'],CF,ZF=add32(regs['rbx'],regs['rbx']); pc=0x415769 elif pc==0x415769: if ZF==0: pc=0x415775 else: regs['rbx']=load32(regs['rsi']); regs['rsi']=u64(regs['rsi']+4); regs['rbx'],CF,ZF=adc32(regs['rbx'],regs['rbx'],CF); regs['rdx']=load8(regs['rsi']); pc=0x415775 elif pc==0x415775: if CF==0: stack.append(0x415765); pc=regs['r11'] else: regs['rax'],CF,ZF=sub32(regs['rax'],3); pc=0x41577a elif pc==0x41577a: if CF==1: pc=0x41578f else: regs['rax']=(regs['rax']<<8)&MASK32; regs['rdx']&=0xff; regs['rax']=(regs['rax']|regs['rdx'])&MASK32; regs['rsi']=u64(regs['rsi']+1); regs['rax']^=MASK32; if regs['rax']==0: pc=0x4157c6 else: regs['rbp']=u64(sext32(regs['rax'])); pc=0x41578f elif pc==0x41578f: regs['rax']=(regs['rcx']+1)&MASK64; stack.append(0x415795); pc=regs['r11'] elif pc==0x415795: regs['rcx'],CF,ZF=adc32(regs['rcx'],regs['rcx'],CF); stack.append(0x41579a); pc=regs['r11'] elif pc==0x41579a: regs['rcx'],CF,ZF=adc32(regs['rcx'],regs['rcx'],CF); if ZF==0: pc=0x4157b6 else: regs['rcx']=regs['rax']&MASK32; regs['rax']=(regs['rax']+2)&MASK32; pc=0x4157a3 elif pc==0x4157a3: stack.append(0x4157a6); pc=regs['r11'] elif pc==0x4157a6: regs['rcx'],CF,ZF=adc32(regs['rcx'],regs['rcx'],CF); regs['rbx'],CF,ZF=add32(regs['rbx'],regs['rbx']); pc=0x4157aa elif pc==0x4157aa: if ZF==0: pc=0x4157b4 else: regs['rbx']=load32(regs['rsi']); regs['rsi']=u64(regs['rsi']+4); regs['rbx'],CF,ZF=adc32(regs['rbx'],regs['rbx'],CF); pc=0x4157b4 elif pc==0x4157b4: if CF==0: pc=0x4157a3 else: pc=0x4157b6 elif pc==0x4157b6: CF,ZF=cmp64(regs['rbp'],0xfffffffffffff300); regs['rcx'],CF,ZF=adc32(regs['rcx'],regs['rax'],CF); stack.append(0x4157c4); pc=0x415702 elif pc==0x415702: regs['rax']=u64(regs['rdi'] + sext32(regs['rbp'])) ecx=regs['rcx']&MASK32 CF=1 if ecx<5 else 0; ZF=1 if ecx==5 else 0 regs['rdx']=load8(regs['rax']) if CF or ZF: pc=0x41572e else: CF=1 if regs['rbp'] < 0xfffffffffffffffc else 0; ZF=1 if regs['rbp']==0xfffffffffffffffc else 0 if not (CF or ZF): pc=0x41572e else: regs['rcx'],CF,ZF=sub32(regs['rcx'],4); pc=0x415716 elif pc==0x415716: regs['rdx']=load32(regs['rax']); regs['rax']=u64(regs['rax']+4); regs['rcx'],CF,ZF=sub32(regs['rcx'],4); store32(regs['rdi'],regs['rdx']); regs['rdi']=u64(regs['rdi']+4); pc=0x415716 if CF==0 else 0x415727 elif pc==0x415727: regs['rcx']=(regs['rcx']+4)&MASK32; regs['rdx']=load8(regs['rax']); pc=stack.pop() if regs['rcx']==0 else 0x41572e elif pc==0x41572e: regs['rax']=u64(regs['rax']+1); store8(regs['rdi'],regs['rdx']); regs['rcx'],CF,ZF=sub32(regs['rcx'],1); regs['rdx']=load8(regs['rax']); regs['rdi']=u64(regs['rdi']+1); pc=0x41572e if ZF==0 else stack.pop() elif pc==0x4157c4: pc=0x41574d elif pc==0x4157c6: print('done'); break else: raise Exception(f'bad pc {pc:x}') except Exception as e: print('Exception',e) for ent in hist: st,pc,r,cf,zf,stk=ent print('step',st,'pc',hex(pc),'cf',cf,'zf',zf,'rax',hex(r['rax']),'rbx',hex(r['rbx']),'rcx',hex(r['rcx']),'rdx',hex(r['rdx']),'rsi',hex(r['rsi']),'rdi',hex(r['rdi']),'rbp',hex(r['rbp']),'stacktop',hex(stk[-1]) if stk else None,'stacklen',len(stk)) PY

Troubleshooting potential offset decoding issue

Hmm, I’m seeing that rbp turning negative could cause issues when copying the source. The calculated offset leads to an impossible memory reference, indicating a bug in the emulation or decoding process. I’m considering whether the algorithm might reference bytes before the output start, but that seems unlikely. I’ll need to reassess earlier copy length handling.### Investigating callback and stack behavior

Not using ja and opting for dword loop makes sense for now. A bigger concern might be the stack handling during the callback at 4156f2. If the stack pops return addresses and nested function calls aren't handled properly, this could create issues. The adc ecx,eax at 7bd might be a bit tricky too with flag usage after comparison.

Understanding memory initialization and header references

It seems there may be an issue with memory initialization. CTF0 is zeroed, but the decompressor location might not be at 0x1000. The RVAs below 0x1000 point to the PE headers, which are loaded with all zeros from file headers. This could explain the negative offsets and back-references. The unpacker might reference data in the header region.### Examining memory layout and unpacking flow

I found that the offset calculation results in a negative value, which suggests the data isn't from the headers. The packed data source uses absolute addresses, including the base. The compressed stream pointer or memory layout might be incorrect, though it's possible the embedded error string is part of an anti-unpacking mechanism.

Exploring unpacking solutions

I’m considering using the actual UPX decompressor if it's UPX-like, or finding a script to extract the decompress function from the stub. Maybe I could also check string patterns with objdump. I wonder if using tools like pyinstxtractor, QEMU with GDB, or possibly angr could help here. Let's investigate!

bash -lc python - <<'PY' import importlib.util mods=['angr','miasm','z3','unicorn','capstone','rizin','pefile','pwntools','pwn'] for m in mods: print(m, bool(importlib.util.find_spec(m))) PY

Exploring unpacking bugs

I’m considering using a static heuristic to extract the compressed CTF0 stream and check for the flag, but that might not lead anywhere. I’m also thinking about comparing our approach with UPX pseudocode on memory to see if we missed a detail, especially with how we reset or handle ecx in the decompression loop.

Debugging bit, stack, and memory handling

I’m thinking that the rbp=-27 comparison works fine here, but there might be an issue with how bits are handled in mov dl,[rsi]. After digging deeper, I suspect that the memory loading could be misinterpreting endianess. Also, something might be off with our stack handling, especially when return addresses are being manipulated.

bash -lc apt-cache policy upx-ucl upx 2>/dev/null | sed -n '1,120p'

bash -lc apt-cache search '^upx' 2>/dev/null | sed -n '1,40p'

Searching for UPX decompression algorithm details

I’m not sure if I can use an apt package here, but maybe I can use the web to look up some details about the UPX decompression algorithm. I’ll search for specifics on the UPX stub assembly or pseudocode since I’m unclear on the exact algorithm. Let's look it up!

