Select Committee on Science and Technology Fifth Report


"So what is special about Digital Images?"

  2.1     A key element of digital technology is the ease with which perfect copies can be made (see Box 2). Attempts to copy a conventional analogue audio tape produce an inferior copy but a digital copy of a digital compact disc (CD) is an identical replica. This is one of the great benefits of digital technology. However it can also create commercial difficulties for the manufacturer trying to prevent the production of counterfeit goods, for example. Other key features of digital technology are the ease with which data can be changed and the ease with which data can be matched with information from other databases (see Chapter 4). As one of our witnesses said: "with recent increases in computing power at affordable prices and improvements in imaging hardware, image manipulation has become far more accessible" (p 150). The Chartered Institute of Arbitrators also cautioned that even colour laser photocopiers commonly featured editing facilities, so that tampering with an image is no longer the preserve of computer science (p 156). This is both a great advantage and a severe problem of the technology.

  2.2     If presented with an 'original' digital image and a copy, perfect in every respect, in the absence of any other information it is impossible to distinguish which one was produced first. In many circumstances this might not matter, but if the data are transferred to a computer, modified in some way and then returned to an identical storage device to the one they were recorded on (eg to an identical looking CD which is passed off as the 'original'), it might matter. It would matter greatly if the item described as an 'original' were a photograph of a defendant apparently involved in a criminal activity. Equally, the Crown Prosecution Service expressed concern that the technology could give defendants "the ability to construct an alibi that is convincing ... using home video equipment to manipulate an image" (p 168).

  2.3     Here we find that there is some confusion over what can be described as the original when considering digital images, in particular between the technical and legal definitions (see Box 2). We prefer the following definition in relation to digital images: the original is the data first recorded in memory. Thus any printed or displayed image created from these data is a copy. Consequently digital recording technology provides no original that could be produced in evidence. All that is available for use as 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.

Box 2: What is an original?
An analogue image always has an original to which it relates in either negative or positive form (ie the negatives from which photographic prints are made, transparencies or the magnetic recording on a video tape). Copies can be made from the original and normally there will be little difference between them, but as further copies are made from these copies then the image quality is likely to deteriorate.
For a digital image the 'original' consists of the data recorded in memory, from which the image can be generated. Because the recorded image is represented as a finite set of numbers, exact copies may be made, and images made from these copies. Each stage of copying is precise and there is no loss of image quality between generations. Thus it becomes impossible to say which is a first generation image: the implication is that any digital image can be thought of as being 'the original' even if it is produced from a copied set of data, unless it is tagged in some way to identify it as the first image made (see Box 4 on authentication).
An original for the purposes of the law of evidence is usually the first manifestation of a document in final form. However, where there are multiple possibilities (eg where a fax is sent, both sender and receiver have 'original' copies) the original for some purposes is that version of a document which was intended by its maker to be legally effective.
Because of the possibility of different interpretation of 'original' in law, we have adopted the definition of paragraph 2.3.

  2.4     In relation to a digital camera, for example, it is probable that the original would be the digital file representing the image which is stored on a memory chip, or series of chips, at the moment that the image is captured. That digital file will immediately be copied to some other form of storage and the memory chip will be overwritten with the next image[4]. 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 in evidence[5] and it is irrelevant that the original was destroyed by the person seeking to produce the copy as evidence[6]. Nor is it a problem in Scotland because although the general rule[7] that copies of documents are admissible whether or not the originals still exist does not apply to visual images[8], copies of a document which no longer exists are admissible under the best evidence rule[9]. 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[10]. But in general, provided it would not prejudice a fair trial, the court is likely to admit the evidence and the judge will direct the jury on the weight they should consider attaching to it.

Altering a digital image

  2.5     Because a digital image is just a visual representation of ones and zeros in computer memory it may be altered, deleted or copied at will in the same way as any other computer data. An image may be altered in much the same way as a word processor can be used to change the text of a document: the final text when displayed need give no clue as to how many words or paragraphs have been changed from the first version, nor who changed them and when (or why). A variety of software packages are available which allow one to sharpen and enhance the image, change colours, contrast and brightness, copy and move around segments of the image, and import image segments from other photographs to make a seamless collage. The sequence of a 'movie' can also be altered or reversed to show events in non-chronological order. As discussed in Box 2, the ultimate modified image may be indistinguishable from the first version in terms of quality or the apparent representation of reality.

  2.6     The potential for making subtle, but significant, alterations to an image gives most cause for concern as such changes may be highly convincing and almost impossible to spot. For example, software is available which can be used to add or subtract shadows and ensure that the sun angle on an object (perhaps a person) imported into a base image is congruent with the rest of the scene. Timing mechanisms on electronic devices often do not keep accurate time (and they are often set incorrectly to start with, Q 104) and visual cues from a scene that might corroborate timings (eg a clock face) are easy to alter or obscure.

