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Our company is being acquired by another company and we are curious on the requirements needed to create a cross-certification / bridge CA solution.

Cross-Certification is issuing a Cross Certification Auth. certificate to the root CA of Fabrikam from Contoso's Issuing CA.

"The effect of this Cross Certification Authority certificate is that the Fabrikam Root CA appears as a subordinate CA of the Contoso Issuing CA when the certificate is presented to a computer at Fabrikam."

Is there a preferred method and why?

  • I think, your understanding of cross-certification and bridge CA is incorrect. – Crypt32 Jul 11 '17 at 17:50
  • Cross certification allows you to issue a Cross Certification Authority certificate from a CA in your organization to a CA in another organization. The advantage of cross certification is that you do not have to reissue any certificates to your organization’s users. – Matt L. Jul 11 '17 at 17:59
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You will need simple cross-certification. It is suitable to "friend" two PKIs in a easy and convenient way. Each organization cross-certify other organization and publish cross-certificates within their organization.

  • there is no need in bidirectional AD trust (assuming that CRLs are accessible via HTTP, not LDAP)
  • CRLs from other organization must be accessbile from your network and vice versa.

Simple cross-certification isn't much scalable. In practice, it will work for no more than 3 fully connected peers. In order to establish a complete trust between all parties, the number of all cross-certificates to be issued is calculated as follows: formula. In other words, it is a complete graph topology.

When there are 4 or more participants, a number of total issued cross-certificates increases dramatically. This is where CA bridging helps. The graph is transformed into star topology, where Bridge CA is a center of the topology and it is connected to all participants with single edge. The total number of required cross-certificates is calculated as follows: formula

Each party issue single cross-certificate to Bridge CA and Bridge CA issue back single cross-certificate to each party. When other organization present you their certificate, its chain is looped through Bridge CA cross-certificate and ends up with your own root CA.

  • That makes sense. At a point Bridge CAs are more useful with more organizations since you can spin up a "branch" CA in each organization. Cross-Certification we will just need to make sure we can talk to our CRL/OCSPs and deploy both Root Certificates to both environments, correct? – Matt L. Jul 17 '17 at 16:54

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