bash -lc file /mnt/data/attachment.exe /mnt/data/attachment2.exe && ls -l /mnt/data/attachment.exe /mnt/data/attachment2.exe

Inspecting PE sections for address mappings

I need to analyze the PE (Portable Executable) sections for address mappings. I could use tools like objdump or radare2 (r2) to do this. These tools should help examine the headers and the structure to get a better understanding of how the executable is mapped in memory.

bash -lc which objdump || true; which r2 || true; which radare2 || true; which rabin2 || true; which llvm-objdump || true; python3 - <<'PY' import pefile, sys for p in ['/mnt/data/attachment.exe','/mnt/data/attachment2.exe']: print(p) pe=pefile.PE(p) print(hex(pe.OPTIONAL_HEADER.ImageBase), hex(pe.OPTIONAL_HEADER.AddressOfEntryPoint), pe.FILE_HEADER.Machine, pe.OPTIONAL_HEADER.Magic) for s in pe.sections: print(s.Name.decode(errors='ignore').rstrip('\x00'), hex(pe.OPTIONAL_HEADER.ImageBase+s.VirtualAddress), hex(s.Misc_VirtualSize), hex(s.SizeOfRawData), hex(s.PointerToRawData), s.Characteristics) PY

Continuing PE inspection with objdump

It looks like pefile isn't installed, so I’ll have to continue with objdump instead. Using the -x flag on objdump should help me display all of the available headers in the PE file and get detailed information about the sections and their addresses. That should work for now.

bash -lc objdump -x /mnt/data/attachment2.exe | sed -n '1,160p'

bash -lc objdump -x /mnt/data/attachment2.exe | sed -n '160,260p'

Inspecting specific bytes and disassembling with objdump

I need to look at the bytes at specific addresses: 0x41650A, 0x4165B9, 0x426020 (BSS), and 0x424040 (data). Since the .bss section isn't in the file, I’ll proceed by using objdump to disassemble and examine these sections, seeing how they relate to the rest of the executable.

bash -lc objdump -d -Mintel --start-address=0x416440 --stop-address=0x4165f0 /mnt/data/attachment2.exe | sed -n '1,220p'

Comparing original with patched disassembly

It looks like attachment2 has already been patched, and the disassembly shows some issues. I’ll need to compare it with the original to see what changes were made. This should help clarify what was altered and why the patching might’ve caused these issues.

bash -lc objdump -d -Mintel --start-address=0x4164f0 --stop-address=0x416620 /mnt/data/attachment.exe | sed -n '1,220p'

Analyzing issues with the patch

The patch seems bad because it mixes 64-bit registers in a 32-bit PE file. It looks like IDA might have incorrectly assembled a 64-bit instruction with a REX prefix in a 32-bit context, causing issues. I’ll inspect the bytes at 41650A, which show unusual instructions, and calculate the patch size. The partial overwrite might have caused a misaligned replacement, affecting the original encryption loop.

Investigating byte sequence error

It looks like the patch has replaced the original mov instruction with an incorrect displacement. The byte sequence at 416512 should correspond to [esp+18h], but the bytes show 0x90 instead of 0x18. This could be due to a wrong interpretation of the instruction or a misalignment in the patch. I'll compare the file bytes with xxd to confirm what's going wrong.

bash -lc python3 - <<'PY' from pathlib import Path for p in ['/mnt/data/attachment.exe','/mnt/data/attachment2.exe']: data=Path(p).read_bytes() # VA to file off: .text va 0x401000 file 0x400 base=0x400000; va=0x4164f8; off=0x400+(va-0x401000) print('\n',p, hex(off)) chunk=data[off:off+0xe0] for i in range(0,len(chunk),16): print(f'{va+i:08x}:', ' '.join(f'{b:02x}' for b in chunk[i:i+16])) PY

Considering dynamic analysis options

The user wants to brute-force byte-by-byte, but can IDAPython call the target function? It can't execute the function in real-time without using debugging APIs to set the program's execution state. Using tools like Frida or x64dbg might be more suitable for this task, potentially with scripting for more flexibility.Breaking down encryption loop and transformations

I need to keep analyzing the encryption steps: starting with the shift-left (shl eax,4), then the XOR with the key, adding the original character, and storing it. There's also a rotation to consider (rol8(x,5)), which seems like a left rotate. I need to progressively inspect how key[] affects the transformation and look at the data modifications.

