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dalctf{y311e8wi11_4m4eypt_32tni274} - flag này kho...

Criado em: 7 de junho de 2026

Respondido usando GPT-5.5 Thinking por Chat01

Pergunta

ScratchingAround.png
output.txt
alphabet.txt

dalctf{y311e8wi11_4m4eypt_32tni274} - flag này khong đúng
Flag format: dalctf{...}
file output.txt + alphabet.txt là của cùng 1 challenge
tìm flag rồi viết write up chi tiết kèm lệnh + script đã dùng

Python

python
from pathlib import Path
bits=Path('/mnt/data/output.txt').read_text().strip()
lines=Path('/mnt/data/alphabet.txt').read_text().splitlines()
items=[]
for i,line in enumerate(lines):
    if not line.strip(): continue
    parts=line.split()
    ch=parts[0]
    freq=float(parts[1])
    items.append((ch,freq,i))
len(bits), bits[:50], len(items), items[:5], items[-5:]


(192,
 '10111010101101100110111111001101111110111101101011',
 41,
 [('z', 0.00021, 0),
  ('j', 0.0003, 1),
  ('q', 0.00033, 2),
  ('x', 0.00051, 3),
  ('{', 0.001, 4)],
 [('0', 0.0336, 36),
  ('e', 0.03609, 37),
  ('3', 0.064, 38),
  ('2', 0.0808, 39),
  ('1', 0.0908, 40)])

Python

python
import heapq, itertools
def huffman_codes(items, tie='index', zero_left=True):
    # items (ch, freq, index)
    heap=[]
    counter=itertools.count()
    for ch,f,idx in items:
        if tie=='index':
            key=idx
        elif tie=='counter':
            key=next(counter)
        heap.append((f,key, {'ch':ch,'left':None,'right':None}))
    heapq.heapify(heap)
    nextkey=max(x[1] for x in heap)+1 if heap else 0
    while len(heap)>1:
        f1,k1,n1=heapq.heappop(heap); f2,k2,n2=heapq.heappop(heap)
        if zero_left:
            node={'ch':None,'left':n1,'right':n2}
        else:
            node={'ch':None,'left':n2,'right':n1}
        heapq.heappush(heap,(f1+f2,nextkey,node)); nextkey+=1
    root=heap

Python

python
from PIL import Image
import numpy as np, matplotlib.pyplot as plt, os, math
img=Image.open('/mnt/data/ScratchingAround.png').convert('RGBA')
img.size


(679, 54)

Python

python
import numpy as np
arr=np.array(img)
# white/nonblack mask maybe alpha>0 and RGB>100
mask=(arr[:,:,3]>0)&(arr[:,:,:3].sum(axis=2)>300)
col_counts=mask.sum(axis=0)
# find gaps cols with no pixels
gaps=np.where(col_counts==0)[0]
# intervals of nonzero
intervals=[]
in_run=False
start=0
for i,c in enumerate(col_counts):
    if c>0 and not in_run:
        start=i; in_run=True
    if (c==0 or i==len(col_counts)-1) and in_run:
        end=i-1 if c==0 else i
        intervals.append((start,end))
        in_run=False
intervals[:20], len(intervals)


([(5, 21),
  (27, 43),
  (49, 52),
  (54, 61),
  (67, 83),
  (89, 100),
  (102, 105),
  (111, 114),
  (121, 123),
  (129, 145),
  (151, 167),
  (173, 176),
  (179, 189),
  (195, 211),
  (217, 221),
  (223, 226),
  (232, 248),
  (254, 270),
  (276, 292),
  (298, 314)],
 38)

Python

python
runs=intervals
def combine(runs, max_gap):
    out=[]
    s,e=runs[0]
    for ns,ne in runs[1:]:
        if ns-e-1 <= max_gap:
            e=ne
        else:
            out.append((s,e)); s,e=ns,ne
    out.append((s,e))
    return out
for gap in range(0,10):
    ints=combine(runs,gap)
    print(gap,len(ints),ints[:10], 'avg width', sum(e-s+1 for s,e in ints)/len(ints))

Python

python
s='dalctf{y311e8wi11_4m4eypt_32tni274}'
len(s), len(s[7:-1]), s[7:-1]


(35, 27, 'y311e8wi11_4m4eypt_32tni274')

Python

python
# create ascii art downsample? print crop with x coords around first 130
mask=(arr[:,:,3]>0)&(arr[:,:,:3].sum(axis=2)>300)
# crop rows 0-54 cols 0-130; but need show as chars
for y in range(0,54,2):
    line=''
    for x in range(0,130):
        if mask[y:y+2,x].any(): line+='#'
        else: line+=' '
    print(line.rstrip())

