There is a widespread need nowadays for individuals to use, and by implication to remember, authentication codes.

These are to typically four digit numeric codes called personal identification numbers or PINs. For security reasons, these codes should not be written down. If the user does find it necessary to write down the code in order to remember it, it should certainly not be kept with the card or document to which it relates.

Cards of various kinds proliferate. They are typically identity cards or benefit cards. In these senses, identity cards serve essentially to identify the card holder, whereas benefit cards will typically also have financial implications, and include credit cards, debit cards, and charge cards. While commonly referred to as cards, these items are typically made of a plastics material, and are usually of a standard size, about 85mm by 54mm.

These cards are often associated with a machine-readable element, such as an encoded magnetic stripe, or may be so- called smart cards with electrical contacts and integrated microcircuitry, which give the card a specific, commonly unique, identity. A PIN may then be required to validate the use of such a card.

Patent Application W093/11511 (also GB2276967) describes personal identification devices and access control systems based on the use of complex images, typically images of human faces. Complex images are considered to be images that are recognisable if already known but not readily capable of unique description to a person to whom they are not known. Reference should be made to W093/11511 for further discussion and understanding of what is meant by complex images in this context, and for a discussion of the abilities of human subjects to memorize and recall images of human faces.

The invention described in W090/11511 is concerned with the integrity of security systems. The present invention, in contrast, is concerned with providing assistance to an individual to record or remember his PIN in a relatively secure manner. In that earlier disclosure, faces stored by a computer are displayed in shifting positions. In the present invention, to be described herein, the images are continually displayed in fixed positions. In one implementation they can be printed permanently on the card itself. The feature in common with W093/11511 is making use of the ability of the human mind to recognise images, and to pick out remembered images from a group of images, even if they are not wholly memorized in all their details.

By means of the present invention, a four digit PIN can in effect be securely recorded on one face of a card by encoding each digit of the PIN in an array of, say, 9 or 10 images of different human faces. Instead of memorizing a sequence of four digits, the PIN holder memorizes four faces. Each array of images is formed in a regular manner, for example as a three by three or five by two grid, so

that each element of the array can be assigned a number from 1 to 9 or from 0 to 9 respectively. The four arrays can be set out in a defined sequence. Then, the set of four arrays serves as an aide memoire: the user can easily learn to recognise the four key faces, pick them out of the different grids, and by their position and sequence reconstruct the PIN.

Having recognised the faces, the user will need to interpret them to a data terminal. The images may be numbered explicitly, or implicitly by the order in which they appear in the arrays. Likewise, the sequence of the arrays can be explicitly numbered or can be implied by the order in which they are displayed. For convenience, a numeric code can be assumed, and this can be entered by a standard numeric keypad marked with the numbers. Of course, the images can be labelled with other characters or symbols, and the keypad marked accordingly, or a separate concordance can be provided.

The term"keypad"is used for convenience in this description and claims, but it should be assumed to have a general functional meaning. It might be a simple array of 9 or 10 keys, operated in a standard way, or part of a larger keyboard, a touch screen, a mouse or track ball and associated screen display, or any other input device capable of performing a keypad function.

The invention can be expressed both as a method of communicating an authentication code, and as a portable display device to show an authentication code. The latter can take many forms, but at its simplest, may be reduced to a printed reminder card, in which the printing displays images of an encoded form of an authentication code, the

card being linked by the code to data in a data processing machine.

According to one aspect of the invention, a method of communicating an authentication code, comprising a plurality of digits in sequence, to a data processing machine, comprises: providing a data processing machine with a terminal including a data entry keypad having a plurality of keys corresponding to different digits including the digits of the authentication code; providing a portable display device adapted to display a sequence of arrays of elements, each element of an array comprising a different image of a human face; providing a concordance between the respective elements of each array and corresponding keys of the keypad; entering the consecutive digits of the authentication code by selecting an image from each array in sequence, identifying the element of the array in which the selected image occurs, and actuating the key corresponding to that element, whereby to communicate the authentication code to the data processing machine.

According to another aspect of the invention, there is provided a portable display device provided with means to store and display an authentication code, consisting of a plurality of digits in sequence, in encoded form to a user of a data processing machine, wherein said data processing machine is provided with a terminal including a data entry keypad having a plurality of keys corresponding to different digits including the digits of the authentication code; said personal display device comprising means for displaying a sequence of arrays of elements, each element of an array comprising a different image of a human face, wherein there is a concordance between the respective elements of each array and corresponding keys of the

keypad; so that by selecting an image from each array in sequence, identifying the element of the array in which the selected image occurs, and actuating the key corresponding to that element, a user can enter the consecutive digits of the authentication code and thereby communicate the code to the data processing machine.

The concordance can be overtly expressed, or implicit in the arrangement of the elements of the arrays.