bash -lc python3 - <<'PY' from pathlib import Path p=Path('/mnt/data/attachment.exe').read_bytes()

map VA to off function

def va_to_off(va): # sections known if 0x401000 <= va < 0x401000+0x22688: return 0x400+(va-0x401000) if 0x424000 <= va < 0x424000+0x2c0: return 0x22c00+(va-0x424000) if 0x425000 <= va < 0x425000+0xe74: return 0x23000+(va-0x425000) if 0x427000 <= va < 0x427000+0x7b4: return 0x24000+(va-0x427000) raise ValueError(hex(va)) for va,l in [(0x424040,32),(0x424080,6*4),(0x425000,64)]: off=va_to_off(va) b=p[off:off+l] print(hex(va), b.hex(' '), b) PY

Clarifying the encryption step and patching process

It seems I'm dealing with a two-step encryption process, where the original and modified targets differ. The given patch may be inverted, with the second step's final output replaced at 0x424040. I think they targeted the first-step encryption by comparing the buffer after the second step and brute-forced the transformation.

Investigating patched behavior and loop process

It seemsCalculating flag from original data and encryption

I’m trying to figure out the flag. At index 14, there might be a non-invertible issue, so I’ll work on the inverse of the encryption step. The key is matching the target data by applying the inverse of the rotation, temp adjustments, and ensuring the structure follows from second to first-step encryption.

bash -lc python3 - <<'PY' orig=[0x6e,0x10,0xec,0x13,0xc1,0xcb,0xf0,0x2d,0xc6,0x32,0xfd,0x86,0xee,0xcb,0x89,0x92,0x3c,0x46,0x49,0x71,0x62,0x57] given=[0x6E,0x70,0x75,0x65,0x7c,0x6e,0x87,0x59,0x44,0x5e,0x5d,0x44,0x77,0x4e,0x5a,0x61,0x4f,0x42,0x4a,0x7b,0x21,0x87] keys=[0x21,0x3f,0xa3,0xe9,0x8f,0x00] def rol8(x,n): return ((x<<n)&0xff)|(x>>(8-n)) def ror8(x,n): return ((x>>n)|((x<<(8-n))&0xff))&0xff def f(c): return ((c|0x0a|0x7b) ^ (c&0x0a) ^ (0x0a&0x7b) ^ (0x7b&c) ^ (c&0x0a&0x7b)) & 0xff print('f examples')

create inverse map

inv={} for x in range(256): inv.setdefault(f(x),[]).append(x) print('f image size', len(inv), 'ambig', sum(1 for v in inv.values() if len(v)>1), 'pre for ?') def inv_stage(target): buf=[None]*len(target) buf[0]=target[0] for i in range(1,len(target)): k=((keys[i%6]>>6) ^ ((keys[(i-1)%6]<<4)&0xffffffff)) & 0xff # Note low 8 only temp=ror8(target[i],5) val=(temp-k)&0xff buf[i]=val return buf for name,t in [('orig',orig),('given',given)]: after_sub=inv_stage(t) print(name,'after_sub bytes:', [hex(x) for x in after_sub], bytes(after_sub)) print('str', ''.join(chr(x) if 32<=x<127 else '.' for x in after_sub)) val=after_sub[14] print('index14 after_sub',hex(val),'preimages', inv.get(val,[]), ''.join(chr(x) if 32<=x<127 else '.' for x in inv.get(val,[]))) for cand in inv.get(val,[]): b=after_sub.copy(); b[14]=cand print('candidate', cand, bytes(b), ''.join(chr(x) if 32<=x<127 else '.' for x in b)) PY

Investigating the encryption functions

Okay, so the given data seems to be the inverse of the original target after the first step, which matches the "second-step" result. Looks like the loop decoded the target and produced an intermediate string. It seems like sub_41644A impacts only index 14, but maybe sub_416431 modifies other bytes recursively. I’ll need to inspect that and nearby code.

bash -lc objdump -d -Mintel --start-address=0x416300 --stop-address=0x41644a /mnt/data/attachment.exe | sed -n '1,240p'

