How is Cross-Certification implemented? I am currently learning about Certificates in Public Key Infrastructure.


  Cross-Certification exists as an alternative because the concept of a single monolithic CA certifying every possible user in the world is quite unlikely. 
  Cross Certification are issued by CA's to create Non hierarchical trust path.


It works on the concept that every root CA has cross certified each other. But how is that even possible?

It is stated that almost every country has it's own root CA (in an abstract way). And there exists a lot of countries in the world, then how can all of them cross certify each other? 

For Example, the concept of Cross Certification is explained by showing how the root CA of England cross certifies the root CA of Africa. And then Alice living in England can verify the chain of trust of a Bob living in Africa. Stating, that Technically this would mean that Alice's root CA has obtained a certificate or itself from Bob's root CA and vice versa. 

This method seems feasible when the number of CA's are limited (like CA of England and japan). But how could this scale up to a large number of CA's(CA of every country)?
In practice Cross Certification is rare, instead, certificate consumers trust multiple CAs. For example, Mozilla includes 154 different trusted CAs by default with their browser.

As to the process of cross-certification, it involves a CA signing other CA's public key. It can be one-way or cross-certified.