GB2109970A - Signature recognition - Google Patents

Abstract

A written signature is validated electronically as in connection with a bank transaction terminal, by comparing predetermined parameters of a waveform derived from pen movements, such as a pen velocity waveform, with upper and lower limit reference levels previously derived from sample signatures and held in a store. For at least one parameter, such as the crossing of a particular threshold voltage value as detected by a threshold detector 2, the waveform is divided into a plurality of successive time segments by gates G1-G6 successively enabled by logic 5 and the parameter as detected by respective counts C1-C6, in each time segment is compared with respective upper and lower limit reference levels. The time segments may be equal in length and of a fixed duration, unequal in length following a selected distribution, for example logarithmic, or equal in length and of a duration determined by the expected overall signature duration. The time division permits different signatures with the same total number of threshold voltage transitions, for example, to be distinguished. <IMAGE>

Description

SPECIFICATION Signature recognition This invention relates to the validation of signatures in an electronic manner.

According to one aspect of the present invention there is provided a method of validating a signature comprising comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, and wherein for at least one parameter the waveform is divided into a plurality of successive time segments and the parameter as detected in each time segment is compared with associated upper and lower unit reference levels for each time segment in the store.

According to another aspect of the present invention there is provided a method of validating a signature comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, and wherein for at least one parameter the waveform is divided into a plurality of successive equal length time segments and the parameter as detected in each equal length time segment is compared with associated upper and lower limit reference levels for each time segment in the store.

According to a further aspect of the present invention there is provided a method of validating a signature comprising comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, and wherein for at least one parameter the waveform is divided into a plurality of successive time segments which are unequal in length and the parameter as detected in each time segment is compared with associated upper and lower limit reference levels for each time segment in the store.

According to yet another aspect of the present invention there is provided a method of validating a signature comprising comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, wherein one stored parameter is related to the overall expected signature duration, and wherein for at least one other parameter the waveform is divided into a plurality of successive time segments whose length is determined by the one stored parameter and the other parameter as detected in each time segment is compared with associated upper and lower limit reference levels for each time segment in the store.

According to a still further aspect of the present invention there is provided an apparatus for use in validating a signature comprising means whereby a waveform is derived from pen movements during the act of signing, means for detecting the occurrence of predetermined parameters in the waveform, means for storing upper and lower limit parameter occurrence reference levels, means for comparing the detected occurrences with the stored upper and lower limit reference levels, and means indicating whether or not the detected occurrences are within the limit levels, and wherein for at least one parameter means are provided to divide the waveform into time segments and to detect the parameter occurrence in each time segment separately, the stored reference levels for the at least one parameter comprising associated levels for each separate time segment.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which; Figure 1 shows a schematic circuit arrangement for uniform time-segmentation of a pen velocity waveform according to one embodiment of the present invention; Figure 2 shows a decoder logic circuit for use in the arrangement of Figure 1, and Figure 3 shows a schematic circuit arrangement for pre-coded time-segmentation of a pen velocity waveform according to another embodiment of the present invention.

In one known electronic signature verification method, as described in our co-pending application No.

7849598 (Serial No. 203911 8A) (A.E. Brewster - A.J. Hicks 79-1), the act of signing generates a waveform representing the velocity of pen movements up and down a writing tablet. The waveform is subjected to a number of tests, each test generating a numerical count typical of a particular signature. These numbers are grouped together as the identifying code for that signature. The tests comprise measurement of signature duration and a count of the number of pen stops or lift-offs, together with counting the number of crossings of several selected voltage thresholds by the velocity waveform. In order to verify a signature, as for example at a bank, the identifying code generated by the act of signing is compared with upper and lower reference values or limits extracted from a store.The reference limits held in the store are established previously from a number of sample or master signatures from the person in question.

This method has two significant disadvantages. Firstly, the crossing of any of the higher voltage thresholds must necessarly be preceded by the crossing of all the lower voltage thresholds as the pen accelerates, creating a relationship between the tests which introduces an element of undesirable redundancy. Furthermore, since the tests accumulate a total count for the full length of the signature, they are unable to discriminate between different signatures in which the test total is the same although, in one instance the majority of threshold crossings might have occurred early in the signature, whilst in another late in the signature, and in yet another according to some other time distribution.

In the method of the present invention as described hereinafter it is assumed that the recognition data (identifying code) which is to be transmitted to a central processor, and the acceptance limits held in a central store, are not a complete representation of the signature waveform but a highly simplified account of certain of its salient features, expressed as a group of binary numbers. Any attempt to add time-related data will inevitably increase the amount of data to be transmitted or stored, however this may be compensated for by eliminating the redundancy mentioned above.

