the account holder by taking the enrolled 256 byte iris code stored as part of the database recorded on the hard disk fig6 56 for that user and carrying out similar operations step 95 on the enrolled iris code to those carried out at the point of sale device 58 on the captured iris code it will be realised by those skilled in the art that this operation could instead be carried out using hardware similar to the module 74 contained within the asic 70 in the point of sale device the server 54 then decrypts the encrypted partial iris code transmitted from the point of sale device step 96 and carries out a comparison of the transmitted partial iris code with the acceptable code it has just computed step 97 the server 54 transmits a signal which indicates the result of that comparison step 98 if the two codes are sufficiently similar then the server outputs a signal representing an approval of the transaction which is received by the point of sale device step 99 the transaction being allowed to continue if however the computer transmits a signal representing disapproval of the transaction then the point of sale device 58 displays a refusal of the transaction in step 100 in the above procedure the thirteen bit value is derived pseudo randomly by the server 54 however alternative arrangements can be envisaged in which the value is dependent on the time of the request for example the point of sale apparatus 58 might additionally forward to the server in step 88 an indication of the time and date of the request both the server and the point of sale device might synchronously follow a sequence of numbers between.