Inspecting function chain and first step encryption

I need the full chain, starting from 0x416000 up to 416350. The goal is to derive the first-step encryption, paying attention to how functions like sub_41644A call 416431 and modify specific bytes. I’ll need to disassemble from 0x401500 and analyze the entire function sequence.

bash -lc objdump -d -Mintel --start-address=0x416000 --stop-address=0x416350 /mnt/data/attachment.exe | sed -n '1,300p'

Gathering disassembly from beginning

I’ll need to start earlier than 0x416000, possibly from the function at 0x415FE4. From there, I’ll trace the chain up to 0x401xxx. I could use linear extraction or parse functions that modify specific bytes, but to be thorough, I’ll grab the whole disassembly starting from the 0x401500 series.

bash -lc objdump -d -Mintel --start-address=0x401500 --stop-address=0x401600 /mnt/data/attachment.exe | sed -n '1,220p'

Collecting additional disassembly from 4015F5 onward

I need to gather more information starting from 0x4015F5 onward. This should help me fill in the gaps and trace the function chain further. It’s essential that I get the full context for the analysis. Let’s focus on this section.

bash -lc objdump -d -Mintel --start-address=0x4015f0 --stop-address=0x401900 /mnt/data/attachment.exe | sed -n '1,360p'

bash -lc objdump -d -Mintel --start-address=0x4018f0 --stop-address=0x401c50 /mnt/data/attachment.exe | sed -n '1,380p'

Evaluating emulation and tools for deeper inspection

I’m considering emulating the program’s first-step function chain, possibly under Wine, though it may not be installed. I wonder if I could statically analyze functions reachable from 0x41644A using objdump disassembly. Alternatively, I could look at using Capstone or Unicorn, though they also aren’t installed. I’ll need to check my options.

bash -lc which wine || true; which qemu-i386 || true; python3 - <<'PY' try: import capstone; print('capstone yes') except Exception as e: print('capstone no', e) try: import unicorn; print('unicorn yes') except Exception as e: print('unicorn no', e) PY

Considering full disassembly analysis and function parsing

I’m thinking through the full disassembly and needing to parse the functions from objdump. The user’s request seems more conceptual, so I may not need to reveal the flag unless explicitly asked. It’s probably better to focus on the function flow, starting from 0x41644a. After identifying the operations, I'll look into interpreting the disassembled text to emulate the functions.

Using recursive descent to parse functions

I’ll use recursive descent to parse instructions starting at 0x41644a. I need to follow the sequence of operations in the functions, which include push, mov, sub, then calls. After executing the direct operations, I’ll recursively execute the called functions. Then, I’ll parse the whole disassembly to find the function boundaries and track calls to the custom functions and helpers.Clarifying bitwise transformations and functions

The expression eax = (const - x) simplifies to x - const - 1. If const = -9, then eax = x + 8 (mod 256). Next, for a complex boolean function, the transformation involves multiple bitwise operations like (x|a|b) ^ (x&a) ^ (a&b) ^ (b&x) ^ (x&a&b). We'll parse constants and carry out the sequence in order, including both custom and helper calls.

bash -lc objdump -d -Mintel /mnt/data/attachment.exe > /mnt/data/attachment.dis && python3 - <<'PY' import re from pathlib import Path lines=Path('/mnt/data/attachment.dis').read_text(errors='ignore').splitlines() inst=[] pat=re.compile(r'^\s*([0-9a-fA-F]+):\s+((?:[0-9a-fA-F]{2} )+)\s*\t?(.*)$') for line in lines: m=pat.match(line) if m: addr=int(m.group(1),16); asm=m.group(3).strip(); inst.append((addr,asm,line)) addr_to_idx={a:i for i,(a,_,__) in enumerate(inst)}

def body(start): i=addr_to_idx[start] out=[] while i<len(inst): a,asm,line=inst[i] out.append((a,asm,line)) if asm == 'ret' or asm.startswith('ret '): break i+=1 return out

