In days of old, introductions were made by means of credentials, issued by a trusted party. In some cases, such credentials were accepted more or less at a global level. A good example of this is the ordinary passport, supported, if necessary, by additional documentation, such as a visa. In this case, the trusted issuer of the credential is a national government or combination of governments. One obvious requirement for such credentials is that they are difficult to forge and yet easy to verify. Today, however, more and more business is conducted electronically, often via the Internet, and as face-to-face contact is reduced and documents are simply bit-strings, we have the problem of what to do about credentials. How do I prove that I am who I say I am, that my “credentials” can be trusted and that my privileges are as claimed?