Cryptohorrific

Technical Walkthrough

Writeup

Challenge

• Name: Cryptohorrific
• Category: Mobile
• Difficulty: Medium
• Mode: file

Summary

The archive contains an iOS Simulator .app bundle. The app stores a Base64 ciphertext in challenge.plist and decrypts it at launch with CommonCrypto.

Static Mach-O analysis showed the decrypt routine uses AES with PKCS7 padding and ECB mode. The hardcoded key is stored as a constant NSString in the binary. Replaying the same decrypt operation against the plist ciphertext recovers the flag.

Artifact Inventory

• files/a12c7373-8d1f-4cf7-94d7-f895d8927018.zip: original HTB archive.
• analysis/extracted/hackthebox.app/: extracted iOS app bundle.
• analysis/extracted/hackthebox.app/hackthebox: Mach-O 64-bit x86_64 executable.
• analysis/extracted/hackthebox.app/challenge.plist: plist containing the encrypted flag value.
• analysis/otool-disasm.txt: Objective-C and CommonCrypto call flow evidence.
• analysis/crypto-params.txt: reconstructed crypto parameters.

Analysis

file identified the executable as a 64-bit Mach-O for x86_64. This matched the bundle metadata in analysis/info-plist.txt, which marks the supported platform as iPhoneSimulator.

challenge.plist contains one dictionary with a Base64-encoded flag field. The binary imports _CCCrypt, and Objective-C metadata exposes a method named SecretManager:key:iv:data:.

The viewDidLoad disassembly in analysis/otool-disasm.txt shows the app:

1. opens challenge.plist;
2. reads index 0;
3. extracts the flag key;
4. Base64-decodes the string into NSData;
5. calls SecretManager:key:iv:data:.

The SecretManager:key:iv:data: disassembly then sets up CCCrypt as:

• operation: decrypt;
• algorithm: AES;
• options: PKCS7 padding plus ECB mode;
• key length: 16 bytes;
• input: Base64-decoded plist ciphertext.

The two constant NSStrings used as key and IV are in __cfstring. The IV is still passed to CCCrypt, but ECB mode does not use it.

Solve

solve/solve.py reproduces the app behavior:

1. loads analysis/extracted/hackthebox.app/challenge.plist;
2. Base64-decodes the flag value;
3. decrypts with <secret redacted> and PKCS7 unpadding;
4. writes the candidate to loot/flag-candidate.txt.

The harness captured the candidate into loot/flag.txt.

Flag

Raw flag is stored in loot/flag.txt and intentionally not reproduced here.

Lessons

• Mobile challenges are not always APKs; inspect the archive before assuming Android tooling.
• CommonCrypto calls can be reconstructed directly from x86_64 Mach-O register setup.
• kCCOptionECBMode makes an IV parameter irrelevant even if the application passes one.
• Treat RAG as advisory only; the decisive proof was the current binary’s CCCrypt call setup.

Source-Backed Dossier

The sections below are merged from companion Markdown notes for the same case. They are rendered after sanitization so the article stays precise without publishing raw flags, credentials, or target-specific secrets.

Notes

Scope

• Challenge: Cryptohorrific
• Category: Mobile
• Difficulty: Medium
• Mode: file
• Remote instance: none
• Start time: 2026-06-15T04:39:28Z
• Operator: harness
• State file: challenge-state.json

Harness Status

• Current phase: see challenge-state.json
• Next allowed actions: see next-action.json
• Raw flags and sensitive material stay in loot/ only. Do not paste them here.

Artifact Inventory

Evidence Ledger

Key Findings

• The archive contains an iPhone Simulator .app bundle, not an Android APK.
• Main binary: analysis/extracted/hackthebox.app/hackthebox, Mach-O 64-bit x86_64.
• challenge.plist stores a Base64-encoded flag value.
• ViewController viewDidLoad loads challenge.plist, reads flag, Base64-decodes it, and passes the bytes into SecretManager:key:iv:data:.
• SecretManager:key:iv:data: wraps CCCrypt with decrypt operation, AES, 16-byte key, PKCS7 padding, and ECB mode.
• The IV string is passed but ignored because ECB mode is set.
• Reproducible solver: solve/solve.py.
• Raw captured flag: loot/flag.txt.

RAG / Advisory Memory

RAG output is advisory only. Record evaluated retrievals with:

scripts/challenge_harness.py rag-record <workspace> --query "..." --tag MATCHED|PARTIAL|MISSING|<secret redacted>|GENERIC --validation "..."

Secrets/Flags

Raw flags and sensitive material stay in loot/ only. Use scripts/challenge_harness.py capture-flag to validate and record flag capture without printing the value.

Memory Summary

Metadata

• Platform: HackTheBox Challenges
• Category: Mobile
• Challenge: Cryptohorrific
• Difficulty: Medium
• Source workspace: <local workspace>

Validated Solve Chain

Concepts only. Do not include raw flags, reusable credentials, tokens, cookies, private keys, or live secrets.

1. Extracted an iOS Simulator .app bundle from the challenge archive.
2. Found challenge.plist containing a Base64-encoded encrypted flag field.
3. Used Mach-O Objective-C metadata and otool disassembly to locate ViewController viewDidLoad and SecretManager:key:iv:data:.
4. Verified the app reads the plist flag, Base64-decodes it, then passes it to CommonCrypto.
5. Reconstructed CCCrypt parameters from register/stack setup: decrypt, AES, PKCS7 padding, ECB mode, 16-byte key.
6. Decrypted the plist ciphertext with a reproducible Python solver.
7. Captured the recovered HTB flag through the harness.

Reusable Lessons

• Do not assume Mobile means Android; this was an iPhone Simulator Mach-O app.
• otool -ov and otool -tvV can be enough for small Objective-C iOS Simulator binaries.
• In CommonCrypto, options value 3 means PKCS7 padding plus ECB mode.
• In ECB mode, an IV argument can be present but unused.

Dead Ends

• APK/JADX path was not applicable after ZIP inventory showed an iOS .app.
• Runtime/emulator tooling was unnecessary because the static CommonCrypto parameters were complete.

Tool Quirks

• apktool, adb, frida, and class-dump were unavailable, but did not block the solve.
• otool -tvV produced usable disassembly; otool -tV alone initially produced no useful output.
• Python crypto libraries were available locally; solver supports both pycryptodome and cryptography.

Evidence Paths

• analysis/challenge-plist.txt
• analysis/otool-objc.txt
• analysis/otool-disasm.txt
• analysis/crypto-params.txt
• solve/solve.py
• loot/flag.txt

Ingestion Decision

• Proposed for LightRAG: yes
• Requires user approval before ingestion: yes

Hypothesis Board

Keep no more than 3 active hypotheses on Easy/Medium and 5 on Hard unless the user explicitly asks for breadth.

Closed Branches