A method for recognition of an identifier, comprising: for each of a plurality of objects, storing a first visual signature, which has been extracted from at least a part of a first image of the object captured at a first location, and a first time stamp associated with the first captured image, wherein each object comprises a vehicle; computing a measure of similarity between a second visual signature and at least some of the first visual signatures to identify a matching one of the first visual signatures, based on the computed measure of similarity, the second visual signature having been extracted from at least a part of a second image of a second object captured at a second location, based on at least a part of the second image, wherein the first and second visual signatures comprise a fixed-length vectorial representation of pixels of at least the part of the first and second images respectively in D-dimensional space, wherein D is at least 30; retrieving the stored first time stamp associated with the matching one of the first visual signatures and a stored second time stamp associated with the second captured image; performing a test which is a function of the retrieved stored first time stamp and the stored second time stamp; and recognizing an identifier of the second object based on a captured image of the second object, only when it is confirmed that the test has been met, wherein recognizing an identifier comprises performing automated license plate character recognition on the second image, and wherein the first and second visual signatures serve as hash keys of the original images in that there is no feasible way to recover the identifier therefrom. for each of a plurality of objects, storing a first visual signature, which has been extracted from at least a part of a first image of the object captured at a first location, and a first time stamp associated with the first captured image, wherein each object comprises a vehicle; computing a measure of similarity between a second visual signature and at least some of the first visual signatures to identify a matching one of the first visual signatures, based on the computed measure of similarity, the second visual signature having been extracted from at least a part of a second image of a second object captured at a second location, based on at least a part of the second image, wherein the first and second visual signatures comprise a fixed-length vectorial representation of pixels of at least the part of the first and second images respectively in D-dimensional space, wherein D is at least 30; retrieving the stored first time stamp associated with the matching one of the first visual signatures and a stored second time stamp associated with the second captured image; performing a test which is a function of the retrieved stored first time stamp and the stored second time stamp; and recognizing an identifier of the second object based on a captured image of the second object, only when it is confirmed that the test has been met, wherein recognizing an identifier comprises performing automated license plate character recognition on the second image, and wherein the first and second visual signatures serve as hash keys of the original images in that there is no feasible way to recover the identifier therefrom.