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When I look at the trusted certificates in my browser, I see that many certificate authorities have multiple root certificates associated with them. For example, org-Amazon has Amazon Root CA 1, Amazon Root CA 2, Amazon Root CA 3, and Amazon Root CA 4.

How do the purposes of these four certificates differ?

I also see that some CAs have root certificates of different "tiers", such as gold, silver, and platinum. What's the deal with these? I imagine there are price differences associated with the different tiers, but what extra service could they be offering?

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    Good question. I gave some details regarding Amazon below. I suggest you split off the gold/silver/platinum part into a question of its own so that it can be answered separately. – StackzOfZtuff Aug 10 at 8:07
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Numerous public CAs have multiple root certificates in trust store for different reasons and largely depends on their business operations. Often roots from same vendor differ by:

  • List of usages
  • Public key or signature algorithm support
  • Assurance level
  • Redundancy

For example, Amazon Root CA 2 and Amazon Root CA 4 allows Code Signing certificates in their chains, while Amazon Root CA 1 and Amazon Root CA 3 do not allow such. This is "EKU" separation.

Amazon Root CA 1 and Amazon Root CA 3 share same list of usages, but they use different public key and signature algorithms: Amazon Root CA 1 and Amazon Root CA 2 use RSA public key, Amazon Root CA 3 and Amazon Root CA 4 use ECC public key.

This setup provides good level of flexibility, compatibility and security.

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Amazon lists 5 root CA on https://www.amazontrust.com/repository/ :

Now I think "Starfield" was the CA that they bought to get into the CA game. And the other 4 Amazon made themselves.

And one difference I see is that CAs 1-4 all have different key types. (Key Type 2048-bit RSA, Key Type 4096-bit RSA, Key Type ECDSA (P-256), Key Type ECDSA (P-384).) That is a significant difference between then. Maybe there also are OTHER differences (such as intended usage maybe). But the key type alone makes the four CAs very different from one another.

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