Authenticating methods and systems.

摘要

Authenticity of a paper document (10) is tested by sensing (40) its transparency and comparing (32) the sequence of representations produced, with a sequence of reference representations read (39) from a record of an earlier-made sensing written (38) on the document (10) in an offset-printed (and therefore non-uniform) magnetic stripe (18). The reference sequence is written on the stripe (18) in the form of an encrypted identifier (156, Figure 4) within a bit string (Figure 4) that is interspersed with framing characters (155) and includes a public encryption key (158) and an error-correction code (162). Each data and control bit of the bit-string format is written as an eight-bit character enabling ready distinction between data and control bits and enhancing error detection and correction of the string when read (39) from the non-uniform stripe (18); the relative timings and the polarities of peaks of the analogue signal from the magnetic reading head 939) determine the data value ("0" or "1" according to timing interval t or t/2) or the control nature (interval 3t/2) or each read bit. The error-correction code is applied to reduce errors and enable decryption of the recorded identifier using the public key, whereby an appropriate degree of correlation for authenticity confirmation, can be achieved with the fresh transparency output (40).