def parse_op(body): # returns possibly op tuple based on body # find store a2 XX 60 42 -> asm mov ds:0x...,al stores=[] for idx,(a,asm,line) in enumerate(body): m=re.match(r'mov\s+ds:0x(4260[0-9a-fA-F]{2}),al', asm) if m: stores.append((idx,int(m.group(1),16))) if not stores: return None idx,addr=stores[0] # find preceding movzx source addr src=None for j in range(0, idx): m=re.match(r'movzx\s+eax,BYTE PTR ds:0x(4260[0-9a-fA-F]{2})', body[j][1]) if m: src=int(m.group(1),16) # detect boolean by local constants c2=c3=None for a,asm,line in body[:idx]: m=re.match(r'mov\s+BYTE PTR $ebp-0x2$,0x([0-9a-fA-F]+)', asm) if m: c2=int(m.group(1),16)&0xff m=re.match(r'mov\s+BYTE PTR $ebp-0x3$,0x([0-9a-fA-F]+)', asm) if m: c3=int(m.group(1),16)&0xff if c2 is not None and c3 is not None: return ('bool', addr, c2, c3) # detect xor/add/not for j in range(0, idx): asm=body[j][1] m=re.match(r'xor\s+eax,0x([0-9a-fA-F]+)', asm) if m: return ('xor', addr, int(m.group(1),16)&0xff) m=re.match(r'add\s+eax,0x([0-9a-fA-F]+)', asm) if m: return ('add', addr, int(m.group(1),16)&0xff) # detect not sequence const in edx const=None for j in range(0, idx): m=re.match(r'mov\s+edx,0x([0-9a-fA-F]+)', body[j][1]) if m: const=int(m.group(1),16)&0xffffffff if const is not None: delta=(- (const & 0xff) - 1) & 0xff # x + delta low-byte return ('add', addr, delta) # identity return ('id', addr)

seen=[] ops=[]

def run_func(addr, depth=0): if addr not in addr_to_idx: print('unknown func',hex(addr)); return b=body(addr) op=parse_op(b) if op: ops.append((addr,op)) for a,asm,line in b: m=re.match(r'call\s+0x([0-9a-fA-F]+)', asm) if m: callee=int(m.group(1),16) # only follow calls in code, excluding likely CRT? But follow if known and below 0x417000 or 0x401000 range if callee in addr_to_idx and (0x401000 <= callee < 0x416600): run_func(callee, depth+1) else: print('skip call',hex(callee),'from',hex(addr))

run_func(0x41644a) print('num ops',len(ops)) for addr,op in ops[:20]: print(hex(addr), op) print('...last') for addr,op in ops[-20:]: print(hex(addr), op)

apply to target intermediate from user (after second stage inverse)

t=[0x6E,0x70,0x75,0x65,0x7c,0x6e,0x87,0x59,0x44,0x5e,0x5d,0x44,0x77,0x4e,0x5a,0x61,0x4f,0x42,0x4a,0x7b,0x21,0x87]

ops transform original input -> t. We need invert ops in reverse order.

def boolf(x,a,b): return ((x|a|b) ^ (x&a) ^ (a&b) ^ (b&x) ^ (x&a&b)) & 0xff

Create inverse maps for bool ops

buf=t.copy() for addr,op in reversed(ops): typ=op[0]; ga=op[1]; idx=ga-0x426020 if idx<0 or idx>=len(buf): continue if typ=='xor': buf[idx] ^= op[2] elif typ=='add': buf[idx]=(buf[idx]-op[2])&0xff elif typ=='bool': a,b=op[2],op[3] inv=[x for x in range(256) if boolf(x,a,b)==buf[idx]] if len(inv)!=1: print('ambig bool',hex(addr),idx,buf[idx],inv[:10]) buf[idx]=inv[0] elif typ=='id': pass print('buf',buf) print(bytes(buf)) print('str', ''.join(chr(x) if 32<=x<127 else '.' for x in buf))

verify by applying ops and second stage to original target

v=buf.copy() for addr,op in ops: typ=op[0]; ga=op[1]; idx=ga-0x426020 if 0<=idx<len(v): if typ=='xor': v[idx] ^= op[2] elif typ=='add': v[idx]=(v[idx]+op[2])&0xff elif typ=='bool': v[idx]=boolf(v[idx],op[2],op[3]) print('forward intermediate', bytes(v), v==t) PY

Analyzing recursion and call chain cycling

I’m thinking through the potential call chain, wondering if recursion happens because of calls like 0x416431. It could cycle through a series of functions, but I need to detect if it truly loops, possibly by examining each function’s return (ret). I also need a way to track function activity to detect any cycles.

