Applying the ACPO guidelines for electronic evidence

The Association of Chief Police Officers (ACPO) publish guidelines for handling electronic evidence and it is important that these are strictly adhered to when investigating computers or digital media. Four principles are involved:

Principle 1: No action taken by law enforcement agencies or their agents, should change data held on a computer or storage media, which may subsequently be relied upon in court.

Principle 2: In exceptional circumstances, where a person finds it necessary to access original data held on a computer or on storage media, that person must be competent to do so and be able to give evidence explaining the relevance and the implications of their actions.

Principle 3: An audit trail or other record of all processes applied to computer based electronic evidence should be created and preserved. An independent third party should be able to examine those processes and achieve the same result.

Principle 4: The person in charge of the investigation (the case officer) has overall responsibility for ensuring that the law and these principles are adhered to.

In practice this means that a chain of custody needs to be established, ensuring that no unauthorised access to the media can occur. When interrogating the media, it is necessary to use a write-blocker, so that the data cannot be altered or overwritten. Only approved forensic tools should be used and all interrogations completed on a forensic clone of the media, not on the original media.

In a minority of cases, it may not be possible to obtain an image using a recognised imaging device. In these circumstances, it may become necessary for the original machine to be accessed to recover the evidence. With this in mind, it is essential that a witness, who is competent to give evidence to a court of law, makes any such access.