Method of authenticating a user, the method comprising automatically determining a measure of similarity or dissimilarity between a drawing input by the user and at least one reference drawing (s uj) previously input by the user, the input drawing being arranged to be compared picture element by picture element with the or each reference drawing (s uj), a picture element being a pixel or a voxel, the input drawing comprising a first plurality of picture elements, and the reference drawing comprising a second plurality of picture elements, the method being characterised by: obtaining at least one user drawn candidate reference drawing and verifying that the at least one candidate reference drawing meets a predetermined quality criterion for use as the at least one reference drawing; for each of the first and second plurality of picture elements, determining location and temporal coordinates of the said each picture element, the location coordinates of a respective picture element being the coordinates of that picture element in a canvas, the temporal coordinate comprising the time at which the respective picture element of the respective drawing was drawn, relative to the time at which the user started drawing the respective drawing, the location and temporal coordinates of a picture element being referred to hereinafter as a symbol, the symbols of the first plurality of picture elements being referred to hereinafter as an input sentence, and the symbols of the second plurality of picture elements being referred to hereinafter as a reference sentence; determining a measure of similarity or dissimilarity between the input sentence and the at least one reference sentence without generating feature vectors from the input sentence, wherein the determination of the measure of similarity or dissimilarity comprises determining cost values for edit operations applied to the symbols of the input sentence symbol by symbol for transforming the input sentence into the respective reference sentence, and determining a minimum edit distance between the symbols of the input sentence of the input drawing (s uj) and the symbols of the reference sentence of the at least one reference drawing, wherein the minimum edit distance is the edit distance of the sequence of edit operations which transforms the input sentence of the input drawing into the reference sentence of one of the reference drawings with minimum cost; and positively authenticating the user if the minimum edit distance is below a given threshold. obtaining at least one user drawn candidate reference drawing and verifying that the at least one candidate reference drawing meets a predetermined quality criterion for use as the at least one reference drawing; for each of the first and second plurality of picture elements, determining location and temporal coordinates of the said each picture element, the location coordinates of a respective picture element being the coordinates of that picture element in a canvas, the temporal coordinate comprising the time at which the respective picture element of the respective drawing was drawn, relative to the time at which the user started drawing the respective drawing, the location and temporal coordinates of a picture element being referred to hereinafter as a symbol, the symbols of the first plurality of picture elements being referred to hereinafter as an input sentence, and the symbols of the second plurality of picture elements being referred to hereinafter as a reference sentence; determining a measure of similarity or dissimilarity between the input sentence and the at least one reference sentence without generating feature vectors from the input sentence, wherein the determination of the measure of similarity or dissimilarity comprises determining cost values for edit operations applied to the symbols of the input sentence symbol by symbol for transforming the input sentence into the respective reference sentence, and determining a minimum edit distance between the symbols of the input sentence of the input drawing (s uj) and the symbols of the reference sentence of the at least one reference drawing, wherein the minimum edit distance is the edit distance of the sequence of edit operations which transforms the input sentence of the input drawing into the reference sentence of one of the reference drawings with minimum cost; and positively authenticating the user if the minimum edit distance is below a given threshold.