bash -lc python3 - <<'PY' import re from pathlib import Path lines=Path('/mnt/data/attachment.dis').read_text(errors='ignore').splitlines() inst=[]; pat=re.compile(r'^\s*([0-9a-fA-F]+):\s+((?:[0-9a-fA-F]{2} )+)\s*\t?(.*)$') for line in lines: m=pat.match(line) if m: inst.append((int(m.group(1),16),m.group(3).strip(),line)) addr_to_idx={a:i for i,(a,_,__) in enumerate(inst)}

def body(start): i=addr_to_idx[start]; out=[] while i<len(inst): a,asm,line=inst[i]; out.append((a,asm,line)) if asm=='ret' or asm.startswith('ret '): break i+=1 return out stack=[] def run(addr,depth=0): if addr in stack: print('cycle', '->'.join(hex(x) for x in stack+[addr])); raise SystemExit if depth>80: print('depth',depth,'stack','->'.join(hex(x) for x in stack)); raise SystemExit stack.append(addr) b=body(addr) calls=[] for a,asm,line in b: m=re.match(r'call\s+0x([0-9a-fA-F]+)', asm) if m: callee=int(m.group(1),16) if callee in addr_to_idx and (0x401000 <= callee < 0x416600): calls.append(callee) for c in calls: run(c,depth+1) stack.pop() run(0x41644a) print('ok') PY

Managing long call chains with iterative recursion

I need to continue parsing this call chain, but I realize the recursion limit might be hit since the chain is pretty long. Maybe I can adjust the recursion limit or use an iterative approach instead to handle the long sequence without running into problems?

bash -lc python3 - <<'PY' import re, sys sys.setrecursionlimit(10000) from pathlib import Path lines=Path('/mnt/data/attachment.dis').read_text(errors='ignore').splitlines() inst=[]; pat=re.compile(r'^\s*([0-9a-fA-F]+):\s+((?:[0-9a-fA-F]{2} )+)\s*\t?(.*)$') for line in lines: m=pat.match(line) if m: inst.append((int(m.group(1),16),m.group(3).strip(),line)) addr_to_idx={a:i for i,(a,_,__) in enumerate(inst)}

def body(start): i=addr_to_idx[start]; out=[] while i<len(inst): a,asm,line=inst[i]; out.append((a,asm,line)) if asm=='ret' or asm.startswith('ret '): break i+=1 return out

def parse_op(b): stores=[] for idx,(a,asm,line) in enumerate(b): m=re.match(r'mov\s+ds:0x(4260[0-9a-fA-F]{2}),al', asm) if m: stores.append((idx,int(m.group(1),16))) if not stores: return None idx,addr=stores[0] c2=c3=None for a,asm,line in b[:idx]: m=re.match(r'mov\s+BYTE PTR $ebp-0x2$,0x([0-9a-fA-F]+)', asm) if m: c2=int(m.group(1),16)&0xff m=re.match(r'mov\s+BYTE PTR $ebp-0x3$,0x([0-9a-fA-F]+)', asm) if m: c3=int(m.group(1),16)&0xff if c2 is not None and c3 is not None: return ('bool',addr,c2,c3) for j in range(0,idx): asm=b[j][1] m=re.match(r'xor\s+eax,0x([0-9a-fA-F]+)', asm) if m: return ('xor',addr,int(m.group(1),16)&0xff) m=re.match(r'add\s+eax,0x([0-9a-fA-F]+)', asm) if m: return ('add',addr,int(m.group(1),16)&0xff) const=None for j in range(0,idx): m=re.match(r'mov\s+edx,0x([0-9a-fA-F]+)', b[j][1]) if m: const=int(m.group(1),16)&0xffffffff if const is not None: delta=(- (const & 0xff) - 1) & 0xff return ('add',addr,delta) return ('id',addr) ops=[]; stack=[] def run_func(addr): if addr in stack: raise Exception('cycle '+ '->'.join(hex(x) for x in stack+[addr])) stack.append(addr) b=body(addr) op=parse_op(b) if op: ops.append((addr,op)) for a,asm,line in b: m=re.match(r'call\s+0x([0-9a-fA-F]+)', asm) if m: callee=int(m.group(1),16) if callee in addr_to_idx and (0x401000 <= callee < 0x416600): run_func(callee) stack.pop() run_func(0x41644a) print('num ops',len(ops)) print('first,last',hex(ops[0][0]),ops[0][1],hex(ops[-1][0]),ops[-1][1])