Python

python
for gap in [1,2,3,4]:
    ints=combine(runs,gap)
    print('gap',gap,'len',len(ints))
    print(ints)

Python

python
sum(f for ch,f,i in items)


0.745

Python

python
from pathlib import Path
data=Path('/mnt/data/ScratchingAround.png').read_bytes()
data[:8], len(data)


(b'\x89PNG\r\n\x1a\n', 3439)

Python

python
import struct, zlib, binascii
pos=8
chunks=[]
while pos<len(data):
    length=struct.unpack(">I",data[pos:pos+4])[0]; typ=data[pos+4:pos+8].decode('latin1')
    payload=data[pos+8:pos+8+length]; crc=data[pos+8+length:pos+12+length]
    chunks.append((typ,length,payload[:50]))
    pos += 12+length
chunks[:20], pos, len(data)


([('IHDR', 13, b'\x00\x00\x02\xa7\x00\x00\x006\x08\x06\x00\x00\x00'),
  ('sRGB', 1, b'\x00'),
  ('gAMA', 4, b'\x00\x00\xb1\x8f'),
  ('pHYs', 9, b'\x00\x00\x0e\xc3\x00\x00\x0e\xc3\x01'),
  ('IDAT',
   3332,
   b'x^\xed\xdd\xbf\x8b\x1b\xe9\x19\x07\xf0\xaf\x93@\x9a-R\x18R\xea\\\x1c+\xc2\x14)R\xb8\x13n\xbcW\xd8\xd5\x88\xe3\x9a\x80*\xabH\x0c\x017\x82\x85\x90F \x02'),
  ('IEND', 0, b'')],
 3439,
 3439)

Python

python
import numpy as np
from PIL import Image
img=Image.open('/mnt/data/ScratchingAround.png').convert('RGBA')
arr=np.array(img)
# unique colors
colors=np.unique(arr.reshape(-1,4), axis=0)
len(colors), colors[:10], colors[-10:]


(40,
 array([[ 39,  39,  39, 255],
        [ 39,  39,  83, 255],
        [ 39,  39, 122, 255],
        [ 39,  83,  83, 255],
        [ 39,  83, 159, 255],
        [ 39, 122, 122, 255],
        [ 39, 122, 159, 255],
        [ 39, 122, 192, 255],
        [ 83,  39,  39, 255],
        [ 83,  39,  83, 255]], dtype=uint8),
 array([[224, 224, 159, 255],
        [224, 255, 224, 255],
        [224, 255, 255, 255],
        [255, 192, 122, 255],
        [255, 192, 159, 255],
        [255, 224, 159, 255],
        [255, 224, 192, 255],
        [255, 255, 192, 255],
        [255, 255, 224, 255],
        [255, 255, 255, 255]], dtype=uint8))

Python

python
from PIL import ImageOps
img=Image.open('/mnt/data/ScratchingAround.png').convert('RGBA')
for name,im in [('orig',img),('flip_lr',ImageOps.mirror(img)),('flip_tb',ImageOps.flip(img)),('rot180',img.rotate(180,expand=True))]:
    im.save(f'/mnt/data/{name}.png')


<<ImageDisplayed>>

Python

python
def huff(items, tiekey_func, internal_key_strategy='sumkey', orient='small0'):
    import heapq
    heap=[]
    for ch,f,idx in items:
        heap.append((f, tiekey_func(ch,f,idx), {'ch':ch,'left':None,'right':None}))
    heapq.heapify(heap)
    nextkey=0
    while heap:
        nextkey=max(nextkey, heap[0][1]+1 if isinstance(heap[0][1],(int,float)) else 0)
        break
    counter=1000000
    while len(heap)>1:
        f1,k1,n1=heapq.heappop(heap); f2,k2,n2=heapq.heappop(heap)
        if orient=='small0':
            left,right=n1,n2
        elif orient=='small1':
            left,right=n2,n1
        elif orient=='lexleft':
            # no
            left,right=n1,n2
        if internal_key_strategy=='counter':
            k=counter; counter+=1
        elif internal_key_strategy=='minchar':
            # collect min char
            def chars(n):
                return [n['ch']] if n['ch'] is not None else chars(n['left'])+chars(n['right'])
            k=min(chars(left)+chars(right))
        elif internal_key_strategy=='idxmin':
            pass
        heapq.heappush(heap,(f1+f2,k, {'ch':None,'left':left,'right':right}))
    root=heap[0][2]
    return decode(bits,root)[0], decode(bits,root)[1]

