Tags: rev
Rating:
# Coverup
**Event:** Nullcon Goa HackIM 2026 CTF
**Category:** Reverse
**Points:** 89
**Files:** `encrypted_flag.txt`, `coverage.json`, `encrypt.php`, `generate_challenge.php`, `Dockerfile.public`
## Overview
The encryptor applies a custom per-byte mapping, XORs with a processed key byte, and applies the same mapping again. The output is returned as `base64(processed) : sha1(processed)`. Branch coverage reveals which key-byte branches executed, so the unordered set of key bytes is leaked.
## Key Observations
- The key length is 9 bytes (`generateRandomKey(9)`).
- Coverage identifies exactly which `if ($keyChar == chr(n))` branches were hit, yielding the 9 raw key bytes (unordered).
- The per-byte mapping converts each raw key byte into the processed key byte used in the XOR.
- The same mapping is applied to ciphertext bytes, so each byte has a small set of preimages.
## Solution
1. Verify integrity with `sha1(base64_decode(data))`.
2. Extract the 9 raw key bytes from coverage and build the per-byte mapping table.
3. Transform them into the 9 processed key bytes.
4. Compute all valid preimages for each ciphertext byte.
5. Brute-force the 9! permutations of the processed key bytes and filter using the `ENO{...}` format and printable output.
## Flag
`ENO{c0v3r4g3_l34k5_s3cr3t5_really_g00d_you_Kn0w?}`