For the purposes of the following description crossings of a single threshold voltage for example zero-crossings, will be considered. In practice corresponding techniques may be applied simultaneously to several voltage thresholds, orto both positive-going and negative-going zero-crossings, for example.

In the known signature verification method mentioned above only one counter is provided for each test parameter. Each counter, starting from zero at the start of the signature, counts the successive occurrences of its related parameter, for example zero crossings, over the full length of the signature, giving a single total for that parameter. However, in accordance with one embodiment of the present invention, we now propose to provide several counters for one parameter, or each of a number of parameters. By arranging for these counters to operate in turn over successive time periods of the signature, a quantised time distribution of the events may be recorded. For example, assuming a typical signature duration to be five seconds, a set of five counters may be provided, each covering a respective one second of the total period. This may be referred to as uniform time segmentation.It will be apparent that although the sum of the five output numbers will be equal to the single total obtained from an overall counter, the several discrete numbers make it possible to achieve a distinction between signatures having the same total count but different event distribution.

The precision with which this distinction may be achieved will depend upon the initial choice of size and number of time segments. Whilst the use of more time segments will give enhanced discrimination, it will also increase the amount of data to be transmitted and the amount of limit storage to be provided at the central processor. Hence a suitable compromise, based on the statistics of a variety of signature, is desirabie.

For example the higher voltage thresholds of our prior method mentioned above may be advantageously abandoned in order to free the counters to serve as time-segment counters operating only on, for example, zero-crossings. This would leave the identification code the same length as in the prior method, but the numbers transmitted would have a different significance.

A particular arrangement of apparatus employing uniform time segmentation is shown schematically in Figure 1. The apparatus comprises a pen velocity signal input 1, a threshold detector 2, six gates G1 to G6, six counters C1 to C6, a pen contact switch 3, an astable 4, a gate 67, a counter C7 and decoder logic 5 with outputs comprising gate control lines L1 to L6 for gates G1 to G6, the interconnections between the decoder logic and the gates being omitted for reasons of clarity. Only one threshold voltage is used so that the six counters C1 to C6 each count the threshold crossings of the pen-velocity signal in one respective segment or time interval. The circuitry shown could be adapted to count crossings of two threshold levels in three time segments, or other combinations.

The operation of the circuit of Figure 1 is as follows. A pen velocity signal applied to input 1 is examined by the threshold detector 2 which produces a voltage pulse for every threshold crossing. The detector output is passed to the counters C1 to C6 via the respective gates G1 to G6. Each gate is opened in turn by a timing sequence determined by the decoder logic 5 in such a way as to define the required segmentaton intervals.

Whenever a gate is opened its respective counter views the detector output and so counts the number of threshold crossings in that time interval. The timing signals required by the gates G1 to G6 are derived from the decoded outputs of counter C7 which is incremented by a pulse train from astable 4. This pulse train is gated (gate 67) under the control of pen contact switch 3, so that counter C7 only increments in the period of actual writing and ignores pauses in a signature.

The decoder logic, shown in more detail in Figure 2, comprises a Nand gate decoder 6 and a signetics type 4022 divide-by-eight counter/decoder circuit 7. Nands N1 to N6 have their inputs connected to the appropriate counter C7 outputs, so that each Nand will sense those values of the count that define the required segmentation intervals. Each Nand N1 to N6 will produce a voltage pulse, of width equal to the astable period, at the beginning of each segment. Nand N7 collects the information from Nands N1 to N6 so that a voltage pulse from any one of Nands N1 to N6 is passed on to circuit 7. The divide-by-eight counter of circuit 7 increments for every pulse it receives from Nand N7, and the decoder of circuit 7 drives one of the gate control lines L1 to L6 high depending on the value of its counters. Eight decoded outputs are available from the type 4022 circuit, but only six are required here, one for each of gates G1 to G6.

As an example of the decoder design, assume that the astable operates at 1 OHz, so that counter C7 increments every 0.1 second. Let counter C7 be a six bit counter enabling a longest expected signature of six seconds to be accommodated by its maximum count of 63 (6.3 seconds). If six uniform time segments are required, then the segments are defined as in the following table.

