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Lets take a random well-known Root CA. This CA allows me to sign my own certificate so that it is immediately trusted by others since they trust the root CA. The only thing, which the CA ever sees is my Certificate Signing Request (CSR), they never get to see my private key.

Assuming the CA gets compromised in the future, is my certificate still secure for encrypting emails to me? In my understanding yes, since the CA never saw my private key which I use to decrypt messages. The only thing which could be faked would be a signature, if I don't check that the signature matches the previous signature each time. And also assuming that the signature was not compromised from the beginning.

Is that assumption correct?

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If you receive mails which are encrypted with your certificate than these mails are still safe against decryption by others since only you have the private key. This is also true for signatures you create with this certificate since in this case the private key is used for the signature so that other could verify the signature with the public key contained in the certificate.

But, if the attacker compromising the CA managed to get another certificate for your identity then he could try to claim that this is the real certificate (and not yours). This way the attacker can sign mails with your identity and also receive encrypted mails which are sent to you as long these use the attackers certificate. The attacker can also work as the man in the middle in this case, i.e. receive mails for you with but encrypted with the attackers certificate, modify the mail and re-encrypt it with your certificate and then forward it to you.

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