Visit the website of a well-known commercial CA to establish (a) what levels of public-key certificate they issue; (b) what credentials they require for registration for these different levels of 8. public-key certificate: els of public-key certificate; (d) how often they publish certificate revocation lists (CRLs) (e) how clients access CRLs; (f) what advice they provide clients concerning the importance of checking CRLs