Self-signed Certificate

In cryptography and computer security, a self-signed certificate is an identity certificate that is signed by the same entity whose identity it certifies. This term has nothing to do with the identity of the person or organization that actually performed the signing procedure. In technical terms a self-signed certificate is one signed with its own private key.

In typical public key infrastructure (PKI) arrangements, a digital signature from a certificate authority (CA) attests that a particular public key certificate is valid (i.e., contains correct information). Users, or their software on their behalf, check that the private key used to sign some certificate matches the public key in the CA's certificate. Since CA certificates are often signed by other, "higher-ranking," CAs, there must necessarily be a highest CA, which provides the ultimate in attestation authority in that particular PKI scheme.

Obviously, the highest-ranking CA's certificate can't be attested by some other higher CA (there being none), and so that certificate can only be "self-signed." Such certificates are also termed root certificates. Clearly, the lack of mistakes or corruption in the issuance of such certificates is critical to the operation of its associated PKI; they should be, and generally are, issued with great care.

In a web of trust certificate scheme there is no central CA, and so identity certificates for each user can be self-signed. In this case, however, it has additional signatures from other users which are evaluated to determine whether a certificate should be accepted as correct. So, if users Bob, Carol, and Edward have signed Alice's certificate, user David may decide to trust that the public key in the certificate is Alice's (all these worthies having agreed by their signatures on that claim). But, if only user Bob has signed, David might (based on his knowledge of Bob) decide to take additional steps in evaluating Alice's certificate. On the other hand, Edward's signature alone on the certificate may by itself be enough for David to trust that he has Alice's public key (Edward being known to David to be a reliably careful and trustworthy person). There is of course, a potentially difficult regression here, as how can David know that Bob, Carol, or Edward have signed any certificate at all unless he knows their public keys (which of course came to him in some sort of certificate)? In the case of a small group of users who know one another in advance and can meet in person (e.g., a family), users can sign one another's certificates when they meet as a group, but this solution does not scale to larger settings. This problem is solved by fiat in X.509 PKI schemes as one believes (i.e., trusts) the root certificate by definition. The problem of trusting certificates is real in both approaches, but less easily lost track of by users in a Web of Trust scheme.