26 Sep, 2016 13:21

An innovative digital forensics technique has been used by Surrey Police in securing the successful conviction of a man for voyeurism.

The technique, which has previously only been used by two other forces in securing convictions for two child sex offence cases, is a scientific method of proving that specific digital images recovered in the course of an investigation come from a specific camera. The method can also be used to prove that images were taken by the same camera in cases where the camera is not recovered, and can also be applied to video footage.

In October 2014 a female member of staff at a school in Surrey located a hidden recording device in a unisex staff toilet. The device was cleverly concealed within a plastic pipe and attached alongside other plumbing to disguise it.

No physical evidence was recovered from either the device or the location from which it was found. After digitally examining the device, it was found to contain both images and videos of male and female members of staff using the toilet. The footage could also be seen to have been recorded at multiple locations within the school including another toilet. However, it appeared that nothing further could be done with the footage evidentially.

All of the victims captured by the device were identified and spoken to. All lines of enquiry were exhausted and the investigation stalled, with no suspect identified.

In February 2015, investigating officer Detective Constable Ben Andreas asked staff at the school to come forward with their suspicions about who might be responsible.

Several female members of staff came forward with the same name and a man who worked at the school was subsequently arrested. His house was searched and electronic devices and media were seized, including a mobile phone and memory card.

The mobile phone was digitally examined, and a series of images was recovered from the memory card within that device. Those images were recovered “deleted” files and appeared to be a series of stills from a video depicting a woman getting changed in a bedroom. They looked similar to the images recovered from the hidden device in terms of the resolution and colour cast but there was still no evidence to support this.

Undeterred, DC Andreas contacted Professor Jessica Fridrich at Binghamton University in New York, an expert in Digital Image Forensics Using Sensor Noise, a technique which uses the unique digital sensor electrical noise produced by a camera when creating an image to extract a “digital fingerprint” which is unique to the specific camera sensor used to create the image. This enables two images, or an image and a camera, to be compared and say whether that camera took those images or whether those images came from the same camera.

The data recovered from the suspect was sent for testing – with compelling results. The recovered deleted files from the mobile phone and the videos recovered from the hidden device were found to have been produced by the same camera, with a chance of error at less than 1:10,000000.
The data was later ratified by further analysis carried out by Professor Chang-Tsun Li at Warwick University.

The suspect was charged with voyeurism and maintained a not guilty plea throughout multiple pre-trial hearings, with a crown court trial set for September. However, after being confronted with the findings from the digital forensics tests, he changed his plea to guilty and was sentenced to 10 months’ imprisonment at Guildford Crown Court last month.

DC Andreas said: “Without this technique, it would have been virtually impossible to bring this man, who violated the privacy of innocent victims in the most intimate of settings, to justice and he most certainly would never have pleaded guilty without the overwhelming evidence against him which it provided. This technique has been used by two other forces on two previous occasions but is still a very niche field that has not yet found its way into mainstream policing use. However, I believe this technique is invaluable and could provide a much-needed breakthrough for similar cases, and I am already considering using the technique for one of my ongoing rape investigations.”