{"search_query":[{"q":"UPX NRV2B decompression algorithm assembly stub adc eax eax jae 762 eax xor ffffffff","domains":["github.com"]},{"q":"UPX unpack stub 415740 adc eax eax jae 762 cmp rbp f300 copy_match", "domains":["github.com"]}],"response_length":"short"}

Using Python decompressor from gist

I found a Python-based full decompressor on a gist, which looks promising. I’ll open it and see if I can use the content for what I need. It seems like a viable option to proceed with for UPX decompression! Let me explore it further.

{"open":[{"ref_id":"turn391082search4"}]}

Adjusting algorithm for main stub

This new algorithm matches well, but there’s a difference in how mOff is calculated. It starts with mOff=2+gb after getBit, whereas in my assembly, eax starts with ecx+1. I might need to adjust by adding 1 to mOff during calculations for accuracy. Let's go step by step through the assembly.

Analyzing mPos and offset behavior

In this assembly, we see how mPos = dPos - mOff is calculated. The borrow during eax -= 3 happens when eax is less than 3, which leads to reusing rbp. It looks like our earlier issue was due to initializing lastMOff as -1 when it should start as 1. Still wondering if a large negative offset could appear earlier on.### Trying a high-level approach

I’m thinking it might be better to rewrite using a high-level gist algorithm and compare it with the exact version. The assembly should be roughly equivalent. We can start by decoding the compressed bytes from 0xf025 up to the end marker. Hopefully, this works and eliminates the need for an exact emulator. Let's give it a try.

bash -lc python - <<'PY' import struct, re from pathlib import Path p=Path('/mnt/data/challenge.exe').read_bytes() enc = p[0x225:0x225+0x6a00-0x25]

def getbit(pos,data,four,count): if count == 0: count = 31 if pos+4>len(data): raise EOFError four = struct.unpack('<I', data[pos:pos+4])[0] pos += 4 else: count -= 1 bit = (four >> count) & 1 return bit,pos,four,count

def decompress(data): out=bytearray() s=0; d=0; lastMOff=1; shift=0; four=0 while True: if s >= len(data): return out gb,s,four,shift=getbit(s,data,four,shift) while gb != 0: out.append(data[s]); s+=1; d+=1 gb,s,four,shift=getbit(s,data,four,shift) gb,s,four,shift=getbit(s,data,four,shift) mOff = 2 + gb gb,s,four,shift=getbit(s,data,four,shift) while gb == 0: gb,s,four,shift=getbit(s,data,four,shift) mOff = 2mOff + gb gb,s,four,shift=getbit(s,data,four,shift) if mOff == 2: mOff = lastMOff else: mOff = (mOff - 3) * 256 + data[s] s += 1 if mOff == 0xffffffff: break mOff += 1 lastMOff = mOff mLen,s,four,shift=getbit(s,data,four,shift) gb,s,four,shift=getbit(s,data,four,shift) mLen = mLen2 + gb if mLen == 0: mLen += 1 gb,s,four,shift=getbit(s,data,four,shift) mLen = 2mLen + gb gb,s,four,shift=getbit(s,data,four,shift) while gb == 0: gb,s,four,shift=getbit(s,data,four,shift) mLen = 2mLen + gb gb,s,four,shift=getbit(s,data,four,shift) mLen += 2 if mOff > 0xd00: mLen += 1 mPos = d - mOff if mPos < 0: raise Exception(f'mPos negative {mPos} d={d} mOff={mOff} s={s}') out.append(out[mPos]); mPos +=1; d +=1 while mLen > 0: mLen -=1 out.append(out[mPos]); d +=1; mPos +=1 return out

out=decompress(enc) print('outlen',len(out)) Path('/mnt/data/decomp.bin').write_bytes(out) for m in re.finditer(rb'[ -~]{4,}', out): s=m.group().decode('latin1') if any(k in s.lower() for k in ['flag','ctf','wrong','correct','input','usage']): print(hex(m.start()), s) print(out[:64]) PY

Checking decompressed data and next steps

The high-level decompressor seems to work, and the output length is 79335, which is quite large. This might correspond to the unpacked image starting from memory at 0x401000. I'll need to check for relocation or import fixes and see if call/jmp fixups are necessary. It looks like there are multiple functions in the first part, so I plan to disassemble the raw binary starting at 0x401000 to locate the OEP. Let's use objdump for this.

bash -lc objdump -D -b binary -m i386:x86-64 -M intel --adjust-vma=0x401000 /mnt/data/decomp.bin | sed -n '300,520p'

Fixing imports and analyzing possible self-checks

The disassembly shows calls with wrong targets, likely because the required fixups and imports aren’t applied yet. I think I see a function around 0x401550, possibly checking for the 'CTF!' signature, which could be part of an anti-unpacking or self-check. I’ll need to locate the main interaction and apply the fixes to make calls valid. I might also search for strings after decompression, so I'll run a strings command on the decompressed file to gather more info.

bash -lc strings -a -n 4 /mnt/data/decomp.bin | sed -n '1,200p'

Analyzing main logic using visible imports

The strings indicate error messages, base64 alphabet, and possibly a flag or password. Now that I can see imports through the IAT strings, it’s time to analyze the main logic. I’ll focus on finding references to strings like "Error reading password" and "Invalid password!". Applying the fixups should resolve the incorrect direct calls. Afterward, I’ll likely reconstruct imports from the IAT and adjust them where necessary. I'll start with a high-level script for simplicity.### Considering UPX filter and possible fixup approach

For the E8/E9 case, before reaching 803, al is byte 0xE8, so it’s either 0 or 1. After that, I subtract 2 from the low byte of eax. If the result isn't zero, I continue; otherwise, I do some other operations like bswap. This part is likely the UPX filter for relative addresses. But maybe I don’t need the exact implementation — instead, I can use a known algorithm for the E8/E9 or 0F8x sequences, checking if the lower byte matches 0 or 1. Alternatively, using fixup assembly on the decompressed bytes might be easier.

Analyzing stack usage and register flow

I’m seeing some odd stack juggling in this code. It starts with rbx and `### Analyzing stack behavior and potential issues

The code seems to load eax from [rsi] and increment rsi. Then sub al, dl; jne 7ed checks a condition. If jne occurs, it suggests rsi was saved on the stack, but no pop happens, leading to possible stack growth. However, after jumping to 7ed, the stack might not be balanced correctly. At 80e, pop rdi happens only when successful. The stack could accumulate values unless lodsd at 809 correctly handles rsi.

Clarifying stack usage and flow

It looks like the code relies on the stack pointer temporarily, with rsi pushed at 808. If the jne path is taken, the old rsi should still be on the stack. However, it seems possible that the function directly ends or most candidates succeed before cleaning up. Later, pop rbx, pop rsi at 81e suggests a balance. Each candidate likely hits success eventually.### Locating string references and addresses

I need to locate the main part of the binary by searching for string references like "Error: Invalid password!". Since the decompressed binary includes the full PE structure, I could use objdump to search for RVA of strings and map addresses. By calculating VA from offsets, I can narrow down relevant code. Time to write a script.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/decomp.bin').read_bytes() for s in [b'Error reading password.', b'Error: Invalid password!', b'Illegal modification or unpacking detected.', b'%s%s', b'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/']: idx=b.find(s) print(s.decode('latin1'), hex(idx), hex(0x401000+idx) if idx!=-1 else None) PY

Searching for xrefs to specific addresses in disassembly

I need to search for cross-references to the address 0x405034 in the disassembly. Since raw binary disassembly doesn't annotate addresses, I can use objdump -D and then grep for 405034. This will help identify where those addresses are used with lea rcx/rdx, rip+disp.

bash -lc objdump -D -b binary -m i386:x86-64 -M intel --adjust-vma=0x401000 /mnt/data/decomp.bin | grep -n '40503|40504|40cb60' | sed -n '1,120p'

