Just curious… I’ve seen this X1/X2 convention before for CA roots. Does anyone know the origin or rationale for this?
Now we have a “Y” generation showing up, but it seems like whoever thought of “X” didn’t anticipate more than three generations, or they would have used A1/A2.
It's a good question! I know that our first root (ISRG Root X1) used that naming scheme simply because it was cross-signed by IdenTrust's root (DST Root CA X3) which used that same scheme. But where they got the scheme from, I don't know.
Using Y to denote the "next generation" of roots is a scheme I came up with in the past year while planning our YE/YR ceremony, so it's certainly not something that people were thinking about when they named the first roots.