Digital Images as Evidence

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At last it is out, the long awaited report from the House of Lords Select Committee on Science and Technology.

Most people in the industry and many end users have been waiting with baited breath for what was expected to be a series of draconian regulations imposing severe restrictions on the use of digital recordings. Everyone I spoke to on the subject had their own views of impending doom and enormous increases in costs to comply with these, as then, unpublished papers.

The result? I found the report to be an extremely down-to earth, pragmatic document that deals with the subject realistically. It recognises the inevitability of progress along the digital route and the problems of creating legislation based on technological criteria. The recommendations do come down very heavily on the need for secure audit trails from initial recording to copies produced as evidence.

Although the report does not require forms of encryption, watermarking or other anti-tamper measures it should be remembered that these techniques may well be necessary for other security requirements. The report deals with digital images as evidence only.

There is one great difference between analogue and digital recording. If a copy is made from a tape recording, the copy will be of a lessor quality than the original, if further copies were made from copies the results may well be unusable. A digital recording however, consists of a series of binary digits which can be copied an unlimited number of times with no degradation of the images compared to the original. If an 'original' set of images was on a CD for instance and then copied several times, it would be impossible to determine which was the original and which were copies.

The report draws on a great deal of experience in relation to the digital storage and copying of documents and applies this to digital images. Following is a selection of extracts from the report.

What is an original? The report prefers; "The original is the data first recorded in memory. Thus any printed or displayed image created from these is a copy. Consequently digital recording technology provides no original that could be produced in evidence. All that is available for evidence is a copy of the first, probably temporary, recording in memory, and this will be admissible as evidence. Its weight as evidence will depend on proper authentication and other matters".

In the case of a digital camera, it is probable that the original would be the digital file representing the image. This would be stored on a memory chip or series of chips and immediately transferred to some other form of storage (hard disc, etc.) and the memory chip being overwritten with the next image. "This does not represent a problem under the Law of England and Wales because if the original of a document no longer exists, copies or even copies of copies are admissible as evidence and it is irrelevant that the original was destroyed by the person seeking to produce the copy as evidence. Nor is it a problem in Scotland because although the general rule that copies of documents are admissible whether or not the originals still exist does not apply to visual images, copies of a document which no longer exists are admissible under the best evidence rule. The fact that a document is a copy goes to its weight as evidence, not its admissibility. It will therefore be necessary for the user to be able to give evidence of the procedures used for generating, processing and storing digital images. So as to be able to prove that the image produced to the court is an accurate copy of the original". ....In general the court is likely to admit the evidence, the judge will direct the jury on the weight they should consider attaching to it.

There is a section of the report dealing with the possibilities of modifying a digital image. There are a variety of inexpensive software packages that can perform sophisticated alterations to digital images. These techniques can be used for several reasons ranging from simple enhancement to making significant and possibly malicious alterations to an image. "There is no clear distinction between acceptable 'enhancement' and unacceptable 'manipulation'. Any changes have to be considered on their merits.

The need for caution. With modern processing and tele-communications techniques even an image that purports to be analogue may have had a 'digital past' For example, it is possible that an image stored digitally may have been generated with a standard analogue camera, the signal or picture may be converted into digital format for transmission and then converted back to analogue again to be displayed. Thus in many circumstances, it can be difficult to make a distinction between what is and what is not a digital image. The ease with which images, when in digital format, can be copied and modified suggests that caution must be exercised when any image is used as evidence: all images, both analogue and digital might be suspect.

But are these concerns real? ......We concluded that before the advent of digital technology it might have been a time consuming and costly exercise to produce a modified image in which it was difficult to detect tampering, but with the present widespread availability of digital technology it could now be a low cost operation to produce an image in which the modifications would be undetectable. The existence of a technology that can be used to modify images in this way need in itself be of no great concern; even the widespread availability of the technology at low cost might not cause concern. .........This means that when presented with an image the observer should have no more, and no less, faith in it than if the information had been text on a sheet of paper of questionable provenance".

The report continues to state that witnesses regarded the differences between digital images and other evidence as being one of 'degree rather than of fundamental kind'.

There is a distinction between the admissibility of evidence between civil and criminal cases. "......This means that in criminal cases, any use of a digital image as evidence must be accompanied by the certificate required under section 69 (of the Police and Criminal Evidence Act 1984) This certificate, given by a person responsible for the computer system in question, must state that either the computer system was at all times operating properly, or that any defect in its operation was not such as to affect the accuracy of the record".

