A method for identifying a user computer from other computers, the method comprising the steps of: in the course of a first online transaction between a computer and a merchant web server: obtaining information (34, 36) about the computer including a first local computer time according to a resident computer clock of the computer; measuring a first local server time of the merchant web server; calculating a first delta of time parameter based upon a difference in time between the first local computer time and the first local server time; generating a first computer identifier based at least in part on the first delta of time parameter; in the course of a second online transaction between the computer and the merchant web server: obtaining information about the computer including a second local computer time according to the resident computer clock; measuring a second local server time of the merchant web server; calculating a second delta of time parameter based upon a difference in time between the second local computer time and the second local server time; generating a second computer identifier based at least in part on the second delta of time parameter; flagging the first computer identifier in response to determining the first online transaction may be a fraudulent transaction; comparing the first computer identifier and the second computer identifier; determining that a measured difference between the delta of time parameter of the first computer identifier and the delta of time parameter of the second computer identifier falls within a selected range; and generating an indication that the computer was used in the first online transaction and the second online transaction, based at least on the determination that the measured difference falls within the selected range. in the course of a first online transaction between a computer and a merchant web server: obtaining information (34, 36) about the computer including a first local computer time according to a resident computer clock of the computer; measuring a first local server time of the merchant web server; calculating a first delta of time parameter based upon a difference in time between the first local computer time and the first local server time; generating a first computer identifier based at least in part on the first delta of time parameter; obtaining information (34, 36) about the computer including a first local computer time according to a resident computer clock of the computer; measuring a first local server time of the merchant web server; calculating a first delta of time parameter based upon a difference in time between the first local computer time and the first local server time; generating a first computer identifier based at least in part on the first delta of time parameter; in the course of a second online transaction between the computer and the merchant web server: obtaining information about the computer including a second local computer time according to the resident computer clock; measuring a second local server time of the merchant web server; calculating a second delta of time parameter based upon a difference in time between the second local computer time and the second local server time; generating a second computer identifier based at least in part on the second delta of time parameter; obtaining information about the computer including a second local computer time according to the resident computer clock; measuring a second local server time of the merchant web server; calculating a second delta of time parameter based upon a difference in time between the second local computer time and the second local server time; generating a second computer identifier based at least in part on the second delta of time parameter; flagging the first computer identifier in response to determining the first online transaction may be a fraudulent transaction; comparing the first computer identifier and the second computer identifier; determining that a measured difference between the delta of time parameter of the first computer identifier and the delta of time parameter of the second computer identifier falls within a selected range; and generating an indication that the computer was used in the first online transaction and the second online transaction, based at least on the determination that the measured difference falls within the selected range.