File Entropy Analysis

File entropy, in a simplified sense, is a mathematical measure of the randomness or disorder within the data of a file. It's calculated based on the distribution of different byte values within the file. In the context of screensharing and malware analysis, file entropy serves as a useful heuristic indicator, particularly for identifying files that might have been intentionally packed, encrypted, or obfuscated.

• Why it Matters: Legitimate, uncompressed executable code and standard data formats tend to have structure and repetition, resulting in relatively lower entropy values. Conversely, processes like compression (lossless or lossy), encryption, and the techniques used by software packers (tools designed to compress executables and protect them from reverse engineering) inherently increase the randomness and unpredictability of the byte distribution within the file.

• Interpreting Entropy Values: Entropy is typically measured on a scale from 0 (perfect order, e.g., a file full of null bytes) to 8 (maximum randomness, typical of well-encrypted or compressed data).

  • Low Entropy (e.g., < 6.0): Often seen in uncompressed text files, simple executables with lots of structured data, or files with large sections of uniform data.

  • High Entropy (e.g., > 7.0 - 7.5): This is a strong indicator that the file contains highly random data, which is characteristic of encrypted content, compressed data, or code obfuscated using sophisticated packers (like Themida, VMProtect). Malware and cheats are frequently packed or encrypted precisely to evade signature-based detection and hinder analysis.

• As a Suspicion Indicator: While high entropy does not definitively prove a file is malicious (legitimate installers, compressed archives, and encrypted documents also have high entropy), finding an executable (.exe, .dll) with unusually high entropy, especially if it's unsigned or found in a suspicious location, serves as a significant red flag. It strongly suggests the file's true nature is being concealed and warrants further investigation using other tools and techniques (e.g., dynamic analysis, unpacking attempts, signature checks).

• Analysis Tools: Several tools can calculate and display file entropy:

  • Detect It Easy (DiE): Includes entropy calculation as part of its PE file analysis features, often showing entropy per section.

  • VirusTotal: The file analysis report on VirusTotal often includes an overall file entropy value.

  • Specialized forensic tools or standalone entropy calculators.

Analyzing file entropy provides a quick way to flag potentially obfuscated executables that require closer scrutiny.