The Law Commission has recently recommended the repeal of Section 69 of PACE because 'it serves no useful purpose'. With the repeal of this section, a presumption of proper functioning would be applied to computers.

The report emphasised many times the need for an accurate recorded audit trail from the initial image to the copy produced in court. "A prosecutor or party to litigation will always need to be prepared to offer further evidence about the source of a digital image and the processing and storage it has undergone since it was first recorded. It has been held that the person adducing a recording as evidence must describe its provenance and history, so as to satisfy the judge that there is a prima facie case that the evidence is authentic".

The next chapter concentrates on the use of digital images and continues the theme of authentication. "The fact that a document is a copy may reduce its weight as evidence, unless there is sufficient authentication evidence to convince the court that is an accurate copy. This authentication evidence would normally be in the form of an audit trail connecting the original image with the computer record which is to be adduced in evidence and recording what has occurred to that record in the interim.

Here the report refers to a British Standard Code of Practice for the Legal Admissibility of Information on Electronic Document Management Systems. (DISC PD 0008, February 1996). This sets out procedures and documentation required for the audit of systems producing documents or other images that may be used as evidence in a court of law. In the absence of any Code of Practice more relevant to CCTV images, it may be that we all need to obtain a copy of this BS.

I was very surprised to note the following comment. "Of considerable interest to us was that no defence teams in the United Kingdom had, as yet, ever requested an audit trail be produced in any case where video images were being used by the prosecution This may change as defendants and their lawyers become more familiar with the technology".

I have a feeling that this situation is going to change rapidly, and, of course will not only apply to digital recordings.

The report goes into some length about watermarking and encryption technologies and the pros and cons of their usefulness in authenticating digital recordings. It continues:

"The arguments against specifying new criteria which must be met before evidence can be admitted are:-

  • It would be very difficult to specify the nature of the authentication technology in such a way that it would not quickly become outdated as the technology advances.
  • It would take an appreciable time for manufacturers of digital image technology to incorporate such measures, and even longer for such technology to become widely used;
  • When technology advances, the courts will be faced with the position that images over which there is no dispute as to their reliability cannot be received as evidence because they were not captured by technology which met the specification; and
  • The clear trend in the development of the law is to remove prior requirements for all forms of documentary evidence, leaving it to the courts to determine whether the evidence is reliable.

For these reasons we are not convinced that some sort of criteria must be met before evidence can be admitted. Rather we agree with the witnesses who said that there should not be different rules about admissibility based on the technology used to capture the evidence".......

"We recommend that evidence should not necessarily be inadmissible because it does not conform with some form of technological requirement".

However, it continues, that although there should not be technological requirements which digital images must meet, it does not mean that the report is against authentication technologies. Quite the reverse, "We support the application of any technology which can help with the verification of an image and provide assistance to the court in assessing its worth".

"We recommend that the Government encourage the use of authentication techniques. Members of the legal profession should be aware of the benefits of these techniques, their value in adding weight to evidence and the possible significance of their omission.".

Technical procedures can only be part of the authentication process, provenance of evidence can be greatly enhanced by the correct procedural measures. "We recommend that the Government produce guidance on the benefits of conformance with procedural measures to establish the reliability of evidence, with particular reference to existing standards. When this guidance is available, we recommend that the trade associations of those organisations likely to be concerned with it produce training material on its use.

The report continues with a chapter on Civil Liberty implications and a final summary and recommendations.

So, digital images are admissible as evidence in court whether or not they have been manipulated. Their weight as evidence will be decided by authentication methods such as encryption or watermarking and particularly a secure audit trail from initial image to copy produced in court.

I found the document quite readable and interesting, bringing out many aspects of digital documents and images when used in evidence. I would commend everyone who has an interest in digital recording, whether end user, manufacturer, installer or consultant to obtain a copy. The full report is 37 pages and at £7.80 it's a steal.

The full report is entitled;
House of Lords, session 1997-98, 5th report
Select Committee on Science and Technology
Digital Images as Evidence
available from;
The Stationary Office, London
Telephone, 0345 023474
Reference HL paper 64, price £7.80. Payment can be made over the telephone by credit card.

Please also see our article "Digital Evidence".