Examining disassembly at specific addresses

I need to examine the disassembly around the addresses 0x4031e0 and 0x403203 to understand the xrefs. I'll focus on the area around 0x4031xx to gather relevant data and gain insights into the surrounding context of these references.

bash -lc objdump -D -b binary -m i386:x86-64 -M intel --adjust-vma=0x401000 /mnt/data/decomp.bin | sed -n '2500,2575p'

Inspecting function around password reading

It seems like the function around address 0x4030?? could be related to password reading, but I need more context. I'll inspect the code starting from above address 0x40314b to gather more details and understand the function's behavior better. This will help clarify what's going on.

bash -lc objdump -D -b binary -m i386:x86-64 -M intel --adjust-vma=0x401000 /mnt/data/decomp.bin | sed -n '2430,2510p'

Investigating function at 4030a0

I need to better understand the function at address 0x4030a0. It likely opens a temporary file and constructs a path using a combination of a temp path and something from 0x40cb30. It might then read a password from a resource. I’ll locate strings around 0x40cb30 to see if I can clarify this.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/decomp.bin').read_bytes() for va in [0x40cb30,0x405080,0x40cb60]: off=va-0x401000 print(hex(va), b[off:off+120]) PY

Investigating base64 and temp file creation

It looks like the function at 0x405080 starts with a base64 encoding of an MZ/PE header. Maybe it decodes an embedded PE or base64-encoded data. The function around 0x4030a0 could handle temp file creation from a base64 string, followed by reading the password. I’ll need to search for references to 0x40cb30 and 0x405080 for more insights, especially around 0x40317d—that might be writing the blob.

bash -lc objdump -D -b binary -m i386:x86-64 -M intel --adjust-vma=0x401000 /mnt/data/decomp.bin | sed -n '2510,2640p'

Exploring embedded PE handling and password extraction

I’m planning on analyzing the PE file, possibly embedded at 0x405080 as base64. The password might be extracted by writing a temp file and executing it. The function around 0x4030a0 seems to do checks and anti-unpacking before proceeding. I'll next analyze the strings near 0x40504c and 0x40506c to explore further.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/decomp.bin').read_bytes() for va in [0x40504c,0x40506c]: off=va-0x401000 print(hex(va), b[off:off+40], b[off:off+40].split(b'\0')[0]) PY

Analyzing password validation and potential flag retrieval

I think the password comparison happens at 0x4030fb, where the input is checked against the string "NGeQwv8eCRpINEcO". If it's correct, the program continues to create a temp file and write the embedded payload. The password seems likely to be that string, but I’ll need to verify if the flag is hidden after entering it. Also, I’ll search for a possible user prompt to ensure I’m not missing a part of the process.

Searching for main or function references

I need to check if 0x4030a0 is referenced from the main function. Since the direct calls are unresolved, it seems like there might be imports or IAT stubs close to 0x403000. I'll search for the address either in raw bytes or by using disassembly references like 0x4030a0.

bash -lc objdump -D -b binary -m i386:x86-64 -M intel --adjust-vma=0x401000 /mnt/data/decomp.bin | grep -n '4030a0|0x4030a0' | sed -n '1,40p'

Analyzing embedded payload and imports

It seems there might be no cross-references due to unresolved relative calls. I want to reconstruct things with a call fixup loop. I'll need to check if an embedded payload holds the real flag, maybe. There's a function at 0x403220 which could be handling base64 decoding, where it takes a string input in rbx and outputs decoded bytes. Let's break down some imports and functions like GetModuleFileNameA to confirm path retrieval.### Inspecting import table and embedded PE

I could examine 0x411274 to see if it's the first MSVCRT import, but it's easier to build the import table from the decompressed PE header embedded in the strings. The raw decompressed file seems to start at 0x401000, so that's tricky. There’s likely a Base64-encoded PE payload hidden elsewhere in the binary. Analyzing that section might help, but the headers seem incomplete for parsing just yet.

Inspecting code around 4030a0 more closely

The import table resolves at start + 0x13000, with IAT entries at 0x15000. I could read the import descriptor from the decompressed data to identify addresses, but I only need key function signatures for code semantics. At 4030a0, there’s an anti-unpack check returning a boolean. I'll also look at 4030b2, and check the GetModuleFileNameA call for the exe path. The logic around the error handling is still unclear but probably involves the "!" string after a newline.### Analyzing password verification and input handling

It looks like rbx doesn't store the exe path — it likely holds the input line from stdin. The program calls fgets to read input, using __iob_func or __acrt_iob_func. Then, strcspn is used to check for a newline, followed by comparison to the password. On success, it creates buffers and calls GetTempPathA. There's also a random string function to be explored.### Decoding and analyzing the potential flag

It seems the password "NGeQwv8eCRpINEcO" is likely used to unlock the payload. There might be a secondary executable or payload in the process. Since it's a CTF challenge, the password might be sufficient, but I'll follow through. I'll decode the base64 blob and check if it reveals something more obvious, like another executable or flag.

bash -lc python - <<'PY' from pathlib import Path import base64 b=Path('/mnt/data/decomp.bin').read_bytes() start=0x405080-0x401000

take until first non-b64? let's find contiguous region

alphabet=b'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=' end=start while end < len(b) and b[end] in alphabet: end +=1 s=b[start:end] print('b64 len',len(s)) data=base64.b64decode(s) print('decoded len',len(data), data[:2]) Path('/mnt/data/stage2.bin').write_bytes(data) PY

Analyzing stage2.bin

I need to inspect the stage2.bin file to determine whether it's a PE file. I'll start by analyzing its contents and looking for any embedded strings to get a sense of its structure and what it could contain. There's a chance it holds the next part of the puzzle.

bash -lc file /mnt/data/stage2.bin && strings -a -n 4 /mnt/data/stage2.bin | sed -n '1,220p'

bash -lc objdump -x /mnt/data/stage2.bin | sed -n '1,220p'

Finding string references

I need to locate the string "Enter the key" and find its cross-references in the code. The first step is to get the virtual address (VA) of this string. Once I have that, I can follow the references to understand how it's being used, which might help in the analysis.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/stage2.bin').read_bytes() for s in [b'Enter the key: ', b'Error reading input.']: idx=b.find(s) print(s, hex(idx), hex(0x400000+idx) if idx!=-1 else None) PY

Searching for references

I need to search for references, specifically cross-references (xrefs) that might point to the target data or function. I'll need to track these references to understand how this particular element is interacted with in the code. This should help guide the next steps in the analysis process.

bash -lc objdump -d -M intel /mnt/data/stage2.bin | grep -n '403c00|403c10' | sed -n '1,80p'

Searching for specific references in disassembly

I want to analyze the PE file and adjust the objdump approach, since it may not show key information. I’ll use grep to look for RIP-relative calls and .rdata references, then use objdump with additional flags to focus on specific addresses. Searching for disassembly lines containing "3c0" might reveal more useful details.

bash -lc objdump -d -M intel /mnt/data/stage2.bin | grep -n '3c00|3c10|3c2' | sed -n '1,120p'

Searching for references and analyzing disassembly

Instead of relying on a disassembler, I'll use strings -t x to check offsets and then search with objdump -drwC. I'll search for specific displacement hex patterns, like be 00 3c 40 00. It’s simpler to use custom xref scanning around known import addresses, like printf at 0x40c3a8. I'll just grep for it all.

bash -lc objdump -d -M intel /mnt/data/stage2.bin | grep -n '40c3a8|40c378|40c3b0|40c3c8' | sed -n '1,200p'

Looking for main function calls

It seems like the main function doesn’t directly call the IAT but instead uses stubs. I'll focus on finding calls to specific addresses—402fc8 for printf, 402fc0 for puts, 402fa8 for strcspn, and 402ff8 for fgets. Searching for calls to these addresses should help pin it down.

