In public-key encryption, the secrecy of the public key is not required, but the authenticity of the public key is necessary to guarantee its integrity and to avoid spoofing and playback attacks. A user’s public key can be authenticated (signed) by a certificate authority that verifies that a public key belongs to a specific user. In this chapter, digital certificates, which are used to validate public keys, and certificate authorities are discussed. When public-key is used, it is necessary to have a comprehensive system that provides public key encryption and digital signature services to ensure confidentiality, access control, data integrity, authentication, and non-repudiation. That system, public-key infrastructure or PKI, is also discussed in this chapter.