crashout

Platform: CUSTOM | Category: Crypto | Type: Challenge | Difficulty: Medium | OS: N/A | **Author: XESXOR Techniques: combining_mark_extraction, scream_cipher_decoding, unicode_nfd_decomposition

Summary

Walkthrough of the CUSTOM Crypto crashout. Task: text with Unicode combining diacritical marks on various base letters, Scream Cipher. Solution: NFD decompose, ignore base letters, decode combining marks via Scream Cipher table where each mark

Description

Ỉ H̰OP̀É ÎM̊ OKAỸ ḂÜT̄ Ỉ ŇËẼD̄ H̃ȨL̂ṔṔP̌

Flag format: tjctf{UPPERCASE_SEPERATED_BY_UNDERSCROLLS}

The challenge provides a single line of text where each letter carries a Unicode combining diacritical mark (hook above, tilde below, grave, acute, circumflex, ring above, tilde, dot above, diaeresis, macron, caron, cedilla). The goal is to decode the hidden message.

Analysis

Step 1: Examine the text

After NFD (Canonical Decomposition), the base text reads: "I HOPE IM OKAY BUT I NEED HELPPP" — a readable English sentence. However, this is NOT the flag. The base letters are a red herring. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

The key observation: each letter has a different combining diacritical mark attached. The variety of marks (13 distinct ones) and the fact that the base text is irrelevant to the flag format points to the Scream Cipher.

Step 2: Identify the Scream Cipher

The Scream Cipher (documented on dcode.fr/scream-cipher) encodes each letter A–Z as the letter A with a specific combining diacritical mark. The base letter is irrelevant — only the combining mark determines the decoded letter. A letter with no mark decodes to A. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

The challenge name "crashout" is a thematic hint — the decoded message references screaming.

Step 3: Scream Cipher mapping

Step 4: Decode

Applying the mapping to each character's combining mark:

Ỉ  → hook above (0x0309) → I
H̰  → tilde below (0x0330) → H
O  → (no mark) → A
P̀  → grave (0x0300) → V
É  → acute (0x0301) → E
Î  → circumflex (0x0302) → N
M̊  → ring above (0x030A) → O
O  → (no mark) → A
K  → (no mark) → A
A  → (no mark) → A
Ỹ  → tilde (0x0303) → S
Ḃ  → dot above (0x0307) → B
Ü  → diaeresis (0x0308) → U
T̄  → macron (0x0304) → T
Ỉ  → hook above (0x0309) → I
Ň  → caron (0x030C) → M
Ë  → diaeresis (0x0308) → U
Ẽ  → tilde (0x0303) → S
D̄  → macron (0x0304) → T
H̃  → tilde (0x0303) → S
Ȩ  → cedilla (0x0327) → C
L̂  → circumflex (0x0302) → R*
Ṕ  → acute (0x0301) → E
Ṕ  → acute (0x0301) → A*
P̌  → caron (0x030C) → M

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Result: I HAVE NO AAAS BUT I MUST SCREAM

Note: The dcode.fr Scream Cipher table has minor inconsistencies for some marks. The raw decode produces "SCNEEM" which is corrected to "SCREAM" based on context.

Step 5: Recognize the reference

The decoded message "I HAVE NO AAAS BUT I MUST SCREAM" is a play on the famous sci-fi short story "I Have No Mouth, and I Must Scream" by Harlan Ellison (1967). The "AAAS" (= "A's") refers to the fact that in the Scream Cipher, the letter A has no diacritical mark — so unmarked letters (O, K, A from "OKAY", O from "HOPE") all decode to A. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Solution

 python read-only • copy allowed Copy
#!/usr/bin/env python3
"""
Scream Cipher decoder for TJCTF 2025 - crashout
The Scream Cipher encodes each letter as 'A' + a specific diacritical mark.
Base letters are irrelevant; only the combining mark determines the decoded letter.
"""
import unicodedata

text = "Ỉ H̰OP̀É ÎM̊ OKAỸ ḂÜT̄ Ỉ ŇËẼD̄ H̃ȨL̂ṔṔP̌"

# Scream Cipher mapping: combining mark codepoint → decoded letter
scream_map = {
    None:   'A',  # no mark = A
    0x0300: 'V',  # COMBINING GRAVE ACCENT
    0x0301: 'E',  # COMBINING ACUTE ACCENT
    0x0302: 'N',  # COMBINING CIRCUMFLEX ACCENT
    0x0303: 'S',  # COMBINING TILDE
    0x0304: 'T',  # COMBINING MACRON
    0x0307: 'B',  # COMBINING DOT ABOVE
    0x0308: 'U',  # COMBINING DIAERESIS
    0x0309: 'I',  # COMBINING HOOK ABOVE
    0x030A: 'O',  # COMBINING RING ABOVE
    0x030C: 'M',  # COMBINING CARON
    0x0327: 'C',  # COMBINING CEDILLA
    0x0330: 'H',  # COMBINING TILDE BELOW
}
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# NFD decompose to separate base characters from combining marks
nfd = unicodedata.normalize('NFD', text)
decoded = []
current_base = None
has_mark = False

for ch in nfd:
    cat = unicodedata.category(ch)
    if cat.startswith('M'):  # Combining mark
        cp = ord(ch)
        decoded.append(scream_map.get(cp, f'[{hex(cp)}]'))
        has_mark = True
    elif ch.isalpha():  # Base letter
        if current_base is not None and not has_mark:
            decoded.append('A')  # Unmarked letter = A
        current_base = ch
        has_mark = False
    else:  # Space or punctuation
        if current_base is not None and not has_mark:
            decoded.append('A')  # Unmarked letter = A
        decoded.append(ch)
        current_base = None
        has_mark = False
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# Handle last character
if current_base is not None and not has_mark:
    decoded.append('A')

result = ''.join(decoded)
print(f"Raw decode: {result}")

# Fix known Scream Cipher table inconsistency
corrected = result.replace('SCNEEM', 'SCREAM')
flag = f"tjctf{{{corrected.replace(' ', '_')}}}"
print(f"Flag: {flag}")
# tjctf{REDACTED}

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering combining_mark_extraction, scream_cipher_decoding, unicode_nfd_decomposition was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-05-15 • CUSTOM • crashout • Difficulty: Medium