variants=[]
for tie_name,tie in [
    ('input', lambda ch,f,idx: idx),
    ('revinput', lambda ch,f,idx: -idx),
    ('char', lambda ch,f,idx: ch),
    ('ord', lambda ch,f,idx: ord(ch)),
    ('revord', lambda ch,f,idx: -ord(ch)),
    ('freqstr', lambda ch,f,idx: (str(f),ch)),
]:
    for istr in ['counter','minchar']:
        for orient in ['small0','small1']:
            try:
                s,ok=huff(items,tie,istr,orient)
                variants.append((tie_name,istr,orient,ok,s))
            except Exception as e:
                variants.append((tie_name,istr,orient,False,str(e)))
for v in variants:
    if v[3] and v[4].startswith('dalctf'):
        print(v[:4], v[4], len(v[4]))
print('all ok count', sum(1 for v in variants if v[3]))
for v in variants[:20]:
    print(v)

Python

python
from dataclasses import dataclass, field
import heapq, itertools
@dataclass(order=True)
class HNode:
    freq: float
    key: str
    symbol: str = field(compare=False, default=None)
    left: object = field(compare=False, default=None)
    right: object = field(compare=False, default=None)
    symbols: str = field(compare=False, default='')
def build(nodes_key='minchar'):
    heap=[]
    for ch,f,i in items:
        heapq.heappush(heap,HNode(f,ch,ch,None,None,ch))
    merges=[]
    while len(heap)>1:
        a=heapq.heappop(heap); b=heapq.heappop(heap)
        syms=a.symbols+b.symbols
        key=min(syms) if nodes_key=='minchar' else ''.join(sorted(syms))
        parent=HNode(a.freq+b.freq,key,None,a,b,syms)
        merges.append((a.freq,a.key,a.symbols,b.freq,b.key,b.symbols,parent.freq,parent.key,parent.symbols))
        heapq.heappush(heap,parent)
    return heap[0], merges
root,merges=build('minchar')
codes={}
def walk(n,p=''):
    if n.symbol is not None:
        codes[n.symbol]=p
    else:
        walk(n.left,p+'0'); walk(n.right,p+'1')
walk(root)
decoded,ok=decode(bits, {'ch':None}) if False else ('',True)
def decode2(bits, root):
    n=root; out=[]
    for b in bits:
        n = n.left if b=='0' else n.right
        if n.symbol is not None:
            out.append(n.symbol); n=root
    return ''.join(out), n is root
s,ok=decode2(bits,root)
s, ok, len(s), codes


('dalctf{y0u_wi11_3ncrypt_4lw4y$}',
 True,
 31,
 {'h': '00000',
  'r': '00001',
  'e': '0001',
  'u': '001000',
  'b': '0010010',
  'k': '00100110',
  '_': '00100111',
  's': '00101',
  '6': '00110',
  '8': '00111',
  '2': '010',
  '9': '01100',
  'n': '01101',
  'i': '01110',
  '$': '011110',
  'p': '0111110',
  'g': '0111111',
  '1': '100',
  'o': '10100',
  'a': '10101',
  'l': '101100',
  'w': '1011010',
  'y': '1011011',
  'd': '101110',
  'v': '10111100',
  'x': '1011110100',
  'q': '10111101010',
  'z': '101111010110',
  'j': '101111010111',
  '{': '1011110110',
  '}': '1011110111',
  'f': '1011111',
  '5': '11000',
  't': '11001',
  '!': '11010',
  '7': '110110',
  'm': '1101110',
  'c': '1101111',
  '4': '11100',
  '0': '11101',
  '3': '1111'})

Python

python
for keytype in ['minchar','sortedchars']:
    root,_=build('minchar' if keytype=='minchar' else 'sortedchars')
    s,ok=decode2(bits,root)
    print(keytype,s,ok)

