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ccna best book. User mail agents using the Secure/Multipurpose Internet Mail Extensions (S/MIME) protocol to support mail protection use X.509. IPSec VPNs that use certificates to provide a public key distribution mechanism for IKERSA-based authentication use X.509. The PrettyGood Privacy (PGP) protocol was originally developed by Phil Zimmerman (a privacy advocate) as an application that enables end users to use encryption to protect the privacy of communications. PGP is most commonly used to secure email. PGP is also capable of recognizing x.509 certificates.Certificates are also used by network devices at the network contact or application layer. Cisco routers, CiscoVPN concentrators, and CiscoPIX firewalls can use certificates to authenticate IPSec peers.Cisco switches can use certificates to authenticate end devices connected to a LAN port. Authentication between adjacent devices is performed using 802.lx. Authentication can be proxied to a central ACS via ExtensibleAuthenticationProtocolwithTLS (EAP-TLS).7.4 Public Key Cryptography 213Cisco routers can also provide TN3270 support that does not include encryption or strong authentication. Cisco routers can now use SSL to establish secure TN3270 sessions.7.4.5.3 Public Key Cryptography StandardsAnother important PKI standard is the Public-Key Cryptography Standards (PKCS). PKCS refers to a set of public-key cryptography standards designed and published by RSA Labs. PKCS provides basic interoperability before applications that use public-key cryptography. PKCS defines low-level formats for the secure exchange of arbitrary data (e.g., encrypted data or signed data).A statement on the RSA Labs website states that "The Public Key Cryptography Standard is a specification developed by RSA Labs in collaboration with security system developers worldwide to facilitate the deployment of public key cryptography."