Fixity checking

What is fixity checking?

Fixity checking, also known as integrity checking, is the process of verifying that a digital file has not been altered or corrupted over time.

It works by generating a unique digital fingerprint (called a ‘checksum’) for a file and comparing it over time. If you generate a new checksum for the file and it is different to the original, it means the file has changed. The change could be intentional or due to corruption, bit rot, hardware failure, or unauthorised modification.

Fixity checking is fundamental to ensuring digital materials remain accessible, usable and trustworthy over the long term.

Why does fixity checking matter?

Even if your files are properly backed up and stored on appropriate platforms, they are still vulnerable to change. Over time, digital materials can degrade or become corrupted without any warning signs.

Fixity checking helps to:

• Detect data corruption or degradation (bit rot)
• Confirm the integrity of files after they are moved between locations
• Ensure the integrity and authenticity of digital materials.

Without regular fixity checking, there’s no reliable way to be sure the file you stored years ago is still the same today.

Checking fixity

An effective approach to fixity checking includes:

• Generating checksums and storing them securely with the file’s metadata
• Routinely comparing the stored checksums with newly generated ones to verify the file’s integrity
• Investigating discrepancies between stored and new checksums and restoring files to earlier versions, if necessary
• Documenting the outcome of fixity checking processes.

Tools for checking fixity

Digital preservation systems will generally perform fixity checking processes for you.

If your digital materials are not stored in a dedicated digital preservation system, you can use a tool like BagIt or Fixity Pro, or you can use a command-line checksum utility.

See Community Owned digital Preservation Tool Registry (COPTR) for a comprehensive list of fixity checking tools.

Frequency of fixity checks

How often to perform fixity checks depends on many factors including the type of storage being used, how well the storage is maintained, and the size of the collection.

Creating and checking checksums is resource intensive, sometimes requiring considerable processing power and load on storage systems. Therefore, costs and environment impact should be considered when determining the frequency of your fixity checks.

According to the Digital Preservation Coalition, as a general guideline, data tapes might be checked annually and hard drive based systems might be done every six months.