Segment No Starting Time Counter C7 value to be decoded 1 0 secs. 000000 ( O) 2 1 001010(10) 3 2 010100(20) 4 3 011110(30) 5 4 101000(40) 6 5 110001(50) TABLE I It has been found that there are large variations in signature duration from person to person. In consequence the uniform time-segmentation approach does suffer from the disadvantage that if the limited number of segments are extended so as to cover the longest signatures, the faster signatures might fall normally into the first segment, and thus the advantage of segmentation would be lost. One solution is to make a progressive increase in the duration of the successive segments, so that, for example, the first segment might be 250 milliseconds, the second 500 milliseconds, the third one second and so on.Thus faster signatures can be subjected to some degree of segmentation without sacrificing the later parts of the slower signatures. This nonlinearity might be made to follow a logarithmic or other suitable law, depending on typical signature statistics. If a logarithmic scale is used then average signatures of three seconds would have five segments, longer signatures six segments and even the shortest should have four segments. An example of the timing requirements for a logarithmic scale are as follows: Segment No.Starting Time Counter C7 value to be decoded 1 0 secs. 000000 ( O) 2 0.4 000100 ( 4) 3 0.9 001001 ( 9) 4 1.6 010000(16) 5 2.6 011010(26) 6 4.0 101000(40) In order to ensure that each person's signature, regardless of duration, is divided into the optimum number of time segments, advantage may be taken of mean signature duration data obtained during the initial collection of a set of master signatures. The process of collecting the set of master signatures will result in the accumulation of a set of upper and lower limits for each test (parameter), with which any subsequent version of that signature will be compared. In a bank transaction terminal system the limits are held in store at a central processor and are selected in response to the relevant account number transmitted prior to the act of signing.

As mentioned above, one of the tests of our previous method provides a measure of signal duration, expressed as a number derived from the basic system clock rate. The related stored numerical limits thus define the upper and lower extremes of duration acceptable for that signature. Thus from this advance knowledge of the expected duration of a signature to be verified, an appropriate time-segmentation sequence (pre-coded) can be initiated. Supposing that, say, six of the remaining tests are to be devoted to counting the zero-crossings in each of six equal segments of the signature. It is thus required that the first zero-crossing test count is connected to the pen velocity signal input for the first one-sixth of the expected signal duration, the second test circuit is connected for the second one-sixth, and soon to the end of the signature.This switching sequence can appropriately be performed by circuits associated with the signature tablet, based on an expected duration figure transmitted back from the central processor. This figure can conveniently be the stored upper-limit value for the signatue duration test and can accompany an invitation to sign' code which is returned to a remote bank transaction terminal immediately after the central processor has processed the account number and the relevant transaction details.

The terminals circuits will divide the expected-du ration figure by six and, under the control of the system clock generator, gate the test circuits successively for the appropriate periods. For example, supposing the expected duration figure to be 48, that is 48 clock pulse intervals, the test circuits would be actuated in turn, each for 8 clock pulse intervals, starting from the commencement of the act of signing. For values not divisible by six, the nearest whole number might be taken. For example, for a briefer signature the expected-duration figure might be 23 and the segments each made 4 clock intervals in duration.

A particular circuit arrangement for such pre-coded time segmentation is shown schematically in Figure 3.

The same reference numerals are used in Figures 1 and 3 for like elements. The circuit of Figure 3 includes a quad latch 8m a four-bit comparator 9, a four-bit counter C17, and a divide-by-eight counter/decoder 10, which can be of the same type as element 7 in Figure 2. The operation of the Figure 3 arrangement is as follows. As previously mentioned, before the signature is written its maximum expected duration is transmitted from a store at the central processor to the local terminal where it is converted to a figure related to the required segment duration. For the case of six segments and a clock frequency of 1OHz this will entail division by six, and should the answer involve a fraction then the next highest integer is taken so as to make the segment slightly too long rather than too short.This assures the clock rate for this example is identical to that initially used to determine the signature duration, so that a simple division by the number of segments results in a number which represents the required segment duration without the need for a scaling factor.

Signature samples we have taken indicate a maximum signature duration of six seconds. If six segments are used the longest segment duration required would be one second. The number that determines the segment duration is stored in the latch 8. An alternative to transmitting the upper limit of the signature duration is to perform the division at the central processor and transmit the result directly to the latch. The number is the latch 8 is constantly compared by comparator 9 with the value of the counter C17, which is incremented by the gated pulse train from the astable 4.When the counter C17 reaches the value stored in the latch 8m the comparator 9 emits a pulse to element 10, which is described previously in connection with Figures 1 and 2, causes the task of threshold crossing counting to be transferred to the next counter of C1 to C6 in line. At the same time counter C17 is reset and counts again to the value of the latch.

in our patent application mentioned above it was mentioned that long-term changes in a given signature might be accommodated by arranging forthe acceptance limit values to advance or recede progressively in the light of such changes. Since the above-described pre-coded time segmentation is based upon the stored upper-limit value for signature duration, if this value is capable of long-term self-adjustment it will simultaneously compensate the time-segmentation parameters.

Selecting the duration upper-limit value as the basis of the time segmentation count ensures that the count will continue for long enough to embrace the longest in time example of the expected signature. This does not, however, preclude the possibility of substituting the lower-limit or some intermediate value if operational advantages are thereby obtained.

