Summary

This challenge buried the real logic inside a native library that was called from obfuscated JavaScript. The package contained two flag.enc files, but only one of them belonged to the actual challenge path.

The encryption flow was layered: a custom byte transform, a fixed marker, PKCS#7 padding, and finally AES-CBC with fixed ASCII key and IV values. Once the native transform was understood, the file decrypted cleanly.

Analysis

The first job was identifying the real artifact. The unpacked package contained extracted/unpacked/flag.enc, which turned out to be the relevant ciphertext. The smaller root-level file was just a decoy.

The shared library exported three interesting functions:

  • EncryptFileHex
  • DeriveRoundFingerprint
  • DescribeCipherSuite

EncryptFileHex was the core entry point. It did not encrypt a raw string; it read bytes from a file path, transformed them, and returned the final ciphertext as hex.

That distinction mattered because it explained why direct string tests looked inconsistent at first.

Exploitation

The solve path was:

  1. Decrypt the outer AES-CBC layer.
  2. Remove the fixed marker.
  3. Recover the internal T(msg) transform.
  4. Reverse the stages in the opposite order.
  5. Recover the original plaintext.

The fixed cryptographic values were:

1
2
3

Key = "021b49755fb4961a40f3a539ee80fa8f"
IV  = "8cc46e76876a55c1"
Marker = "67e672f4049b06ee"

Once the native transform was inverted, the plaintext revealed the flag.

Flag

UNR{cu_m454l4r174-m1r3454_54-1_713_d3_1mp4r473454_f4abc8120c3d2a57}

Tags: nodejs aes-cbc reverse-engineering