There are a number of requirements that businesses have with respect to implementing effective public key infrastructures. First and foremost, if users cannot take advantage of encryption and digital signatures in applications, a PKI is not valuable. Consequently, the most important constraint on a PKI is transparency. The term transparency means that users do not have to understand how the PKI manages keys and certificates to take advantage of encryption and digital signature services. An effective PKI is transparent. In addition to user transparency, a business must implement the following items in a PKI to provide the required key and certificate management services: Public key certificates A certificate repository Certificate revocation Key backup and recovery Support for non-repudiation of digital signatures Automatic update of key pairs and certificates Management of key histories Support for cross-certification Client-side software interacting with all of the above in a secure, consistent, and trustworthy manner Note: The term “client-side” refers to application clients and application servers. PKI requirements are the same for both application clients and servers, and both are “clients” of the infrastructure services. All of these requirements must also be met to have an automatic, transparent, usable PKI.