According to this disclosure, a user is identified (and selectively granted access to protected resources) by using information that describes the user's interpersonal relationships. This information typically is stored in a datastore, such as a digital address book, an online profile page, or the like. The user's digital address book carries an “acquaintance pattern” that changes dynamically in time. This pattern comprises the information in the user's contact list entries. In this approach, the entropy inherent in this information is distilled into a unique acquaintance digest (or “fingerprint”) by normalizing the contact list data, and then applying a cryptographic function to the result.