Signature verification algorithm

• US 4,789,934 A
• Filed: 01/21/1986
• Issued: 12/06/1988
• Est. Priority Date: 01/21/1986
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Worthington, Thomas K., Gundersen, Steven C.
• Primary Examiner(s): Ruggiero, Joseph
• Assistant Examiner(s): BUI, KIM T
• Application Number:
  US06/820,846
• Time in Patent Office: 1,050 Days
• Field of Search: 364/419, 382/3, 382/37, 382/34, 235/379, 381/41
• US Class Current: 382/120
• CPC Class Codes: G06V 40/30 Writer recognition; Reading and verifying signatures

Abstract

An improved signature verification method using dynamic signature characteristics in the frequency domain. Analysis of a signature in the frequency domain is employed when time domain analysis does not adequately distinguish between true signatures and forgeries. The preferred signature characteristics that are measured are two acceleration components of the pen, and the rate of change of axial pressure on the pen. More specifically, correlation functions operating on a sample and reference signature produce a correlation score. If the total correlation score is in a selected interval, then the above measured characteristics are analyzed in the frequency domain with the magnitude squared coherence function.

Claims

1. A method of verifying a sample signature of a putative signer, which comprises:

(a) measuring a rate of change of pressure transmitted by the signer'"'"'s hand to a writing instrument during the writing on a writing surface of at least one sample signature of the putatuve signer, to obtain sample pressure signals over a plurality of segments;

(b) measuring acceleration of a writing instrument'"'"'s movement produced by the motion of a putative signer'"'"'s hand during the writing on a writing surface of at least one sample signature of the putative signer, to obtain acceleration signals over a plurality of segments;

(c) storing reference pressure signals and reference acceleration signals representative of a known signer'"'"'s signature, the reference pressure signals and the reference acceleration signals being obtained from a known signer in the same manner as the sample pressure and sample acceleration signals, respectively;

(d) computing a correlation score for the sample signature by operating on segments of the sample and reference pressure signals and segments of the sample and reference acceleration signals with a correlation function;

(e) computing a total coherence score when the correlation score is within a selected interval by operating on paired segments of sample and reference pressure signals with a coherence function and by operating on paired segments of sample and reference acceleration signals with a coherence function; and

(f) comparing the total coherence score with a coherence threshold.

2. A method as recited in claim 1, wherein the selected interval is bounded above by the correlation threshold+winhi and below by the correlation threshold-winlo.

3. A method as recited in claim 2, wherein the correlation threshold approximately equals an average correlation score of a set of enroll signatures minus 0.1.

4. A method as recited in claim 1 wherein the coherence threshold is determined by an average coherence score of a set of enroll signatures minus 0.1.

5. A method of verifying a sample signature of a putative signer, which comprises:

(a) measuring rate of change of pressure transmitted by the signer'"'"'s hand to a writing instrument during the writing on a writing surface of at least one sample signature of the putative signer, to obtain sample pressure signals over a plurality of segments, the rate of change of pressure being measured in a direction essentially parallel to the axis of the pen;

(b) measuring acceleration of a writing instruments'"'"'s movement produced by the motion of the putative signer'"'"'s hand during the writing on a writing surface of at least one sample signature of the putative signer, to obtain acceleration signals over a plurality of segments, the acceleration signals comprising two acceleration components which are essentially orthogonal to each other and which are in a plane essentially orthogonal to the axis of the writing instrument;

(c) storing reference pressure signals and reference acceleration signals representative of a known signer'"'"'s signature, the reference pressure signals and the reference acceleration signals being obtained in the same manner as the sample pressure and sample acceleration signals, respectively;

(d) computing a correlation score for the sample signature by operating on paired segents of the sample and reference pressure signals and on paired segments of the sample and reference acceleration signals with a correlation function;

(e) computing a total coherence score when the correlation score is within a selected interval by operating on paired segments of sample and reference pressure signals and paired segments of sample and reference acceleration signals with a coherence function, the paired segments having been truncated if the segments are greater than 32 points and padded if the segments are less than 32 points, the paired segments then being tapered thereafter;

(f) comparing the total coherence score with a coherence threshold; and

(g) accepting the signature as valid if the total coherence score exceeds the coherence threshold.

6. A method of verifying a sample signature of a putative signer, which comprises:

(a) measuring rate of change of pressure transmitted by the signer'"'"'s hand to a writing instrument during the writing on a writing surface of at least one saple signature of the putative signer, to obtain sample pressure signals in analog format, the rate of change of pressure being measured in a direction essentially parallel to the axis of the pen;

(b) measuring acceleration of a writing instrument'"'"'s movement produced by the motion of the putative signer'"'"'s hand during the writing on a writing surface of at least one sample signature of the putative signer, to obtain acceleration signals in analog format, the acceleration signals comprising two acceleration components which are essentially orthogonal to each other and which are in a plane essentially parallel to the writing surface;

(c) changing the format of pressure and acceleration signals from analog to digital format by sampling at the rate of 80 hertz;

(d) storing reference pressure signals and reference acceleration signals in digital format representative of a known signer'"'"'s signature, the reference pressure signals and the reference acceleration signals being obtained in the same manner as the sample pressure and acceleration signals, respectively;

(e) computing a correlation score for the sample signature by operating on paired segments of the sample and reference pressure signals and on paired segments of the sample and reference acceleration signals with a correlation function;

(f) computing a total coherence score when the correlation score is within a selected interval by operating on paired segments of sample and reference pressure signals and paired segments of sample and reference acceleration signals, the segments having been truncated if the segments are greater than 32 points and padded if the segments are less than 32 points, the segments then being tapered thereafter the total coherence score (cscore) being calculated by the formula

space="preserve" listing-type="equation">cscore=1/3 cpscore+2/3 cascore

where cpscore is the pressure coherence score and cascore is the acceleration coherence score where ##EQU12## where f_o is the fundamental frequency where γ

_p² (nf_o) is the pressure coherence function and γ

_a² (nf_o) is the acceleration coherence function, the pressure coherence function and the acceleration coherence function being merely the coherence function with the sample and reference signals being pressure and acceleration signals, respectively, the coherence function being defined by ##EQU13## where f is the frequency, G_RS (f) is the cross spectral density function between the reference and sample signals, and G_RR (f) and G_SS (f) are auto spectral densities of the reference and sample signals respectively;

(g) comparing the total coherence score with a coherence threshold; and

(h) accepting the signature as valid if the total coherence score exceeds the coherence threshold.