Summarize indices touched

from collections import Counter cnt=Counter(op[1][1]-0x426020 for op in ops if 0<=op[1][1]-0x426020<22) print('touched counts',sorted(cnt.items()))

def boolf(x,a,b): return ((x|a|b) ^ (x&a) ^ (a&b) ^ (b&x) ^ (x&a&b)) & 0xff t=[0x6E,0x70,0x75,0x65,0x7c,0x6e,0x87,0x59,0x44,0x5e,0x5d,0x44,0x77,0x4e,0x5a,0x61,0x4f,0x42,0x4a,0x7b,0x21,0x87] buf=t.copy() for addr,op in reversed(ops): typ=op[0]; idx=op[1]-0x426020 if not (0<=idx<len(buf)): continue if typ=='xor': buf[idx]^=op[2] elif typ=='add': buf[idx]=(buf[idx]-op[2])&0xff elif typ=='bool': a,b=op[2],op[3] inv=[x for x in range(256) if boolf(x,a,b)==buf[idx]] if len(inv)!=1: print('ambig bool',hex(addr),idx,hex(buf[idx]),inv) buf[idx]=inv[0] elif typ=='id': pass print('flag?', bytes(buf), ''.join(chr(x) if 32<=x<127 else '.' for x in buf))

verify first step -> t, second step -> original target

v=buf.copy() for addr,op in ops: typ=op[0]; idx=op[1]-0x426020 if 0<=idx<len(v): if typ=='xor': v[idx]^=op[2] elif typ=='add': v[idx]=(v[idx]+op[2])&0xff elif typ=='bool': v[idx]=boolf(v[idx],op[2],op[3]) print('v==t',v==t, bytes(v))

second stage

keys=[0x21,0x3f,0xa3,0xe9,0x8f,0] def rol8(x,n): return ((x<<n)&0xff)|(x>>(8-n)) w=v.copy() for i in range(1,22): k=((keys[i%6]>>6) ^ ((keys[(i-1)%6]<<4)&0xffffffff)) & 0xff w[i]=rol8((w[i]+k)&0xff,5) orig=[0x6e,0x10,0xec,0x13,0xc1,0xcb,0xf0,0x2d,0xc6,0x32,0xfd,0x86,0xee,0xcb,0x89,0x92,0x3c,0x46,0x49,0x71,0x62,0x57] print('final==orig',w==orig, [hex(x) for x in w]) PY

Investigating nonprintable flag and potential recursion issues

It looks like we're hitting a weird issue with the flag being nonprintable in a few spots. Maybe our parser is over-following the function calls, especially with the large number of operations. The recursive call chain might have misinterpreted the body boundaries. Could the flag be non-ASCII, or is it just a byte-by-byte misstep in our parsing?

Analyzing non-printable result and inversion step

It seems like after the first step, the inversion might yield non-printable results, possibly because of a parser error or a first-step that's non-invertible. I need to verify the function execution, but the emulator isn't available. Maybe the issue lies in how the call graph was parsed or how recurring calls were handled in the sequence.Investigating boolean function and operations

The current operation seems to be working fine with sub edx, eax and not, giving the result of x + 8. However, there may be an issue with the boolean function using 8-bit operations, where AL is involved. I need to ensure the high byte doesn't interfere, especially when dealing with movzx. The recursive calls in the function chain also need to be carefully traced.

Verifying steps and handling non-ASCII input

Okay, if function C is called at the end of B, that might affect the output. The transformation per byte could lead to non-printable values in the flag. I can run the original input through to validate, and the challenge could involve a non-standard encoding instead of ASCII. Let’s also verify the order of operations.Revisiting execution flow and non-ASCII flag possibility

It looks like executing child function calls after the primary operation is correct, but maybe direct helper functions were treated as no-ops while modifying bytes. The flag might not be printable due to non-ASCII content. I could try brute-forcing in the binary using a disassembler to inspect the first step.

