Decrypting Data with Overwrite Analysis

TL;DR

This guide shows how to potentially recover data from encrypted files by exploiting accidental overwrites. We’ll look at identifying overwritten sections, reconstructing partial data, and using this information to break the encryption or reveal plaintext.

Understanding Accidental Overwrites

When a file is ‘deleted’ or partially overwritten, it doesn’t always mean the original data is gone. Often, only parts of the file are replaced. If an encrypted file has been accidentally overwritten with known data (e.g., from another file copy operation), we can potentially use this to our advantage.

Step-by-Step Guide

1. Identify Potential Overwrites: The first step is finding files that might have been partially overwritten. Look for files with modified timestamps close to the time of a known data transfer or copy operation.
   • Use file system metadata (timestamps, sizes) to narrow down candidates.
   • Consider files that were open at the same time as the overwrite event.
2. Create a Disk Image: Before doing anything else, create a full disk image of the drive containing the encrypted file. This preserves the original data and allows you to work on a copy.

   dd if=/dev/sdX of=disk_image.img bs=4096 conv=sync,noerror

   (Replace /dev/sdX with the correct device identifier.)

3. Hex Dump and Analysis: Use a hex editor (like HxD on Windows or `xxd` on Linux) to examine the disk image. Look for repeating patterns of known data within the encrypted file.

   xxd disk_image.img | less

   • Focus on areas where the encryption appears ‘broken’ – sections with recognisable text or common file headers.
   • Look for fragments of files that were copied *to* the drive around the time of the overwrite event.
4. Identify Overwritten Blocks: Pinpoint the exact blocks within the encrypted file that contain overwritten data.
   • Compare sections of the hex dump to known files or data sources.
   • Note the starting and ending offsets of these overwritten blocks.
5. Reconstruct Partial Data: Extract the overwritten blocks from the disk image.

   dd if=disk_image.img of=overwritten_data.bin skip=$START_OFFSET bs=1 count=$BLOCK_SIZE

   (Replace $START_OFFSET with the starting offset and $BLOCK_SIZE with the size of the overwritten block.) Repeat for each block.

6. Attempt Encryption Key Recovery (Simple Cases): If the overwrite included parts of a key file or initialization vector (IV), you might be able to recover them directly.
   • Search the extracted data for known key patterns or IV formats.
   • If found, try using these recovered values with decryption tools.
7. Statistical Analysis: If direct key recovery fails, perform statistical analysis on the reconstructed partial data.
   • Look for common byte sequences or patterns that might indicate the encryption algorithm used.
   • Frequency analysis can sometimes reveal clues about the plaintext (especially with simple ciphers).
8. Brute-Force/Dictionary Attacks: Use the partial data as a starting point for brute-force or dictionary attacks against the encryption.
   • Tools like John the Ripper or Hashcat can be used to test potential keys based on the recovered fragments.
   • Focus your efforts on key spaces that are likely given the context of the encrypted data.
9. Consider File Type: Knowing the original file type (e.g., PDF, DOCX) can help.
   • File format structures have predictable headers and footers. Overwritten sections might still contain enough of these to identify parts of the plaintext.
   • Specialised tools for recovering data from specific file types may be useful.

Important Considerations

• Encryption Strength: Strong encryption algorithms are much harder to break, even with overwritten data. This technique is most effective against weaker or older ciphers.
• Overwrite Extent: The more complete the overwrite, the less likely you are to recover useful information.
• Legal Implications: Ensure you have the legal right to access and analyse the encrypted file before proceeding.