All digital evidence is subject to the same rules and laws that apply to documentary evidence.
In other words, the onus is on the prosecution to show to the court that the evidence produced is no more and no less now than when it was first taken into the possession of the investigator.
No action taken by (law enforcement) agencies, persons employed within those agencies or their agents should change data which may subsequently be relied upon in court.
In circumstances where a person finds it necessary to access original data, that person must be competent to do so and be able to give evidence explaining the relevance and the implications of their actions.
An audit trail or other record of all processes applied to digital evidence should be created and preserved. An independent third party should be able to examine those processes and achieve the same result.
The person in charge of the investigation has overall responsibility for ensuring that the law and these principles are adhered to.
The source document for theses principles is available here