bash -lc objdump -d -M intel /mnt/data/stage2.bin | grep -n '402fc8|402fc0|402fa8|402ff8' | sed -n '1,200p'

Inspecting main function location

Okay, looks like the main function is around address 401550. I’ll inspect the range from 401550 to 4016bb in the stage2 PE file to get more details. This should help me understand how the program's execution flows in that part. Let’s dive into it.

bash -lc objdump -d -M intel --start-address=0x401540 --stop-address=0x4016c0 /mnt/data/stage2.bin | sed -n '1,260p'

Investigating function and strings

The function at 401550 seems to take an HWND or pointer, prints a prompt, reads input, checks its length, and if size is <=0xf0, it calls 404ef0 on the buffer. If it's successful, maybe it uses MessageBoxW. I’ll need to inspect strings around 407000 and dump them for further analysis.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/stage2.bin').read_bytes() for va in [0x407000,0x407010,0x407026,0x407034,0x407042,0x40705e,0x40706e,0x407082]: off=va-0x400000 s=b[off:off+80].split(b'\0')[0] print(hex(va), s) PY

Mapping virtual addresses to file offsets

I realize I need to map virtual addresses (VA) to file offsets, as these may be different due to how PE sections are organized. To do this, I'll use the section headers to map the VA to the correct file offsets. I can use objdump -h for this.

bash -lc objdump -h /mnt/data/stage2.bin | sed -n '1,80p'

Computing the file offset

To compute the offset correctly, I need to use the formula: offset = fileoff + (VA - section.vma). I'll use Python to handle this computation, which should allow me to work with the virtual addresses and section information to get the right file offset.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/stage2.bin').read_bytes() secs=[(0x401000,0x2298,0x400),(0x404000,0x1010,0x2800),(0x406000,0xd0,0x3a00),(0x407000,0x7d0,0x3c00),(0x408000,0x80,0x4400),(0x409000,0x3b4,0x4600),(0x40a000,0x32c,0x4a00),(0x40c000,0x87c,0x4e00),(0x40d000,0x68,0x5800),(0x40e000,0x10,0x5a00)] def get(va,n=80): for v,s,fo in secs: if v<=va< v+s: off=fo+(va-v) return b[off:off+n] return None for va in [0x407000,0x407010,0x407026,0x407034,0x407042,0x40705e,0x40706e,0x407082]: data=get(va,80) print(hex(va), data.split(b'\0')[0]) PY

Inspecting possible wide string references

I’m noticing some interesting single letters at certain addresses. They might be wide strings, which makes sense if MessageBoxW is involved, as it uses wide-character pointers. I’ll need to inspect the bytes around those addresses to confirm if they’re related to wide strings. Let’s dive into that.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/stage2.bin').read_bytes() secs=[(0x407000,0x7d0,0x3c00)] def off(va): return 0x3c00 + (va-0x407000) for va in [0x407020,0x407026,0x407030,0x407034,0x407040,0x407042,0x40705c,0x40705e,0x40706c,0x40706e,0x407080,0x407082]: o=off(va) print(hex(va), b[o:o+16]) PY

Understanding function flow with MessageBoxW

So, the function at 401550 prompts, reads input, checks the length, and calls 404ef0 as a validator. If it’s valid, it shows a MessageBoxW with “Success!” and the title “Success.” If invalid, it shows an error message. Now, I need to focus on solving what happens at 404ef0 and inspect it further.

bash -lc objdump -d -M intel --start-address=0x404ef0 --stop-address=0x405300 /mnt/data/stage2.bin | sed -n '1,360p'

Analyzing possible encrypted or obfuscated data in .hello section

It seems like 404ef0 isn’t code, but could be data or an encrypted blob, maybe part of a custom virtual machine. I need to check the .hello section closely. It might contain a validator or some encoded data fed into an interpreter. I'll also review calls around 401620 and 401623. This might involve examining disassembly, potential decoding, and determining if VirtualProtect or helpers are involved to decode this section.

bash -lc objdump -d -M intel --start-address=0x404000 --stop-address=0x404300 /mnt/data/stage2.bin | sed -n '1,320p'

Investigating possible custom VM and decryption routines

404ef0 could be an obfuscated blob, possibly interpreted by a custom virtual machine starting at 4016c0 or other relevant functions in the .text section. I'll need to further inspect this area for any decryption routines. This might involve searching for functions involved with the .hello section addresses, using tools like grep with relevant values.

bash -lc objdump -d -M intel /mnt/data/stage2.bin | grep -n '404ef0|404000|4041|4042|4043|4044|4045|4046|4047|4048|4049|404a|404b|404c|404d' | sed -n '1,200p'

Solving without full disassembly by focusing on data and algorithms

Instead of a full disassembly, I'll focus on embedded data and mydata. It might involve base64 decoding or a custom VM interpreter processing the .hello section. I’ll search for writes to this region, especially around addresses like 404000-405000, to uncover what could happen at runtime.

bash -lc objdump -d -M intel /mnt/data/stage2.bin | grep -n '404[0-9a-f][0-9a-f][0-9a-f]|4050' | sed -n '1,200p'

Focusing on algorithmic solution rather than full disassembly

Instead of dissecting the obfuscated blob, I'll focus on finding relevant functions around 4016c0. It might involve a generic transformation based on input. I'll inspect the area between 4016c0 and 4017d8 to identify patterns or functions that may lead to a solution.

bash -lc objdump -d -M intel --start-address=0x4016c0 --stop-address=0x401840 /mnt/data/stage2.bin | sed -n '1,260p'

Identifying potential RC4 usage in function

It looks like 4016c0 is not directly relevant since it pertains to PE export sections. However, 4017e0 seems to have an RC4 Key Scheduling Algorithm (KSA) function, which could indicate RC4 is being used in the validator. I need to continue by searching for where 4017e0 is being called or used.

bash -lc objdump -d -M intel /mnt/data/stage2.bin | grep -n '4017e0|401840|4018' | sed -n '1,120p'

Investigating RC4 encryption logic and possible decryption

It looks like 4018b0 handles the RC4 encryption and decryption using a specified key and data length. 4018f0, on the other hand, might be locating the imported API from the modules, potentially using that information to decrypt the blob. I need to examine the code between 4018f0 and 1a43 to confirm how it works.

bash -lc objdump -d -M intel --start-address=0x4018f0 --stop-address=0x401a50 /mnt/data/stage2.bin | sed -n '1,260p'

Exploring PE section header iteration and matching

So, 4018f0 is iterating through section headers, comparing names to a constant at 0x407094. If the section name isn’t ".mydata", it continues. It appears to be checking the section table, with VirtualAddress at an offset. I’ll need to review the strings at 407094 and surrounding addresses for further clarity.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/stage2.bin').read_bytes()

helper convert VA to file offset for .rdata

for va in [0x407094,0x40709c,0x4070c0]: off=0x3c00 + (va-0x407000) print(hex(va), b[off:off+64]) PY

Understanding section decryption mechanism

Okay, 401a10 calls 4018f0 to process sections, checking names like ".mydata" and ".hello". If it matches ".mydata", it starts a pattern comparison and decrypts the section using RC4 with a 32-byte key at 0x4070c0. It uses section header fields to determine the appropriate section sizes. This means that before decryption, sections are encrypted — their disassembled form confirms this. Now, I just need to emulate this behavior and find the section sizes to continue.### Decryption process for stage2.bin

Alright, looks like the goal is to restore VirtualProtect and decrypt .mydata and .hello sections in place using the 32-byte RC4 key. After that, I’ll inspect function 404ef0 again. Now it’s clear—time for the script to handle the decryption. I just need to implement this next step!

