Electronic watermark processing apparatus

Information representing whether a user is included in arbitrary two of all users who may receive digital data is embedded in the digital data. Even when a plurality of users collude to alter the difference data to data of a third party other than themselves, the data representing themselves remains in the altered data, so collusion fails. An electronic watermark processing apparatus embeds watermark such that the watermark can hardly be altered even when a plurality of users collude to compare their data.