Electric Network Frequency Analysis Authenticates Forensic Records from the Cloud

Forensic researchers have learned that the electromagnetic hum made by modern power grids leaves a kind of watermark on digital recordings like audio and video recorded with a computer.

This watermark hum subtly, randomly changes over time.  Analysis of this Electric Network Frequency (ENF) hum can assist in establishing the time a recording was made and whether pieces of the recording were doctored or spliced together at different times.

Corroborate Recording in Criminal Court

Electromagnetic Watermark
Based on this watermark hum, a criminal court in the UK concluded that an audio recording by police was not manipulated by the police.  The police recording stood up in court under analysis by an ENF expert.  The expert proved that the recording was what it purported to be: a single continuous recording, not a collection of spliced snippets.

Application to Forensic Recordings in the Cloud

In the age cloud computing, continuous recordings of audio and video are growing more valuable for capturing and preserving digital evidence.  The reason is that investigators who gather evidence from “the cloud” often don’t have control over the cloud computers or permission from the people who do have control (i.e., the service providers).

What do I mean by “the cloud?”  My definition of the cloud here broadly includes social media like Facebook and LinkedIn, as well as a lot of the web and many mobile apps.  Today it is very common for an investigator to gather evidence from (for example) a social network, with no cooperation or permission from the social network provider.  This investigator is like an eyewitness.  He or she sees and hears (ascertains) some evidence (text, images, video, links, audio) from the cloud; he or she wants to capture the evidence.  The evidence the investigator sees or hears can be gone in an instant.

The investigator needs to make a credible record of what he/she witnesses.  A way to do that is to make a “screencast” video of what the investigator encounters moment to moment.  As the video record is made, the investigator can narrate as a realtime eyewitness, explaining what is happening as it unfolds.

Corroborating Date and Time

In a well-made screencast, the investigator states date and time with the investigator’s voice and moving lips.  The investigator corroborates date and time by storing the screencast video in a system (like Sharepoint) that logs the date and time.



Yet, now date and time is corroborated by an additional method.  The electromagnet hum of the power grid watermarks the record to help show the record was created as a continuous stream at a particular time.

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