bash -lc python - <<'PY' from pathlib import Path import struct b=bytearray(Path('/mnt/data/stage2.bin').read_bytes())

section table parse

helper for VA->file offset

peoff=struct.unpack_from('<I',b,0x3c)[0] num=struct.unpack_from('<H',b,peoff+6)[0] opt=struct.unpack_from('<H',b,peoff+20)[0] sec_off=peoff+24+opt secs=[] for i in range(num): off=sec_off+i*40 name=b[off:off+8].rstrip(b'\0').decode('latin1') vsize,va,rawsize,rawptr=struct.unpack_from('<IIII',b,off+8) secs.append((name,va,vsize,rawptr,rawsize,off)) print(secs)

key at VA 0x4070c0

for name,va,vsize,rawptr,rawsize,off in secs: if name=='.rdata': rdata_rawptr=rawptr; rdata_va=va key_off=rdata_rawptr + (0x70c0 - rdata_va) key=bytes(b[key_off:key_off+0x20]) print('key',key.hex())

RC4

def rc4_crypt(data,key): S=list(range(256)) j=0 keylen=len(key) for i in range(256): j=(j + S[i] + key[i % keylen]) & 0xff S[i],S[j]=S[j],S[i] i=j=0 out=bytearray(data) for idx in range(len(out)): i=(i+1)&0xff j=(j+S[i])&0xff S[i],S[j]=S[j],S[i] K=S[(S[i]+S[j])&0xff] out[idx]^=K return out

for secname in ['.mydata','.hello']: for name,va,vsize,rawptr,rawsize,off in secs: if name==secname: print('decrypting',name,hex(va),hex(rawptr),hex(vsize),hex(rawsize)) data=b[rawptr:rawptr+rawsize] dec=rc4_crypt(data,key) b[rawptr:rawptr+rawsize]=dec # print first 16 bytes print(dec[:16].hex()) Path('/mnt/data/stage2_decrypted.bin').write_bytes(b) PY

Inspecting decrypted .hello disassembly

Now that I've decrypted the sections, it's time to inspect the disassembly of the .hello section. I'll focus specifically on address 404ef0—something is surely happening there, and I need to understand it fully. Let's dive in and check what’s going on there now that the data is decrypted!

bash -lc objdump -d -M intel --start-address=0x404ef0 --stop-address=0x405300 /mnt/data/stage2_decrypted.bin | sed -n '1,360p'

Analyzing validator logic

It looks like the validator function copies the input to a buffer starting at rsp+0x30, possibly adding a prefix byte. I need to inspect the function at 404cb0, which might be performing an MD5 operation. To confirm, I should check this function and analyze surrounding functions for more context.

bash -lc objdump -d -M intel --start-address=0x404c00 --stop-address=0x404ef0 /mnt/data/stage2_decrypted.bin | sed -n '1,360p'

Understanding buffer and function flow

After examining 404ef0, the function seems to copy input bytes into a buffer starting from rsp+0x30, when r9 is set to rsp+0x2f. The loop stores characters at specific positions until it reaches the null. Then, the function pads the input to a multiple of 16 bytes. It might be easier to replicate this process in Python using the decrypted sections and reverse the algorithm from these functions.### Understanding encryption and block comparison

The 404cb0 function takes data, a length, and keys (maybe key1 and key2) to encrypt the input buffer, producing ciphertext. It ensures only inputs that yield a total length of 16, after padding, are accepted. Then, it compares the 16-byte output with constants. Replicating the encryption directly can help, possibly utilizing symbolic or targeted methods, since the block cipher is deterministic. Using key properties might allow inversion or brute-forcing over a character set.

Examining .mydata for encryption constants

I’m considering whether this algorithm is custom, but I’m thinking brute-forcing may not be effective here, unless the charset is small. I should inspect data around 0x408001 for clues after decrypting the .mydata section. I’ll focus on dumping .mydata and the associated addresses to look for tables or constants.

bash -lc python - <<'PY' from pathlib import Path b=Path('/mnt/data/stage2_decrypted.bin').read_bytes()

section mapping

secs=[('.hello',0x404000,0x2800),('.data',0x406000,0x200),('.rdata',0x407000,0x800),('.mydata',0x408000,0x200)]

helper

for va in [0x408000,0x408001,0x408021,0x408031,0x408039,0x408040]: off=0x4400 + (va-0x408000) print(hex(va), b[off:off+64].hex()) PY

Investigating functions and decrypted data

I need to dive into the functions around 404070 and 404190, as they might involve permutations or substitutions. It seems key to understand how data is manipulated here. I’ll inspect the decrypted .hello section starting around 404070 to see what’s happening with the data and these functions.

bash -lc objdump -d -M intel --start-address=0x404070 --stop-address=0x404cb0 /mnt/data/stage2_decrypted.bin | sed -n '1,420p'

{"path":"","only_tools":false}

bash -lc ls -l /mnt/data && file /mnt/data/challenge.exe /mnt/data/decomp.bin /mnt/data/stage2.bin /mnt/data/stage2_decrypted.bin 2>/dev/null || true

Exploring tools for analysis

I need to get the final key, so I'm considering tools like Capstone or angr for analyzing this. Since it's a PE x64 Windows file, could emulate stage2 with Unicorn. I could also potentially use Qiling for calls. Maybe angr can handle symbolic execution for the validator. I'll inspect more functions to choose the best approach. If radare2 isn’t installed, I might use objdump instead.

bash -lc objdump -d -M intel --start-address=0x404cb0 --stop-address=0x405300 /mnt/data/stage2_decrypted.bin | sed -n '1,220p'

bash -lc objdump -d -M intel --start-address=0x404f75 --stop-address=0x405120 /mnt/data/stage2_decrypted.bin | sed -n '1,220p'

Inferring encryption block mode

I need to figure out the block mode for encryption. It looks like 404cb0 might be using CBC encryption. To verify, I should inspect surrounding addresses, like 404b60 and 404a60, to gather more context. This should help clarify the encryption method being used.

bash -lc objdump -d -M intel --start-address=0x404070 --stop-address=0x404cb0 /mnt/data/stage2_decrypted.bin > /tmp/hello_dis.txt && sed -n '1,260p' /tmp/hello_dis.txt

Investigating AES-like encryption

I need to gather more information from the disassembly to confirm if AES is being used. 404190 seems to handle MixColumns, 404070 likely does ShiftRows, and 404410 may be responsible for SubBytes. To confirm the full AES-like encryption, I'll inspect 404410 and the surrounding functions for their roles in the process.

bash -lc sed -n '261,520p' /tmp/hello_dis.txt

Investigating AES-CBC encryption process

I’m considering whether we have enough information based on the idea that this is AES-CBC encryption, where one block is encrypted with an IV and key schedule. The function 404cb0 likely handles the CBC-like process. I should take a look at 404b60 to check the key schedule handling and inspect 404a60 for any potential inverse or alternative steps. I'll inspect these later.

bash -lc objdump -d -M intel --start-address=0x404950 --stop-address=0x404cb0 /mnt/data/stage2_decrypted.bin | sed -n '1,260p'

Continuing from function 404c99

I'll focus on analyzing what happens from function 404c99 onwards. It seems like the next step might involve just the return operation, but I'll need to inspect further to be sure and make sure I don't miss anything important related to the flow.

bash -lc objdump -d -M intel --start-address=0x404ca0 --stop-address=0x404cb0 /mnt/data/stage2_decrypted.bin

Inspecting function 404940 for clarity

From what I'm seeing, 404b60 seems to handle AES encryption with a custom key schedule starting at 404940, while 404a60 appears to be related to decryption. I need to closely examine 404940 from the beginning, as there seems to have been an issue with the prior disassembly. I'll start by reviewing it properly.

bash -lc objdump -d -M intel --start-address=0x404900 --stop-address=0x404a60 /mnt/data/stage2_decrypted.bin | sed -n '1,220p'

Verifying AES-CBC with AES-256 encryption process

So, 404940 is responsible for AES key expansion, referencing tables at 4071e0 for the S-box and 4072e0 for Rcon. The key S-box (4071e0) appears to be regular AES. The function calls for encryption at 404cb0 fit AES-CBC, using IV from r9. It encrypts with AES-256, updating the IV with ciphertext after each block.

