PKI (Public Key Infrastructure) is the organization and application of public-key cryptography in digital certificates and certification authorities. PKI is mainly concerned with secure identification, verification and key exchange between parties.
Traditional PKIs rely on the sender and recipient (relying party) to be in the same “trust domain”: they both must trust the same root Certificate Authority. Cross Certification allows the relying party to map trust between trust domains, allowing the replying party to verify a certificate issued by a CA that he/she does not normally trust. A Bridge Certificate Authority joins many trust domains, by cross-certifying with the separate root CAs, and providing an array of certification paths, easing the process of cross-certification.