Challenge / Crypto

BabyEncryption

By Jennofrie Daguil · Cybersecurity and Red Team Operator

BabyEncryption is a sanitized challenge note from the local HTB archive, organized for quick review by category, difficulty, evidence flow, and reusable operator

Very EasyPublished 2024-03-21Sanitized local writeup

Scenario

BabyEncryption attack path

BabyEncryption is a sanitized challenge note from the local HTB archive, organized for quick review by category, difficulty, evidence flow, and reusable operator

Objective

Challenge walkthrough focused on Crypto evidence, validation, and reusable operator lessons.

Walkthrough flow

Challenge parameters

Weak assumption

Recovered secret state

Proof captured

Source coverage

High source coverage

Status: complete. This article is generated from 6 sanitized Markdown sources and keeps raw flags, credentials, keys, cookies, and reusable secrets out of the rendered blog.

99% coverage

Evidence verdict

High confidence: the page is reconstructed from a primary walkthrough plus multiple supporting notes or evidence sources. Treat the chain as source-backed, while still checking the listed source files for sensitive values.

Crypto/BabyEncryption/writeup.md
htb-challenge/Crypto/BabyEncryption/notes.md
htb-challenge/Crypto/BabyEncryption/memory-summary.md
htb-challenge/Crypto/BabyEncryption/hypothesis-board.md
HTB/_knowledge/exports/ctf-lightrag-latest-203412/documents/challenge__Crypto__BabyEncryption__memory-summary.md.446a149bff.md
HTB/_knowledge/exports/ctf-lightrag-latest-203412/documents/challenge__Crypto__BabyEncryption__notes.md.58cac7c041.md

Technical Walkthrough

Writeup

Challenge

Name: BabyEncryption
Category: Crypto
Difficulty: Very Easy
Mode: file

Summary

This challenge is a direct byte-wise affine cipher. The source file provides the exact encryption rule, so the solve is just modular inversion plus hex decoding.

Artifact Inventory

The challenge consists of two local files:

files/extracted/chall.py: the encryption source
files/extracted/msg.enc: the ciphertext, stored as ASCII hex

analysis/artifact-inventory.json records the hashes and file metadata.

Analysis

files/extracted/chall.py defines a per-byte transform:

python

c = (123 * m + 18) % 256

where m is a plaintext byte and c is the ciphertext byte.

This is an affine transformation over integers modulo 256. Since gcd(123, 256) = 1, the multiplier is invertible. The inverse of 123 mod 256 is 179, so each byte decrypts as:

python

m = 179 * (c - 18) % 256

files/extracted/msg.enc is not raw bytes; it is a hex string. The ciphertext therefore has to be decoded with bytes.fromhex(...) before applying the inverse transform byte by byte.

Solve

Run:

bash

python3 solve/solve.py

The solver:

reads files/extracted/msg.enc
hex-decodes the ciphertext
applies the inverse affine transform to each byte
prints the recovered plaintext

The raw flag was then captured into loot/flag.txt with:

bash

cd <local workspace>
python3 scripts/challenge_harness.py capture-flag Crypto/BabyEncryption --from analysis/flag-candidate.txt

Flag

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

Lessons

If the source gives you a linear transform directly, derive the inverse first instead of reaching for brute force.
Byte-wise affine ciphers over modulus 256 are trivial to reverse whenever the multiplier is coprime with 256.

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: BabyEncryption
Category: Crypto
Difficulty: Very Easy
Mode: file
Remote instance: none
Start time: 2026-06-07T15:52:52Z
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

File	Size	SHA256	Type	Notes
`files/a12c7376-5639-45bb-9a52-e8d8649e9954.zip`	631	`<hash redacted>`	Zip archive data, at least v2.0 to extract, compression method=deflate	zip entries: 2 shown in artifact inventory JSON
`files/extracted/chall.py`	234	`<hash redacted>`	Python script text executable, ASCII text
`files/extracted/msg.enc`	160	`<hash redacted>`	ASCII text, with no line terminators

Evidence Ledger

Time	Action	Output/File	Finding	Confidence	Next
2026-06-07T15:52:52Z	harness init	`challenge-state.json`	Workspace initialized with deterministic state file	High	Inventory artifacts
2026-06-07T15:53:12Z	artifact inventory	`analysis/artifact-inventory.json`	3 artifact(s) inventoried	High	Build or update hypotheses
2026-06-07T15:53:12Z	hypothesis recorded	`hypothesis-board.md`	Reverse chall.py encryption formula and apply the inverse transform to msg.enc to recover the plaintext flag.	High	Read chall.py, identify byte-wise transformation, then implement inverse in solve/solve.py.
2026-06-07T15:53:30Z	research skip	`analysis/research/research-skip.md`	Research intentionally skipped with recorded reason	Medium	Gate before exploit
2026-06-07T15:53:30Z	checkpoint recorded	`analysis/checkpoint-hypothesis_ready-20260607T155330690608Z-3f359780.md`	Checkpoint for <secret redacted>	High	Use checkpoint to drive next decision
2026-06-07T15:54:51Z	flag capture	`loot/flag.txt`	HTB-format flag captured; raw value kept in loot only	High	Write solution and run completion gate
2026-06-07T15:56:08Z	completion gate	`challenge-state.json`	Completion gate passed; state marked COMPLETE	High	Optional sanitized memory summary approval

Key Findings

chall.py implements a byte-wise affine cipher: multiply by 123, add 18, then reduce modulo 256.
msg.enc stores ciphertext as ASCII hex, so the first decode step is bytes.fromhex(...).
Because gcd(123, 256) = 1, the modular inverse exists and is 179, giving decryption formula m = 179 * (c - 18) mod 256.
The plaintext is recoverable offline with a short deterministic script; no brute force or external research is needed.

