What is an RSA “key ID”?

Question

I\'ve seen key IDs used in several places and would like to use them in my program, but I haven\'t been able to find a description of them. How are they generated?

Answer 1

In different formats (PGP, SSH, X.509 certificates) key ID has different meaning. Neither SSH nor X.509 have a "dedicated" concept of key ID, but some people use this term (including their software) - in this case it's usually a hash of the public key or of the certificate in whole.

Update: the comments reminded me that "key identifier" extensions exist in X.509 certifiactes, and they sometimes are being referred to as key IDs. Yet, this is not common - usually the hash (also sometimes called the fingerprint) is referenced as key ID.