A method and system for verifying a check that is being used for an on-line transaction, utilizes a hash code value either printed directly on the check, or obtained from an insert card provided by a check printer. To conduct an on-line transaction using a check, the customer enters in data obtained from a MICR line of the check, whereby the data includes a one-way hash value that is based on the data provided on the MICR line as well as private data not provided on the MICR line. A web server of an e-tailer for which the customer seeks to make the on-line transaction, receives the data entered by the customer. The web server of the e-tailer transmits, to a check verifier, the data entered by the customer. The check verifier verifies whether or not the check is valid, by comparing the hash code value entered in by the customer with a hash code value that is separately calculated by the check verifier, based on private data of the customer obtained by the check verifier from a database.