Claims (1)

1. A method of validating a signature comprising comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, and wherein for at least one parameter the waveform is divided into a plurality of successive time segments and the parameter as detected in each time segment is compared with associated upper and lower unit reference levels for each time segment in the store.

2. A method of validating a signature comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, and wherein for at least one parameter the waveform is divided into a plurality of successive equal length time segments and the parameter as detected in each equal length time segment is compared with associated upper and lower limit reference levels for each time segment in the store.

3. A method of validating a signature comprising comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, and wherein for at least one parameter the waveform is divided into a plurality of successive time segments which are unequal in length and the parameter as detected in each time segment is compared with associated upper and lower limit reference levels for each time segment in the store.

4. A method of validating a signature comprising comparing predetermined parameters of a waveform derived from pen movements during the act of signing with upper and lower limit reference levels for the parameters, which reference levels are held in a store, wherein one stored parameter is related to the overall expected signature duration, and wherein for at least one other parameter the waveform is divided into a plurality of successive time segments whose length is determined by the one stored parameter and the other parameter as detected in each time segment is compared with associated upper and lower limit reference levels for each time segment in the store.

5. A method as claimed in any one of the preceding claims, wherein the or each parameterforwhich the waveform is divided into a plurality of successive time segments comprises the crossing of a respective threshold voltage value.

6. A method as claimed in claim 1 wherein the waveform comprises a pen velocity waveform, wherein the or each parameter for which the waveform is divided into a plurality of successive time segments comprises the crossing of a respective threshold voltage value, and wherein the output of a respective threshold detector is applied to respective event counter for each time segment, via a respective gate.

8. A method as claimed in claim 7, wherein the gates are enabled successively and for respective time intervals such that time segments are uniform and of predetermined fixed length for all signatures, uniform and of a length determined by the overall expected signature duration for a particular signature, or non-uniform and in accordance with a predetermined distribution.

9. A method as claimed in claim 8 wherein the time segments are non-uniform and shorter at the beginning of a signature than at the end thereof.

10. Apparatus for use in validating a signature comprising means whereby a waveform is derived from pen movements during the act of signing, means for detecting the occurrence of predetermined parameters in the waveform, means for storing upper and lower limit parameter occurrence reference levels, means for comparing the detected occurrences with the stored upper and lower limit reference levels, and means indicating whether or not the detected occurrences are within the limit levels, and wherein for at least one parameter means are provided to divide the waveform into time segments and to detect the parameter occurrence in each time segment separately, the stored reference levels for the at least one parameter comprising associated levels for each separate time segment.

11. Apparatus as claimed in claim 10 and wherein at least one parameter comprises the crossing of a respective threshold voltage value, further comprising a respective threshold detector, a respective event counter for each time segment connectable to the respective threshold detector via a respective gate, and means for enabling the gates whereby a different respective event counter is connected to the threshold detector for each time segment.

12. . Apparatus as claimed in claim ii wherein the means for enabling the gates are such that the gates are enabled for time intervals which are uniform and of a predetermined fixed length for all signatures, uniform and of a length determined by the overall expected signature duration for a particular signature, or non-uniform and in accordance with a predetermined distribution.

13. Apparatus as claimed in claim 11 wherein the means for enabling the gates are such that the gates are enabled for time intervals which are uniform and of a predetermined fixed length for all signatures, and wherein the gate enabling means includes an astable multivibrator, a further counter incremented by the astable multivibrator output during the act of signing, and a logic element for decoding the further counter output and enabling each of the gates in turn for equal time intervals.

14. Apparatus as claimed in claim 13 wherein the further counter is gated whereby it is only incremented during periods of actual writing of the signature and ignores pauses therein.

15. Apparatus as claimed in claim 11 wherein the enabling means are such that the gates are enabled for time intervals which are non-uniform and are shorter at the beginning of a signature than at the end thereof.

16. Apparatus as claimed in claim ii wherein the means for enabling the gates are such that the gates are enabled for time intervals which are uniform and of a length determined by the overall expected signature duration for a particular signature, including a latch, means for supplying the latch with a number from the storage means indicative of the required uniform time interval, an astable multivibrator, another counter incremented by the astable multivibrator during the act of signing, a comparator for comparing the latch number with the other counter output and a further logic element which enables the gates in turn on receipt from a respective pulse from the comparator.

17. Apparatus as claimed in claim 16, wherein the other counter is gated whereby it is only incremented during periods of actual counting of the signature and ignores pauses therein.

18. A method of validating a signature substantially as herein described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.

19. Apparatus for use in validating a signature substantially as herein described with reference to and as illustrated in Figures 1 and 2 or Figure 3 of the accompanying drawings.