Considering non-ASCII bytes and analyzing first step handling

The challenge flag could indeed contain bytes outside the conventional ASCII range. If I reversed operations using the user’s intermediate result, the first target might have been wrongly manipulated. I’ll focus on a byte-by-byte transformation for index0 and try to calculateInvestigating conditional function calls and parsing concerns

I’m wondering if functions have conditional jumps that we aren't handling. For example, functions might include branches we haven't parsed yet. I'll need to examine the body of each function for possible conditionals or fallthroughs, particularly looking for unhandled jumps and using a script to detect them.

bash -lc python3 - <<'PY' import re, sys sys.setrecursionlimit(10000) from pathlib import Path lines=Path('/mnt/data/attachment.dis').read_text(errors='ignore').splitlines() inst=[]; pat=re.compile(r'^\s*([0-9a-fA-F]+):\s+((?:[0-9a-fA-F]{2} )+)\s*\t?(.*)$') for line in lines: m=pat.match(line) if m: inst.append((int(m.group(1),16),m.group(3).strip(),line)) addr_to_idx={a:i for i,(a,_,__) in enumerate(inst)} def body(start): i=addr_to_idx[start]; out=[] while i<len(inst): a,asm,line=inst[i]; out.append((a,asm,line)) if asm=='ret' or asm.startswith('ret '): break i+=1 return out visited=[]; stack=[] def run(addr): stack.append(addr); visited.append(addr) for a,asm,line in body(addr): if re.match(r'j\w+',asm): print('jump in',hex(addr),line) m=re.match(r'call\s+0x([0-9a-fA-F]+)', asm) if m: callee=int(m.group(1),16) if callee in addr_to_idx and (0x401000<=callee<0x416600): run(callee) stack.pop() run(0x41644a) print('visited',len(visited),len(set(visited))) PY

Investigating function call to 0x410c10 with loop

I noticed a function call to 0x410c10 that includes a loop. I need to figure out how this happens—maybe it’s not part of the first step, or the call chain is longer than expected. I also need to avoid unnecessary calls within functions that contain branches and inspect the area around 0x410c10.

bash -lc objdump -d -Mintel --start-address=0x410bd0 --stop-address=0x410c80 /mnt/data/attachment.exe

Recognizing loop's role in first-step transformation

Ah, so the first step isn’t purely byte-by-byte—it involves a loop at 0x410c10 that modifies every byte. The transformation works as buf[i] = (buf[i] + i) ^ i. I realize the parser ignored this loop, so the inversion was wrong. I need to emulate this loop in the function body and account for other potential loops. I'll adjust the operations accordingly.

Updating function parsing to include loops

I need to modify our operations to include the special loop after the first operation in function 0x410c10. Right now, our parser only returns the first operation and skips over the loop. I should simulate function operations in instruction order rather than by function. I'll also add the loop operation between the XOR and the call for 0x410c10, then invert as necessary.

bash -lc python3 - <<'PY'

Reuse code from previous but with special loop op after op for 410c10

import re, sys sys.setrecursionlimit(10000) from pathlib import Path lines=Path('/mnt/data/attachment.dis').read_text(errors='ignore').splitlines() inst=[]; pat=re.compile(r'^\s*([0-9a-fA-F]+):\s+((?:[0-9a-fA-F]{2} )+)\s*\t?(.*)$') for line in lines: m=pat.match(line) if m: inst.append((int(m.group(1),16),m.group(3).strip(),line)) addr_to_idx={a:i for i,(a,_,__) in enumerate(inst)} def body(start): i=addr_to_idx[start]; out=[] while i<len(inst): a,asm,line=inst[i]; out.append((a,asm,line)) if asm=='ret' or asm.startswith('ret '): break i+=1 return out

