Chain of custody for digital evidence: a practical guide
Chain of custody is the continuous, documented record of everyone who has handled a piece of evidence, when they handled it, and what they did. For physical evidence the concept is old and well understood. For digital evidence, files, exports, logs, messages, the same principle applies, but the mechanics are different and easy to get wrong.
Why it matters
Digital evidence is trivially easy to copy and, without the right controls, to alter. If an investigation cannot demonstrate that an item is authentic and unchanged since collection, the evidence can be challenged and the conclusions built on it undermined.
A defensible chain of custody answers three questions for every item: is it authentic, is it unaltered, and can we prove who handled it throughout its life.
Practical controls
Hashing on intake: generating a cryptographic hash when an item is collected creates a fingerprint. If the item changes, the hash changes, making tampering detectable rather than assumed.
Append-only logging: recording every view, download, annotation and transfer in a log that cannot be edited or deleted preserves an unbroken history.
Access control and legal holds: scoping evidence to need-to-know, recording each access, and freezing items so they are not disposed of while a matter is live.
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