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Community / AGENT SKILL
measuring-section-entropy-to-detect-packing
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Detects packing and encryption by measuring Shannon entropy across a binary's sections and regions: flagging high-entropy executable sections, entropy spikes, and size/raw-vs-virtual anomalies that indicate compression or encryption. Activates for requests to measure entropy, detect packing, or assess whether a sample is compressed/encrypted.
Understand the trade-offs.
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Original instructions from the publisher’s SKILL.md
# Measuring Section Entropy to Detect Packing ## When to Use - You want a quick statistical signal of whether a sample is packed or encrypted. - You are triaging a PE and need to flag suspicious high-entropy executable sections. - You are deciding whether to route a sample to an unpacking workflow. **Do not use** entropy as a verdict — legitimate compressed resources and installers also show high entropy; combine with imports, section names, and raw/virtual size anomalies. ## Prerequisites - Python (`math` stdlib); optionally `pefile` for per-section analysis. ## Safety & Handling - Compute entropy on the inert file; never execute the sample. - Keep the sample password-protected at rest and reference it by hash. ## Workflow ### Step 1: Compute whole-file and sliding-window entropy Calculate Shannon entropy (0–8 bits/byte). Sliding-window entropy reveals localized high-entropy regions even when the overall value is moderate. ```bash python scripts/analyst.py entropy sample.exe ``` ### Step 2: Compute per-section entropy (PE) For each PE section, compute entropy and compare raw vs. virtual size. Executable sections with entropy > ~7.0 are a packing indicator. ### Step 3: Correlate structural anomalies Flag classic packer signs: high-entropy `.text`, unusual/renamed sections (`UPX0`, random names), tiny raw size but large virtual size, and an entry point outside `.text`. ### Step 4: Decide routing If indicators stack (high entropy + thin IAT + odd sections), route to unpacking; otherwise proceed with normal static analysis. ## Validation - High-entropy executable sections are corroborated by other packing signs before concluding. - Benign high-entropy cases (compressed resources) are not misclassified as packers. - The routing decision (unpack vs. proceed) is justified by combined indicators. ## Pitfalls - Calling any high-entropy file "packed" without structural corroboration. - Ignoring sliding-window entropy and missing a localized encrypted blob. - Overlooking raw-vs-virtual size mismatch, a strong unpacking-at-runtime hint. ## References - See [`references/api-reference.md`](references/api-reference.md) for the entropy tool. - Shannon entropy and the PE/COFF spec (linked in frontmatter).