Who do you trust?
One of the earliest internet protocols was http. Very quickly, engineers discovered a need to secure content that they sent/received. This lead to the development of the HTTPS, SSL, and TLS protocols. One of the main features to this protocol is the reliance on X.509 Certificates1. The idea behind certificates is that developers can go to a central authority, request authorization, and have a certificate that others can trust, if they trust that central authority.
This works just fine if every connecting agent has trusts that central authority, however, due to the complexities of X.509 Certificates, trust verification, and the diversity of revocation checking methods, certificate revocation is effectively broken for websites, with most major browsers performing a “best effort” check if they check at all.
Sometimes Certification Authorities are compromised though. In 2011, a reseller (RA) for Comodo (one of the largest CAs) was compromised, allowing several fraudulent certificates to be issued and trusted for google.com and other major domains. Later that year, DigiNotar was also compromised, issuing more fraudulent certs. The mitigation was to revoke the intermediate cert/root cert that was compromised, and remove the certificate from each computer’s trust store. Doing so, however, eliminated trust for all sites who could be trusted, but had certificates issued by Comodo’s reseller or DigiNotar. So those sites were effectively offline until their certificates were re-issued (which is not a definite time frame).
Cross-signing a certificate allows for a private/public key pair to be signed by multiple CA’s. By getting a cross-signed certificate, you effectively have two certificates that can be used interchangeably depending on trust. In a distributed environment, each node could have a trust authority, with which it issues a certificate. That certificate can then be used to verify trust as, if we trust our peer, and they trust another node, we can trust that node.
Along the same lines, having multiple certificates allows for redundancy in a system. If our noe has a certificate issued by Comodo and Digicert, and our system was operating during 2011, once Comodo was removed from trust on the different nodes, the nodes would continue to operate uninterrupted because of the redundant certificate.
This issue is rare, however, with computational power increasing every year, as well as connectivity speeds, it is increasingly difficult to detect a compromise. This leads many systems to rotate certificates on a regular interval so that even if the certificate is compromised, it can’t damage the system for any significant period of time.