Final stage encryption process and key computation

The final encryption stage pads and encrypts input using AES-256-CBC, with the key at 0x408001 and the IV at 0x408021. To get the plaintext, I decrypt the target block, XOR the result with the IV, and remove the PKCS7 padding. I’ll use Python’s cryptography library, maybe even with OpenSSL or PyCryptodome, to compute this.

bash -lc python - <<'PY' from pathlib import Path import struct p=Path('/mnt/data/stage2_decrypted.bin').read_bytes()

map VA to file offset using pefile if available, else manual from section headers

import pefile pe=pefile.PE('/mnt/data/stage2_decrypted.bin') for va_name in [0x408001,0x408021,0x408031,0x408039]: rva=va_name-pe.OPTIONAL_HEADER.ImageBase off=pe.get_offset_from_rva(rva) print(hex(va_name), hex(rva), hex(off), p[off:off+32].hex()) PY

bash -lc objdump -h /mnt/data/stage2_decrypted.bin

bash -lc python - <<'PY' from pathlib import Path from binascii import hexlify p=Path('/mnt/data/stage2_decrypted.bin').read_bytes() base_va=0x408000 base_off=0x4400 for va in [0x408001,0x408021,0x408031,0x408039]: off=base_off + (va-base_va) print(hex(va), hex(off), p[off:off+32].hex()) PY

bash -lc python - <<'PY' from Crypto.Cipher import AES from pathlib import Path p=Path('/mnt/data/stage2_decrypted.bin').read_bytes() base_va=0x408000;base_off=0x4400 get=lambda va,n: p[base_off+(va-base_va):base_off+(va-base_va)+n] key=get(0x408001,32) iv=get(0x408021,16) ct=get(0x408031,16) print('key',key.hex()) print('iv',iv.hex()) print('ct',ct.hex()) pt=AES.new(key,AES.MODE_CBC,iv).decrypt(ct) print('pt hex',pt.hex()) print('pt bytes',pt) PY

bash -lc python - <<'PY' mods=['cryptography','Crypto','pyaes'] for m in mods: try: import(m) print(m,'ok') except Exception as e: print(m,'no',e) PY

bash -lc python - <<'PY' from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes from cryptography.hazmat.backends import default_backend from pathlib import Path p=Path('/mnt/data/stage2_decrypted.bin').read_bytes() base_va=0x408000;base_off=0x4400 get=lambda va,n: p[base_off+(va-base_va):base_off+(va-base_va)+n] key=get(0x408001,32) iv=get(0x408021,16) ct=get(0x408031,16) c=Cipher(algorithms.AES(key), modes.CBC(iv), backend=default_backend()) pt=c.decryptor().update(ct)+c.decryptor().finalize() if False else None

do properly single decryptor

ctx=Cipher(algorithms.AES(key), modes.CBC(iv), backend=default_backend()).decryptor() pt=ctx.update(ct)+ctx.finalize() print('pt hex',pt.hex()) print('pt raw',pt) print('pad',pt[-1]) print('unpadded',pt[:-pt[-1]]) PY

bash -lc python - <<'PY' from pathlib import Path p=Path('/mnt/data/stage2_decrypted.bin').read_bytes()

.rdata VMA 0x407000 off 0x3c00

base_va=0x407000;base_off=0x3c00 for va in [0x4070e0,0x4071e0,0x4072e0]: off=base_off+(va-base_va) print(hex(va), p[off:off+32].hex()) PY

bash -lc xxd -g1 -l 128 -s 0x3ce0 /mnt/data/stage2_decrypted.bin

bash -lc python - <<'PY' from pathlib import Path p=Path('/mnt/data/stage2_decrypted.bin').read_bytes() start=0x3ce0 for i in range(start,start+128,16): chunk=p[i:i+16] print(hex(i), chunk.hex(), ''.join(chr(b) if 32<=b<127 else '.' for b in chunk)) PY

bash -lc python - <<'PY' from pathlib import Path p=Path('/mnt/data/stage2_decrypted.bin').read_bytes() start=0x3ee0 for i in range(start,start+64,16): chunk=p[i:i+16] print(hex(i), chunk.hex(), ''.join(chr(b) if 32<=b<127 else '.' for b in chunk)) PY

bash -lc python - <<'PY' mods=['unicorn','angr'] for m in mods: try: import(m) print(m,'ok') except Exception as e: print(m,'no',e) PY

bash -lc cat > /tmp/harness.c <<'C' #define _GNU_SOURCE #include <stdio.h> #include <stdint.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <fcntl.h> #include <unistd.h>

struct sec { const char *name; uint64_t vma; uint64_t off; uint64_t size; int prot; };

int main(){ const char *path="/mnt/data/stage2_decrypted.bin"; int fd=open(path,O_RDONLY); if(fd<0){perror("open"); return 1;} off_t fsz=lseek(fd,0,SEEK_END); if(fsz<0){perror("lseek"); return 1;} uint8_t *file=malloc(fsz); lseek(fd,0,SEEK_SET); if(read(fd,file,fsz)!=fsz){perror("read"); return 1;} close(fd);

text
size_t map_size=0x100000;
void *base=mmap((void*)0x400000,map_size,PROT_READ|PROT_WRITE|PROT_EXEC,
                MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0);
if(base==MAP_FAILED){perror("mmap"); return 1;}
memset(base,0,map_size);

struct sec secs[] = {
    {".text",  0x401000, 0x0400, 0x2298, PROT_READ|PROT_EXEC},
    {".hello", 0x404000, 0x2800, 0x1010, PROT_READ|PROT_EXEC},
    {".data",  0x406000, 0x3a00, 0x00d0, PROT_READ|PROT_WRITE},
    {".rdata", 0x407000, 0x3c00, 0x07d0, PROT_READ},
    {".mydata",0x408000, 0x4400, 0x0080, PROT_READ},
    {".pdata", 0x409000, 0x4600, 0x03b4, PROT_READ},
    {".xdata", 0x40a000, 0x4a00, 0x032c, PROT_READ},
    {".idata", 0x40c000, 0x4e00, 0x087c, PROT_READ|PROT_WRITE},
    {".CRT",   0x40d000, 0x5800, 0x0068, PROT_READ|PROT_WRITE},
    {".tls",   0x40e000, 0x5a00, 0x0010, PROT_READ|PROT_WRITE},
};
for(size_t i=0;i<sizeof(secs)/sizeof(secs[0]);i++){
    memcpy((void*)secs[i].vma, file+secs[i].off, secs[i].size);
    uint64_t page=secs[i].vma & ~0xfffULL;
    uint64_t end=(secs[i].vma+secs[i].size+0xfffULL)&~0xfffULL;
    if(mprotect((void*)page,end-page,secs[i].prot)) { perror("mprotect"); return 1; }
}