Python

python
# generate tokenization for correct and wrong candidates
def tokenize(bits, root):
    toks=[]; cur=root; acc=''
    for b in bits:
        acc+=b
        cur=cur.left if b=='0' else cur.right
        if cur.symbol is not None:
            toks.append((cur.symbol, acc)); acc=''; cur=root
    return toks
root_correct,_=build('minchar')
toks_correct=tokenize(bits, root_correct)
# wrong root from input/counter but HNode? implement quick
root_wrong,_= None,None
# Use previous dict node
codes_wrong,root_d=huffman_codes(items, zero_left=True)
# make decoder tokenization for dict root
def toks_dict(bits, root):
    toks=[]; n=root; acc=''
    for b in bits:
        acc+=b
        n=n['left'] if b=='0' else n['right']
        if n['ch'] is not None:
            toks.append((n['ch'],acc)); acc=''; n=root
    return toks
toks_wrong=toks_dict(bits, root_d)
print(len(toks_correct), ''.join(c for c,_ in toks_correct))
print(toks_correct[:12])
print(len(toks_wrong), ''.join(c for c,_ in toks_wrong))
print(toks_wrong[:12])

Pensamento

Preparing final Vietnamese write-up with citations

I need to craft the final write-up in Vietnamese, including the flag, commands, and script—solve.py should be linked as well. I'll be careful with markdown, especially for backticks around raw $. The flag should be in code format, and I'll cite output.txt and alphabet.txt for context. No need for external citations.

Thought for 6m 59s

Resposta

Flag đúng

text
dalctf{y0u_wi11_3ncrypt_4lw4y$}

output.txt là bitstream nhị phân, còn alphabet.txt là bảng ký tự + trọng số/tần suất để dựng Huffman tree. (output.txt) (alphabet.txt)

Vì sao flag bạn thử bị sai?

Chuỗi sai:

text
dalctf{y311e8wi11_4m4eypt_32tni274}

xuất hiện khi dựng Huffman tree với tie-break theo thứ tự dòng trong alphabet.txt.

Nhưng Huffman không duy nhất khi có các node cùng trọng số. Challenge này cần tie-break ổn định theo ký tự, cụ thể là sort theo:

text
(freq, smallest_character_in_subtree)

Khi dùng tie-break đúng, cùng bitstream sẽ decode ra flag hợp lệ:

text
dalctf{y0u_wi11_3ncrypt_4lw4y$}

Lệnh đã dùng

bash
cd /mnt/data
python3 solve.py

Output:

text
[input-order tie break]  dalctf{y311e8wi11_4m4eypt_32tni274}
[lexicographic tie break] dalctf{y0u_wi11_3ncrypt_4lw4y$}

Script solve.py

python
#!/usr/bin/env python3
from dataclasses import dataclass, field
from heapq import heappush, heappop

@dataclass(order=True)
class Node:
    freq: float
    key: str
    left: object = field(compare=False, default=None)
    right: object = field(compare=False, default=None)
    ch: str = field(compare=False, default=None)
    chars: str = field(compare=False, default='')

def load_alphabet(path):
    items = []
    with open(path, 'r', encoding='utf-8') as f:
        for line in f:
            line = line.strip()
            if not line:
                continue
            ch, freq = line.split()
            items.append((ch, float(freq)))
    return items

def build_tree(items, tie_break='lexicographic'):
    heap = []

    for i, (ch, freq) in enumerate(items):
        if tie_break == 'input_order':
            key = f'{i:04d}'
        else:
            key = ch

        heappush(heap, Node(freq=freq, key=key, ch=ch, chars=ch))

    serial = len(heap)

    while len(heap) > 1:
        a = heappop(heap)
        b = heappop(heap)

        merged_chars = a.chars + b.chars

        if tie_break == 'input_order':
            key = f'{serial:04d}'
            serial += 1
        else:
            # Mấu chốt của challenge:
            # nếu freq bằng nhau, node nào có ký tự nhỏ hơn sẽ được ưu tiên.
            key = min(merged_chars)

        heappush(
            heap,
            Node(
                freq=a.freq + b.freq,
                key=key,
                left=a,
                right=b,
                chars=merged_chars
            )
        )

    return heap[0]

def decode(bits, root):
    out = []
    cur = root

    for bit in bits:
        if bit not in '01':
            continue

        cur = cur.left if bit == '0' else cur.right

        if cur.ch is not None:
            out.append(cur.ch)
            cur = root

    if cur is not root:
        raise ValueError('Dangling bits at the end of the stream')

    return ''.join(out)

def main():
    bits = open('output.txt', 'r', encoding='utf-8').read().strip()
    items = load_alphabet('alphabet.txt')

    wrong = decode(bits, build_tree(items, tie_break='input_order'))
    print('[input-order tie break] ', wrong)

    correct = decode(bits, build_tree(items, tie_break='lexicographic'))
    print('[lexicographic tie break]', correct)

if __name__ == '__main__':
    main()

Mình cũng lưu script ở đây: solve.py

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