Challenge 18: Encrypted Payload

Difficulty: Medium
Category: Non-Printable Ratio Bypass

Objective

Use encryption to completely transform a malicious payload into something the scanner cannot signature-match. Navigate both the entropy check and the non-printable byte ratio check to avoid all flags.

Scanner Behavior

The scanner performs two relevant checks on file contents:

Non-Printable Byte Ratio: If more than 40% of bytes (in files >= 64 bytes) are non-printable, the file is flagged as suspicious.
Shannon Entropy Check: The scanner calculates the Shannon entropy of the file’s byte distribution. If entropy exceeds 7.2 bits/byte, it issues a warning. However, this check only warns — it does not block the file.

The scanner does not attempt decryption. It has no knowledge of keys, algorithms, or encrypted data structures. It only sees raw bytes.

Rules

Start with a payload that contains known malicious signatures (words from the signature list).
Encrypt the payload so that no signatures are detectable in the file on disk.
The final file must not be blocked by the non-printable ratio check.
The entropy warning is acceptable (it doesn’t block), but eliminating it is a bonus.
Include a decryption mechanism or document how the payload would be recovered.

Hints

AES (and most encryption) produces output that looks random — high entropy, uniformly distributed bytes.
High entropy triggers the warning (>7.2 bits/byte), but the warning doesn’t block. The real problem is the non-printable ratio.
Encrypted output is essentially random bytes — most of them will be non-printable. That will trigger the 40% ratio check… unless you do something about the representation.
What if you took the ciphertext and re-encoded it into something entirely printable before writing it to disk?

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