Chain of Custody: Data Integrity for Justice

When a photo is captured at a crime scene, it must be the same one that is presented in court. 
When a bystander records a video, it must be the same one that the jury sees. 
When an investigator captures an audio file, it must be the same one that the judge hears. 

In digital evidence, this simple truth defines the chain of custody - the unbroken record of where evidence has been, who handled it, and whether it has changed. Every moment of uncertainty between capture and presentation can become a point of legal challenge. 

The Science of Verification 

To prove that a file has remained unaltered, digital forensics teams use hash functions and hash checks. 

A hash function is a mathematical process that converts the contents of a digital asset, such as an image, into a unique digital fingerprint: a string of numbers and letters that will always be the same for that exact file. Even the smallest change, like a single pixel, a fraction of a second trimmed from a video, or an edited word in a document, produces a completely different result. 

For example, the commonly used SHA-2 hash function generates a 64-character fingerprint such as: 

2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824 

Once a hash string has been generated, investigators can later run the same hash function on the file again and perform a hash check, which is the comparison between the original hash and the new one. If the two match, the file is identical to the original. If they differ, even by one character, something in the file has changed. 

This process allows investigators, prosecutors, and defence teams to independently verify that a file is exactly the same as when it was first collected. In court, a matching hash check is one of the few absolute proofs of digital integrity.

When the Chain Breaks 

Consider this scenario: 

Detective Smith records a body-worn video of an arrest. Back at the station, the footage is copied onto a desktop, then to a USB drive for the lead investigator. Later, another copy is made for the attorney’s office, who puts the file into an online sharing tool so that it can be access by defence. There’s no audit log of the transfer, no recorded hash from the device, and no central repository governing who accessed the file or when. 

When the case reaches court, the defence argues: 

“There’s no proof this video is the same one captured on the day. It could have been edited.” 

Without a verifiable chain of custody, the judge admits the footage only for limited use. A key piece of evidence loses its weight - not because of what it showed, but because the process failed to guarantee its authenticity. 

The New Challenge for Digital Evidence Management 

The issue of data verification and chain of custody has become more pressing with the rise of digital evidence management systems (DEMs) that not only store evidence, but also include features for analysis, enhancement, and watermarking. These capabilities can add operational value - yet they disrupt the most critical principle in the justice process: the integrity of the chain of custody. 

Such changes to digital files will never pass a hash check and, as a result, may be ruled inadmissible in court if there is any doubt that the modified version differs from the original. Accurate judgements and fair assessments in the legal system depend on absolute confidence that evidence has not been tampered with. 

There are already examples of DEM platforms being restricted or precluded from legal use precisely because their built-in enhancement features introduce the potential for modification - even if they are not used. 

The Role of Centralisation

Modern Digital Evidence Management (DEM) systems promise convenience: cloud storage, search, even image enhancement and watermarking. But these same capabilities can threaten admissibility if they allow untracked modification or decentralised handling. 

True integrity demands centralisation. 
All evidence - whether photo, video, audio, or document - must enter a controlled environment the moment it’s collected. From there, every access, transfer, and download must be logged. Every file must be hashed and verified. No copy should ever exist outside that auditable ecosystem. 

Only a centralised system can guarantee that what was captured is what is presented. 
Only a system built around immutability, auditability, and transparency can protect justice from uncertainty.

Why It Matters 

The goal of any justice process is accuracy to ensure decisions are made on untainted facts. If the evidence itself cannot be trusted, the foundation of fairness collapses. 

Centralised, verifiable digital evidence management isn’t just good practice; it’s essential. In a world where a single pixel can alter a verdict, chain of custody is the last line of defence between truth and doubt.