RAG / Advisory Memory

RAG output is advisory only. Record evaluated retrievals with:

bash

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: Crypto
Challenge: BabyEncryption
Difficulty: Very Easy
Source workspace: <local workspace>

Validated Solve Chain

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

Reusable Lessons

Dead Ends

Tool Quirks

Evidence Paths

Ingestion Decision

Proposed for LightRAG: yes/no
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.

Rank	Path	Evidence	Missing Proof	Cheapest Validation	Confidence	Status
1	Reverse chall.py encryption formula and apply the inverse transform to msg.enc to recover the plaintext flag.	Archive contains chall.py and msg.enc only; challenge title is BabyEncryption and scenario asks to decrypt a confidential message.		Read chall.py, identify byte-wise transformation, then implement inverse in solve/solve.py.	High	Active

Closed Branches

Branch	Evidence Tested	Failure Output	Reason Closed	Revisit Condition

Memory Summary

approval_required: true

Sanitized Memory Summary

Metadata

Platform: HackTheBox Challenges
Category: Crypto
Challenge: BabyEncryption
Difficulty: Very Easy
Source workspace: <local workspace>

Validated Solve Chain

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

Reusable Lessons

Dead Ends

Tool Quirks

Evidence Paths

Ingestion Decision

Proposed for LightRAG: yes/no
Requires user approval before ingestion: yes

Notes

Scope

Challenge: BabyEncryption
Category: Crypto
Difficulty: Very Easy
Mode: file
Remote instance: none
Start time: 2026-06-07T15:52:52Z
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

File	Size	SHA256	Type	Notes
`files/a12c7376-5639-45bb-9a52-e8d8649e9954.zip`	631	`<hash redacted>`	Zip archive data, at least v2.0 to extract, compression method=deflate	zip entries: 2 shown in artifact inventory JSON
`files/extracted/chall.py`	234	`<hash redacted>`	Python script text executable, ASCII text
`files/extracted/msg.enc`	160	`<hash redacted>`	ASCII text, with no line terminators

Evidence Ledger

Time	Action	Output/File	Finding	Confidence	Next
2026-06-07T15:52:52Z	harness init	`challenge-state.json`	Workspace initialized with deterministic state file	High	Inventory artifacts
2026-06-07T15:53:12Z	artifact inventory	`analysis/artifact-inventory.json`	3 artifact(s) inventoried	High	Build or update hypotheses
2026-06-07T15: <REDACTED>, identify byte-wise transformation, then implement inverse in solve/solve.py.
2026-06-07T15:53:30Z	research skip	`analysis/research/research-skip.md`	Research intentionally skipped with recorded reason	Medium	Gate before exploit
2026-06-07T15:53:30Z	checkpoint recorded	`analysis/checkpoint-hypothesis_ready-20260607T155330690608Z-3f359780.md`	Checkpoint for <secret redacted>	High	Use checkpoint to drive next decision
2026-06-07T15: <REDACTED>
2026-06-07T15:56:08Z	completion gate	`challenge-state.json`	Completion gate passed; state marked COMPLETE	High	Optional sanitized memory summary approval

Key Findings

chall.py implements a byte-wise affine cipher: multiply by 123, add 18, then reduce modulo 256.
msg.enc stores ciphertext as ASCII hex, so the first decode step is bytes.fromhex(...).
Because gcd(123, 256) = 1, the modular inverse exists and is 179, giving decryption formula m = 179 * (c - 18) mod 256.
The plaintext is recoverable offline with a short deterministic script; no brute force or external research is needed.

RAG / Advisory Memory

RAG output is advisory only. Record evaluated retrievals with:

bash

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.

Technical analogy

How to remember this solve

Think of the challenge like a locked box where the lock is mathematical but slightly flawed. The goal is not to smash the box; it is to notice which part of the lock repeats, leaks, or trusts the wrong assumption.

For BabyEncryption, keep the mental model simple: identify the trusted assumption, prove it with the smallest safe test, then automate or repeat only the part that directly leads to the flag.

BabyEncryption

BabyEncryption attack path

High source coverage

Technical Walkthrough

Writeup

Challenge

Summary

Artifact Inventory

Analysis

Solve

Flag

Lessons

Source-Backed Dossier

Notes

Scope

Harness Status

Artifact Inventory

Evidence Ledger

Key Findings

RAG / Advisory Memory

Secrets/Flags

Memory Summary

Metadata

Validated Solve Chain

Reusable Lessons

Dead Ends

Tool Quirks

Evidence Paths

Ingestion Decision

Hypothesis Board

Closed Branches

Memory Summary

Sanitized Memory Summary

Metadata

Validated Solve Chain

Reusable Lessons

Dead Ends

Tool Quirks

Evidence Paths

Ingestion Decision

Notes

Notes

Scope

Harness Status

Artifact Inventory

Evidence Ledger

Key Findings

RAG / Advisory Memory

Secrets/Flags

How to remember this solve

Related insights