  2.7     In the course of criminal proceedings it may be necessary to use computer techniques to enhance an image, making the subject clearer (eg by removing signal noise) or isolating the subject from a cluttered background to make identification easier. If the image is to be used for evidential purposes, the question that might be asked is, what represents justifiable enhancement? Does this differ from manipulation which might have, or be perceived to have, malicious intent, and where can the distinction be drawn?

  2.8     Changing the contrast or colour balance of a computer monitor may be considered visual 'enhancement' and is no different from what might be done to an analogue image. However, any alterations which affect the image data, no matter how innocuous, should be treated with caution. Sharpening the outline of an object is a standard option on image-processing software which usually involves changing the contrast at the edge of the image (eg black pixels against grey pixels may become black against white). After sharpening, the object is easier to see, but the image data have been changed.

  2.9     Some modifications are made routinely for image processing, for example de-skewing a page as a document is scanned electronically. The process is automatic, uses a set algorithm to make simple adjustments and if agreed limits are placed on the scope of the operation then this perhaps should be accepted as a justifiable modification. Cleaning-up unwanted signals that arise from outside interference might also be acceptable, although it would be better to ensure that the imaging system is immune from such signals in the first place.

  2.10     If human judgement is involved and the process is more complicated, then the resulting image is of questionable evidential value. For example, improving the clarity of a face in a still from a grainy security video will depend on the software and equipment used, the assumptions made about that face (and this in turn will depend on the bias of the operator), and the composition of any comparative data set used as a reference. Value judgements will be made during the process. There is no clear distinction between acceptable 'enhancement' and unacceptable 'manipulation'. Any changes have to be considered on their merits.

The need for caution

  2.11     With modern processing and tele-communication 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 (Abbey National Q 353). The ease with which images, when in digital format, can be copied and modified suggests that caution must be exercised when any image is being used as evidence: all images, both analogue and digital might be suspect.

But are these concerns real?

  2.12     We took much evidence as to the power of image modification technology (eg Mr Sommer, Mr Davies and Mr Smith, and witnesses from IBM), saw it for ourselves thanks to the assistance of the Forensic Science Service and the Greater Manchester Police, and heard about its widespread availability, low cost and the difficulty of detecting modifications (eg Mr Sommer, Q 115). 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[11]. 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. But an apparent lack of understanding of the implications of both these facts should cause concern and warrants further study. The public and all those in the legal profession (Mr Smith Q 84) should be made more aware of the technology, what it can do, and what its limitations are. It was suggested that criminal convictions that were dependent on evidence captured by digital cameras could be at risk if defence lawyers began to realise how vulnerable such images are to manipulation. 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.

  2.13     However, we also heard that digital images presented opportunities: Mr Marcus, Senior Legal Counsel for IBM, held the view that digital images presented "positive evidentiary opportunities ... for the identification of content owners (owners of copyright) and of unauthorised modifications in ways which are not available to traditional analogue technologies" (Q 261). For example, images could be time-stamped or given other authenticating marks.

The current law on the use of images as evidence

  2.14     Although there is no legislation which expressly covers digital images used as evidence, nor any reported cases in which the fact that an image was collected in digital form was at issue (Crown Prosecution Service p 167) there is a substantial body of the law of evidence which deals with the problems arising from technological recordings. This body of law will equally apply to digital images. Indeed, witnesses including the Senior Counsel from IBM (Q 263) and solicitors from Bird & Bird (Q 78) regarded the differences between digital images and other evidence as being one of "degree rather than of fundamental kind" (Q 78).

Special requirements for admissibility

  2.15     The rules of evidence of common law jurisdictions normally exclude from evidence certain types of document. The most unreliable kind of evidence, under these rules, is 'hearsay', which is not admissible as evidence unless it falls within one of the numerous exceptions to the rule. By contrast, information captured by a recording device, with no human intervention, is not normally hearsay and can be admitted in evidence. Video and image recordings are not therefore excluded as hearsay.

  2.16     In civil litigation, there are no prior requirements which must be met before a document (which would include a digital image) can be admitted in evidence. Section 1 of the Civil Evidence Act 1995 abolishes the hearsay rule for civil proceedings, and Section 8 provides that where a statement in a document is admissible, it may be proved by producing a copy of the document (even if the original is still in existence) and that the number of removes between a copy and the original is irrelevant (ie it may be an nth generation copy). Furthermore, under Section 9, documents which form part of the records of a business (defined very widely) are automatically admissible and the absence of an entry in those records can be proved by an appropriately signed certificate[12].

  2.17     In criminal cases the position is a little different, as the House of Lords has decided[13] that the provisions of Section 69 of the Police and Criminal Evidence Act 1984 (see Box 3) apply to all computer records, whether they are hearsay or not. This means that in criminal cases, any use of a digital image as evidence must be accompanied by the certificate required under Section 69. 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.

