Analyzing Ransomware Encryption Behavior

When to Use

• You need to understand how a ransomware sample encrypts data and whether recovery without the key is feasible.
• You are documenting file targeting, markers, and the ransom workflow for an IR report.
• You want to identify the crypto primitives to assess decryptability and detection points.

Do not use this to attempt key recovery on production data without backups; analyze in the lab on disposable test files only.

Prerequisites

• Isolated lab with disposable test files of varied types/sizes.
• Static/RE tools (see the Ghidra and crypto-identification skills) and the dynamic-analysis setup.
• A clean snapshot to revert.

Safety & Handling

• Detonate only against throwaway files in the isolated victim VM; never real data.
• Treat the sample and any dropped components as live; revert after each run.

Workflow

Step 1: Identify the crypto scheme statically

Locate crypto constants and APIs to classify the scheme:

Symmetric only (AES/ChaCha/RC4)  -> key may be derivable/recoverable if mishandled
Asymmetric (RSA/ECC) wrapping    -> per-file symmetric key wrapped with attacker pubkey
Hybrid (typical)                 -> AES per file, RSA-wrapped key in the encrypted file/footer

Look for AES S-boxes, ChaCha constants ("expand 32-byte k"), and CryptImportKey/BCrypt*.

Step 2: Trace key generation and handling

Determine where the symmetric key comes from (CSPRNG vs weak source), whether it is stored, and how it is protected. Weak/reused keys or keys left in memory are recovery opportunities.

Step 3: Map file targeting

Identify which directories/extensions are targeted or skipped (allowlist of system paths), size thresholds, and partial-vs-full encryption (many encrypt only the first N bytes/blocks for speed).

Step 4: Capture markers and metadata

Most families append an extension, write a per-file footer/magic marker, and store the wrapped key or IV there. The script fingerprints before/after test files to extract markers.

python scripts/analyst.py compare plain/ encrypted/

Step 5: Document recovery-relevant facts

Note shadow-copy/backup deletion (vssadmin delete shadows), the ransom note, and any cryptographic mistakes that affect decryptability.

Validation

• The identified scheme matches observed file changes (e.g. hybrid → high-entropy body + a fixed-size footer).
• File markers/extensions are consistent across encrypted test files.
• Shadow-copy deletion and note-drop are confirmed dynamically.

Pitfalls

• Assuming decryptability from the algorithm name; correct AES/RSA usage is not recoverable. Recovery hinges on implementation flaws or leaked keys.
• Missing partial-encryption schemes, leading to wrong impact assessment.
• Running against real files instead of disposable test data.

References

• See references/api-reference.md for the file-marker comparison tool.
• No More Ransom and NIST crypto standards (linked in frontmatter).