// Buffers in normal process memory are fine.
uint8_t in[16];
uint8_t out[16];
memcpy(in, (void*)0x408031, 16); // target ciphertext
typedef void (__attribute__((ms_abi)) *cbc_dec_t)(uint8_t*, int, uint8_t*, uint8_t*, uint8_t*);
cbc_dec_t dec = (cbc_dec_t)(0x404d80);
dec(in, 16, (uint8_t*)0x408001, (uint8_t*)0x408021, out);
printf("decrypted block: ");
for(int i=0;i<16;i++) printf("%02x", out[i]);
printf("\n");
printf("ascii: ");
for(int i=0;i<16;i++) putchar((out[i]>=32&&out[i]<127)?out[i]:'.');
printf("\n");

// Try validate on possible unpadded strings 0..15
typedef int (__attribute__((ms_abi)) *val_t)(const char*);
val_t validate=(val_t)(0x404ef0);
for(int pad=1; pad<=16; pad++){
    int ok=1;
    for(int i=16-pad;i<16;i++) if(out[i]!=pad) ok=0;
    if(ok){
        char s[17]; memset(s,0,sizeof(s)); memcpy(s,out,16-pad);
        int v=validate(s);
        printf("candidate pad=%d len=%d validate=%d str=", pad, 16-pad, v);
        for(int i=0;i<16-pad;i++) putchar((s[i]>=32&&s[i]<127)?s[i]:'.');
        printf("\n");
    }
}
return 0;

} C cc -O2 -fPIC -pie /tmp/harness.c -o /tmp/harness && /tmp/harness

bash -lc cat > /tmp/harness2.c <<'C' #define _GNU_SOURCE #include <stdio.h> #include <stdint.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <fcntl.h> #include <unistd.h> struct sec { uint64_t vma, off, size; int prot; }; int main(){ int fd=open("/mnt/data/stage2_decrypted.bin",O_RDONLY); off_t fsz=lseek(fd,0,SEEK_END); lseek(fd,0,SEEK_SET); uint8_t file=malloc(fsz); read(fd,file,fsz); close(fd); mmap((void)0x400000,0x100000,PROT_READ|PROT_WRITE|PROT_EXEC,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0); memset((void*)0x400000,0,0x100000); struct sec s[]={ {0x401000,0x0400,0x2298,PROT_READ|PROT_EXEC},{0x404000,0x2800,0x1010,PROT_READ|PROT_EXEC},{0x406000,0x3a00,0xd0,PROT_READ|PROT_WRITE},{0x407000,0x3c00,0x7d0,PROT_READ},{0x408000,0x4400,0x80,PROT_READ},{0x409000,0x4600,0x3b4,PROT_READ},{0x40a000,0x4a00,0x32c,PROT_READ},{0x40c000,0x4e00,0x87c,PROT_READ|PROT_WRITE},{0x40d000,0x5800,0x68,PROT_READ|PROT_WRITE},{0x40e000,0x5a00,0x10,PROT_READ|PROT_WRITE} }; for(int i=0;i<10;i++){ memcpy((void*)s[i].vma,file+s[i].off,s[i].size); uint64_t pg=s[i].vma&~0xfffULL; uint64_t end=(s[i].vma+s[i].size+0xfffULL)&~0xfffULL; mprotect((void*)pg,end-pg,s[i].prot);}
typedef void (attribute((ms_abi)) cbc_dec_t)(uint8_t, int, uint8_t*, uint8_t*, uint8_t*); typedef void (attribute((ms_abi)) cbc_enc_t)(uint8_t, int, uint8_t*, uint8_t*, uint8_t*); cbc_dec_t dec=(cbc_dec_t)0x404d80; cbc_enc_t enc=(cbc_enc_t)0x404cb0; uint8_t ct[16], pt[16], ct2[16]; memcpy(ct,(void*)0x408031,16); dec(ct,16,(uint8_t*)0x408001,(uint8_t*)0x408021,pt); enc(pt,16,(uint8_t*)0x408001,(uint8_t*)0x408021,ct2); printf("pt="); for(int i=0;i<16;i++) printf("%02x",pt[i]); printf("\n"); printf("ct ="); for(int i=0;i<16;i++) printf("%02x",ct[i]); printf("\n"); printf("ct2="); for(int i=0;i<16;i++) printf("%02x",ct2[i]); printf("\n"); return 0; } C cc -O2 -fPIC -pie /tmp/harness2.c -o /tmp/harness2 && /tmp/harness2

bash -lc cat > /tmp/harness3.c <<'C' #define _GNU_SOURCE #include <stdio.h> #include <stdint.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <fcntl.h> #include <unistd.h> struct sec { uint64_t vma, off, size; int prot; }; int main(){ int fd=open("/mnt/data/stage2_decrypted.bin",O_RDONLY); off_t fsz=lseek(fd,0,SEEK_END); lseek(fd,0,SEEK_SET); uint8_t file=malloc(fsz); read(fd,file,fsz); close(fd); mmap((void)0x400000,0x100000,PROT_READ|PROT_WRITE|PROT_EXEC,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0); memset((void*)0x400000,0,0x100000); struct sec s[]={ {0x401000,0x0400,0x2298,PROT_READ|PROT_EXEC},{0x404000,0x2800,0x1010,PROT_READ|PROT_EXEC},{0x406000,0x3a00,0xd0,PROT_READ|PROT_WRITE},{0x407000,0x3c00,0x7d0,PROT_READ},{0x408000,0x4400,0x80,PROT_READ},{0x409000,0x4600,0x3b4,PROT_READ},{0x40a000,0x4a00,0x32c,PROT_READ},{0x40c000,0x4e00,0x87c,PROT_READ|PROT_WRITE},{0x40d000,0x5800,0x68,PROT_READ|PROT_WRITE},{0x40e000,0x5a00,0x10,PROT_READ|PROT_WRITE} }; for(int i=0;i<10;i++){ memcpy((void*)s[i].vma,file+s[i].off,s[i].size); uint64_t pg=s[i].vma&~0xfffULL; uint64_t end=(s[i].vma+s[i].size+0xfffULL)&~0xfffULL; mprotect((void*)pg,end-pg,s[i].prot);}
typedef int (attribute((ms_abi)) val_t)(const char); val_t validate=(val_t)0x404ef0; const char *tests[]={"dart{c3d4f5cc-8a", "dart{c3d4f5cc-8", "dart{c3d4f5cc-8a}", "", NULL}; for(int i=0; tests[i]; i++){ int v=validate(tests[i]); printf("%s => %d\n", tests[i], v); } return 0; } C cc -O2 -fPIC -pie /tmp/harness3.c -o /tmp/harness3 && /tmp/harness3

bash -lc objdump -d -M intel --start-address=0x401550 --stop-address=0x401620 /mnt/data/stage2_decrypted.bin

bash -lc objdump -d -M intel --start-address=0x401620 --stop-address=0x4016b0 /mnt/data/stage2_decrypted.bin