In a further embodiment of the invention, the portable display device is provided in combination with a mask and with registration means for positioning the mask with respect to the displayed sequence of arrays of elements, so that, when the mask is registered with the arrays, the mask identifies those images that correspond to the authentication code. In this manner, the mask acts as a key to reveal the images to be remembered by the user, and to assist the user in memorizing the correct images.

The mask may be a perforated or at least partially transparent overlay and may identify the images by exposing them through apertures in the overlay or through transparent portions of the overlay. A card supplier can use such a mask to communicate the PIN to the user.

Alternatively, the portable display device can be combined with a blank mask having a plurality of window portions that are either temporarily occluded or transparent, whereby a user can either clear selected occluded window portions or mark selected transparent window portions to make a mask to subsequently identify the images that correspond to the authentication code. This is useful where the user wishes to choose his own PIN. The occluded window portions can be cleared by pressing them out of the

mask, if they are delineated by perforations or scores, or by locally peeling a removable latex or similar coating from a transparent base, or transparent window portions can be marked by ringing them round, or tinting them, with a drawing or writing instrument such as a pen or the like.

Registration may make use of the edges of the card and of the mask, and the mask may comprise an envelope dimensioned to receive the device in a defined relative position.

In a still further embodiment, the invention provides the combination of a portable display device as aforesaid and a data processing unit provided with a terminal including a data entry keypad having a plurality of keys corresponding to different images including the digits of the authentication code.

An embodiment of the invention is illustrated by way of example in the accompanying drawings, in which Figure 1 shows one face of a printed card, with an insert indicating the numerical concordance between the elements of each of four 3 x 3 arrays of faces and the numeric keys of a keypad (not shown); and Figure 2 illustrates a mask placed over the card of Figure 1 in correct registration to show one image of a human face from each of the four arrays.

As shown in Figure 1, four 3 x 3 arrays of nine images of human faces each are imprinted on a standard credit card sized card 6 and occupy between two thirds and three quarters of the length of the card. This leaves blank space 8 towards one end of the card for the electrical

contacts, in the case of a smart card, and graphic information including writing and promotional material (not present in Fig 1). The sequence of arrays 1,2,3,4, corresponds to western writing, i. e. left to right, line by line, starting at the top. The same sequence of images is followed within each array of nine images, as illustrated in insert A. The corresponding keypad may also appear as in insert A, or may differ in appearance, but will have means for entering the nine digits 1 to 9.

The card may be supplied with a cover 10 serving as a mask (Figure 2) which when properly registered with the card 6 allows the user to see only four particular images through four transparent window portions 14, one from each array, which in this case correspond to an existing PIN number 4217 as defined by the concordance. That is to say, image 4 in array 1, image 2 in array 2, image 1 in array 3, and image 7 in array 4. The eight other window portions in each array are occluded and hide the underlying images. Alternatively, the user can choose a new PIN according to which faces the user wants to use. In such a case, the mask can be supplied to order with the chosen faces showing, or the mask may be formed from a peelable coating over a transparent base, so that the user can scratch or peel the coating from the positions at which the selected faces occur. In either case, when the user has learned the key faces, the mask may be destroyed, leaving only the card with its printed faces. An unauthorized user cannot deduce the PIN from the imprinted card alone any more easily than for an all-numeric code using the digits 1 to 9 without the presence of the faces on the card.

Registration is defined in this instance as the alignment of the edges of the mask 10 with the edges of the card 6.

The mask may be a simple overlay, or may be an envelope for the card.

All the faces shown in Figure 1 are different, and are taken from both sexes. The faces can be as distinctive one from another as desired. It is preferred that each image is of a different face, rather than just a different image of the same face.

It will be possible, but less secure, to use the same faces in each of the four arrays, with only their positions changed. The advantage is that the user only has to learn to recognise one face. The disadvantage is that, once the significance of the face is appreciated, it can be seen that there are only 9 possible 4-digit codes, since any one face can be traced through the sequence and defines the start of a four digit code defined by the positions of that same face in the subsequent three arrays.

The display of arrays of elements, each element comprising a different image of a human face, may be a printed surface in any convenient form, bearing the sequence of arrays visibly imprinted thereon, and can of course be generated by other means. However, the printed surface is currently the preferred form, and can be on a separate card to be stored apart from the identify card, benefit card, or other device to which it relates. In the form of separate printed cards or sheets, such display devices can be collected together in the form of a book with multiple pages, one for each different PIN of the user. Separate display devices may also be printed in larger sizes than standard credit cards, for the benefit of those with poor eyesight. If not constrained by the size, shape or surface features of a card, the number of arrays can easily be

increased without losing visibility, to enhance the security level.

A separate display device can be used and distributed as a promotional gift.

The display of the faces serves as a PIN reminder to be carried by the user without revealing that authentication code to a third party.