def parse_op(b): stores=[] for idx,(a,asm,line) in enumerate(b): m=re.match(r'mov\s+ds:0x(4260[0-9a-fA-F]{2}),al', asm) if m: stores.append((idx,int(m.group(1),16))) if not stores: return None idx,addr=stores[0] c2=c3=None for a,asm,line in b[:idx]: m=re.match(r'mov\s+BYTE PTR $ebp-0x2$,0x([0-9a-fA-F]+)', asm) if m: c2=int(m.group(1),16)&0xff m=re.match(r'mov\s+BYTE PTR $ebp-0x3$,0x([0-9a-fA-F]+)', asm) if m: c3=int(m.group(1),16)&0xff if c2 is not None and c3 is not None: return ('bool',addr,c2,c3) for j in range(0,idx): asm=b[j][1] m=re.match(r'xor\s+eax,0x([0-9a-fA-F]+)', asm) if m: return ('xor',addr,int(m.group(1),16)&0xff) m=re.match(r'add\s+eax,0x([0-9a-fA-F]+)', asm) if m: return ('add',addr,int(m.group(1),16)&0xff) const=None for j in range(0,idx): m=re.match(r'mov\s+edx,0x([0-9a-fA-F]+)', b[j][1]) if m: const=int(m.group(1),16)&0xffffffff if const is not None: delta=(- (const & 0xff) - 1) & 0xff return ('add',addr,delta) return ('id',addr) ops=[] def run_func(addr): b=body(addr) op=parse_op(b) if op: ops.append((addr,op)) if addr==0x410c10: ops.append((addr,('loop_add_xor_index',))) for a,asm,line in b: m=re.match(r'call\s+0x([0-9a-fA-F]+)', asm) if m: callee=int(m.group(1),16) if callee in addr_to_idx and (0x401000 <= callee < 0x416600): run_func(callee) run_func(0x41644a) print('ops',len(ops)) def boolf(x,a,b): return ((x|a|b) ^ (x&a) ^ (a&b) ^ (b&x) ^ (x&a&b)) & 0xff

def apply(buf): buf=buf.copy() for addr,op in ops: typ=op[0] if typ=='loop_add_xor_index': for i in range(22): buf[i]=((buf[i]+i)&0xff) ^ i continue idx=op[1]-0x426020 if 0<=idx<len(buf): if typ=='xor': buf[idx]^=op[2] elif typ=='add': buf[idx]=(buf[idx]+op[2])&0xff elif typ=='bool': buf[idx]=boolf(buf[idx],op[2],op[3]) return buf

def invert(t): buf=t.copy() for addr,op in reversed(ops): typ=op[0] if typ=='loop_add_xor_index': for i in range(22): buf[i]=((buf[i]^i)-i)&0xff continue idx=op[1]-0x426020 if 0<=idx<len(buf): if typ=='xor': buf[idx]^=op[2] elif typ=='add': buf[idx]=(buf[idx]-op[2])&0xff elif typ=='bool': inv=[x for x in range(256) if boolf(x,op[2],op[3])==buf[idx]] if len(inv)!=1: print('ambig',hex(addr),idx,hex(buf[idx]),inv[:5]) buf[idx]=inv[0] return buf

intermediate

t=[0x6E,0x70,0x75,0x65,0x7c,0x6e,0x87,0x59,0x44,0x5e,0x5d,0x44,0x77,0x4e,0x5a,0x61,0x4f,0x42,0x4a,0x7b,0x21,0x87] buf=invert(t) print(bytes(buf), ''.join(chr(x) if 32<=x<127 else '.' for x in buf), [hex(x) for x in buf]) print('verify int', apply(buf)==t)

final verify

keys=[0x21,0x3f,0xa3,0xe9,0x8f,0] def rol8(x,n): return ((x<<n)&0xff)|(x>>(8-n)) w=apply(buf) for i in range(1,22): k=((keys[i%6]>>6) ^ ((keys[(i-1)%6]<<4)&0xffffffff)) & 0xff w[i]=rol8((w[i]+k)&0xff,5) orig=[0x6e,0x10,0xec,0x13,0xc1,0xcb,0xf0,0x2d,0xc6,0x32,0xfd,0x86,0xee,0xcb,0x89,0x92,0x3c,0x46,0x49,0x71,0x62,0x57] print('verify final',w==orig) PY

Investigating first byte and loop transformations

It seems the first byte is still not printing correctly. Maybe the flag starts with 0xaa, or it's encoded differently due to intermediate transformations. The rest of the flag looks almost printable now. I also wonder if we've overlooked any loops or function calls. I'll need to double-check everything.Analyzing byte-level control flow and patching

The byte at 0x41650A appears to be overwriting control flow. Originally, the jump target was in the middle of a loop condition, but after patching, it's turning into a jump with incomplete instructions. There are issues due to patch leftovers and disassembly being corrupted. It seems like ASLR isn’t affecting the problem here. Direct patching of addresses is complex, especially with .bss addresses not being on disk.

Thought for 4m 11s