bash -lc cat > /tmp/offsetscan.c <<'C' #define _GNU_SOURCE #include <stdio.h> #include <stdint.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <fcntl.h> #include <unistd.h> struct sec { uint64_t vma, off, size; int prot; }; int main(){ int fd=open("/mnt/data/stage2_decrypted.bin",O_RDONLY); off_t fsz=lseek(fd,0,SEEK_END); lseek(fd,0,SEEK_SET); uint8_t file=malloc(fsz); read(fd,file,fsz); close(fd); mmap((void)0x400000,0x100000,PROT_READ|PROT_WRITE|PROT_EXEC,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0); memset((void*)0x400000,0,0x100000); struct sec s[]={ {0x401000,0x0400,0x2298,PROT_READ|PROT_EXEC},{0x404000,0x2800,0x1010,PROT_READ|PROT_EXEC},{0x406000,0x3a00,0xd0,PROT_READ|PROT_WRITE},{0x407000,0x3c00,0x7d0,PROT_READ},{0x408000,0x4400,0x80,PROT_READ},{0x409000,0x4600,0x3b4,PROT_READ},{0x40a000,0x4a00,0x32c,PROT_READ},{0x40c000,0x4e00,0x87c,PROT_READ|PROT_WRITE},{0x40d000,0x5800,0x68,PROT_READ|PROT_WRITE},{0x40e000,0x5a00,0x10,PROT_READ|PROT_WRITE} }; for(int i=0;i<10;i++){ memcpy((void*)s[i].vma,file+s[i].off,s[i].size); uint64_t pg=s[i].vma&~0xfffULL; uint64_t end=(s[i].vma+s[i].size+0xfffULL)&~0xfffULL; mprotect((void*)pg,end-pg,s[i].prot);}
typedef void (attribute((ms_abi)) cbc_dec_t)(uint8_t, int, uint8_t*, uint8_t*, uint8_t*); typedef int (attribute((ms_abi)) val_t)(const char); cbc_dec_t dec=(cbc_dec_t)0x404d80; val_t validate=(val_t)0x404ef0; uint8_t out[16]; for(int ko=-1; ko<=1; ko++) for(int io=-1; io<=1; io++) for(int co=-1; co<=1; co++) { uint8_t ct[16]; memcpy(ct,(void*)(0x408031+co),16); dec(ct,16,(uint8_t*)(0x408001+ko),(uint8_t*)(0x408021+io),out); int printable=1; for(int i=0;i<16;i++) if(out[i]<32||out[i]>126) printable=0; int pkcs=0; for(int pad=1; pad<=16; pad++){ int ok=1; for(int i=16-pad;i<16;i++) if(out[i]!=pad) ok=0; if(ok){ pkcs=pad; break; }} if(printable || pkcs){ printf("ko=%d io=%d co=%d out=",ko,io,co); for(int i=0;i<16;i++) putchar((out[i]>=32&&out[i]<127)?out[i]:'.'); printf(" hex="); for(int i=0;i<16;i++) printf("%02x",out[i]); if(pkcs){ char s[17]={0}; memcpy(s,out,16-pkcs); printf(" pkcs=%d val=%d",pkcs, validate(s)); } printf("\n"); } } return 0; } C cc -O2 -fPIC -pie /tmp/offsetscan.c -o /tmp/offsetscan && /tmp/offsetscan

bash -lc python - <<'PY' from pathlib import Path p=Path('/mnt/data/stage2_decrypted.bin').read_bytes() chunk=p[0x4400:0x4480] for i in range(0,0x80,16): c=chunk[i:i+16] print(f'{0x408000+i:08x}:', c.hex(), ''.join(chr(b) if 32<=b<127 else '.' for b in c)) PY

bash -lc cat > /tmp/multidec.c <<'C' #define _GNU_SOURCE #include <stdio.h> #include <stdint.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <fcntl.h> #include <unistd.h> struct sec { uint64_t vma, off, size; int prot; }; int main(){ int fd=open("/mnt/data/stage2_decrypted.bin",O_RDONLY); off_t fsz=lseek(fd,0,SEEK_END); lseek(fd,0,SEEK_SET); uint8_t file=malloc(fsz); read(fd,file,fsz); close(fd); mmap((void)0x400000,0x100000,PROT_READ|PROT_WRITE|PROT_EXEC,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0); memset((void*)0x400000,0,0x100000); struct sec s[]={ {0x401000,0x0400,0x2298,PROT_READ|PROT_EXEC},{0x404000,0x2800,0x1010,PROT_READ|PROT_EXEC},{0x406000,0x3a00,0xd0,PROT_READ|PROT_WRITE},{0x407000,0x3c00,0x7d0,PROT_READ},{0x408000,0x4400,0x80,PROT_READ},{0x409000,0x4600,0x3b4,PROT_READ},{0x40a000,0x4a00,0x32c,PROT_READ},{0x40c000,0x4e00,0x87c,PROT_READ|PROT_WRITE},{0x40d000,0x5800,0x68,PROT_READ|PROT_WRITE},{0x40e000,0x5a00,0x10,PROT_READ|PROT_WRITE} }; for(int i=0;i<10;i++){ memcpy((void*)s[i].vma,file+s[i].off,s[i].size); uint64_t pg=s[i].vma&~0xfffULL; uint64_t end=(s[i].vma+s[i].size+0xfffULL)&~0xfffULL; mprotect((void*)pg,end-pg,s[i].prot);}
typedef void (attribute((ms_abi)) cbc_dec_t)(uint8_t, int, uint8_t*, uint8_t*, uint8_t*); cbc_dec_t dec=(cbc_dec_t)0x404d80; for(int start=0x408031; start<=0x408040; start++){ for(int len=16; len<=64; len+=16){ uint8_t in[64]={0}, out[64]={0}; memcpy(in,(void*)start,len); dec(in,len,(uint8_t*)0x408001,(uint8_t*)0x408021,out); int pr=0; for(int i=0;i<len;i++) if(out[i]>=32&&out[i]<127) pr++; if(pr>=len/2){ printf("start=%x len=%d out=",start,len); for(int i=0;i<len;i++) putchar((out[i]>=32&&out[i]<127)?out[i]:'.'); printf("\n"); } } } } C cc -O2 -fPIC -pie /tmp/multidec.c -o /tmp/multidec && /tmp/multidec

bash -lc cat > /tmp/multidec2.c <<'C' #define _GNU_SOURCE #include <stdio.h> #include <stdint.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <fcntl.h> #include <unistd.h> struct sec { uint64_t vma, off, size; int prot; }; int main(){ int fd=open("/mnt/data/stage2_decrypted.bin",O_RDONLY); off_t fsz=lseek(fd,0,SEEK_END); lseek(fd,0,SEEK_SET); uint8_t file=malloc(fsz); read(fd,file,fsz); close(fd); mmap((void)0x400000,0x100000,PROT_READ|PROT_WRITE|PROT_EXEC,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0); memset((void*)0x400000,0,0x100000); struct sec s[]={ {0x401000,0x0400,0x2298,PROT_READ|PROT_EXEC},{0x404000,0x2800,0x1010,PROT_READ|PROT_EXEC},{0x406000,0x3a00,0xd0,PROT_READ|PROT_WRITE},{0x407000,0x3c00,0x7d0,PROT_READ},{0x408000,0x4400,0x80,PROT_READ},{0x409000,0x4600,0x3b4,PROT_READ},{0x40a000,0x4a00,0x32c,PROT_READ},{0x40c000,0x4e00,0x87c,PROT_READ|PROT_WRITE},{0x40d000,0x5800,0x68,PROT_READ|PROT_WRITE},{0x40e000,0x5a00,0x10,PROT_READ|PROT_WRITE} }; for(int i=0;i<10;i++){ memcpy((void*)s[i].vma,file+s[i].off,s[i].size); uint64_t pg=s[i].vma&~0xfffULL; uint64_t end=(s[i].vma+s[i].size+0xfffULL)&~0xfffULL; mprotect((void*)pg,end-pg,s[i].prot);}
typedef void (attribute((ms_abi)) cbc_dec_t)(uint8_t, int, uint8_t*, uint8_t*, uint8_t*); cbc_dec_t dec=(cbc_dec_t)0x404d80; uint8_t in[64]={0}, out[64]={0}; memcpy(in,(void*)0x408031,48); dec(in,48,(uint8_t*)0x408001,(uint8_t*)0x408021,out); printf("hex="); for(int i=0;i<48;i++) printf("%02x", out[i]); printf("\nascii="); for(int i=0;i<48;i++) putchar((out[i]>=32&&out[i]<127)?out[i]:'.'); printf("\n"); return 0; } C cc -O2 -fPIC -pie /tmp/multidec2.c -o /tmp/multidec2 && /tmp/multidec2

Thought for 23m 37s