A CA which automatically signs public keys which match some requirement.
Typically RobotCAs are set up to validate that the a public key belonging to an email address does actually belong to the email address. This is achieved by the RobotCA signing each uid on the public key and sending the signed copy to the email address, encrypted with the public key. If the public key belongs to whoever reads the email address, they recieve the signed copy, can decrypt it and then publish it to the public KeyServers. If the public key does not belong to whoever reads the email address, they recieve are unable to decrypt the encrypted key, but the accompanying message gives them sufficient information to let them know that that someone is attempting to impersonate them.
RobotCAs are considered significantly less secure that other CAs, which typically require multiple forms of photograph identification. In particular most are only as strong as the underlying Mail infrastructure: anyone who can read another persons mail can impersonate them and anyone who can read and delete another persons mail can get the signature without the person knowing.
A RobotCA also has the side effect of serving as a TimeStampServer for keys---because a time stamp is included in the signature added to the key, the signature is evidence that the key existed at a certain point in time.
Currently there are three RobotCAs in widespread use:
http://www.toehold.com/robotca/http://pgpkeys.telering.at/robotca/http://www.imperialviolet.org/keyverify.htmlThe first two use the same implementation, but all three are wrappers around GPG. (I've used all these RobotCAs -- StuartYeates)
There are some RobotCAs which offer a a higher level of trust than simply verifying that email sent to the address list in the uid gets delivered to a holder of the secret key. Generally these are run by organisations and require some form of identification such as a passport.