Box 3: Police and Criminal Evidence Act 1984 (PACE)
Section 69 of PACE refers to the admissibility of evidence from a computer, and requires assurances about whether the equipment was being used properly and was functioning normally at the time. The implication of Section 69 is that any party relying on computer evidence has to provide oral or written certification of the reliability of the computer, or for each computer if a number have been involved.
Section 69 Evidence from computer records
(1)In any proceedings, a statement in a document produced by a computer shall not be admissible as evidence of any fact stated therein unless it is shown:
    (a)  that there are no reasonable grounds for believing that the statement is inaccurate because of improper use of the computer;
    (b)  that at all material times the computer was operating properly, or if not, that any respect in which it was not operating properly or was out of operation was not such as to affect the production of the document or the accuracy of its contents; and
    (c)  that any relevant conditions specified in rules of court under subsection (2) below are satisfied.
(2)Provision may be made by rules of court requiring that in any proceedings where it is desired to give a statement in evidence by virtue of this section such information concerning the statement as may be required by the rules shall be provided in such form and at such time as may be required.

  2.18     The Law Commission has recently recommended the repeal of Section 69 of PACE because it "serves no useful purpose". The Law Commission said that it was impractical to certify all the intricacies of computer operation and experience showed that most computer error was either easy to detect immediately or had resulted from human error in data entry rather than from malfunction. With the repeal of Section 69, a presumption of proper functioning would be applied to computers (Evidence in Criminal Proceedings: Hearsay and Related Topics, LAW COM No. 245, June 1997).

  2.19     As new technology (and in particular information technology) finds a place outside the research laboratory, its reliability and accuracy is established through use and public acceptance grows through familiarity (or indifference). Debate over the use of the technology moves on from whether the hardware and software are reliable to how the technology might be used to greatest effect, hence the move by the Law Commission. If the proposal of the Law Commission is accepted, legal challenge to new technology may now be reoriented to establishing authenticity and suitable weight for the evidence which it generates.

  2.20     The underlying problem that remains is the potential for image tampering: how to be sure that an image has not been unfairly modified and what, if any, image processing in the name of 'enhancement' is justified. The Home Office already gives 'type approval' for systems where the only evidence of a crime is an image taken by an enforcement camera (eg speeding and bus lane violations), but the general concept of a 'self authenticating image' and how it might be used in a court of law is still something which needs to be evaluated.

  2.21     The courts always have discretion to exclude evidence which has doubtful probative value. 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[14].

4   US law provides that printouts of data stored in a computer are `originals' under Fed. R. Evid. 1001(3), and even if not original are normally admissible as duplicates unless there is some doubt as to their authenticity (rule 1003). The records are always admissible if the originals have been destroyed (rule 1004). Back

5   There is a misconception, presumably derived from the old best evidence rule which is now largely superseded, which stated that where the original of a document existed it must be produced before the courts in preference to a copy. R v. Wayte (1983) 76 Cr. App Rep 110. "It is now well-established that any application of the best evidence rule is confined to cases in which it can be shown that the party has the original and could produce it but does not" (per Beldam J. at p 176). See also Civil Evidence Act 1995 s. 8(2). Back

6   Unless, of course, there is evidence that the original was destroyed to suppress evidential weaknesses. Back

7   Paragraph 1 of Schedule 8 to the Criminal Procedure (Scotland) Act 1995. Back

8   Paragraph 8 of Schedule 8 to the Criminal Procedure (Scotland) Act 1995. Back

9   Lord Advocate's Reference No. 1 of 1992 1992 SLT 1010 at p 1018 J-K. Back

10   This evidence will not be necessary in civil proceedings if the authenticity of the image is admitted during discovery under RSC Ord. 27 r. 4(1), which provides that where lists of documents are exchanged a document which is asserted on the list to be a copy is presumed to be a true copy unless its authenticity is specifically disputed by the other party. If the document is disputed, however oral evidence as to authenticity will be required. In criminal proceedings, it will always be necessary to be able to produce some founding testimony as to the source and authenticity of the image, otherwise the courts may refuse to admit the evidence. Back

11   If the technology exists then it is inevitable that it will be used: for example, the Newport Beach Police Department in Anaheim, California, have recently been testing Adobe Photoshop image processing software to isolate and give greater clarity to suspects in CCTV images. Back

12   Note that not all business documents are `records'. H v. Schering Chemicals (1983) 1 All ER 849, the leading case on the meaning of `record', indicates that a document, irrespective of its form, will only be a record if (a) it effects a transaction or is a contemporaneous compilation of information derived from primary sources which is intended to serve as a record of events; and (b) it is a comprehensive compilation, rather than a selection of source information. If a document is not a comprehensive record of what has occurred, or, even if comprehensive, was not intended to serve as a primary source of information on that matter, it will not be a record. Thus the majority of computer records will fall under s.8 rather than the more favourable provisions of s. 9. A certificate signed by an officer of the business is required under s. 9(2) so as to demonstrate that the document forms part of its records.  Back

13   R v. Shepherd (1993) 1 All ER 225. Back

14   R v. Robson & Harris (1972) 1 WLR 651, a case concerning an audio recording. Back

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