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Can anyone with access to root CA private key decrypt HTTPS traffic of sites using certificate signed by that CA ?

and if yes do he need to be man-in-the-middle or just monitoring traffic is sufficient?

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No, this is not the case. A certificate signed by a CA contains only the public key, but for decrypting you need the private key too. This private key is not needed for the CA to sign the key, so they usually don't have it either.

But, some CA offer to simplify the process of certificate generation by generating a key pair for the certificate too. In this case the CA knows the private key at least for a short time and the owner of the certificate has to trust the CA to forget the key after certificate and key were downloaded by the owner of the certificate. This is much more trust than just the signing of an existing CSR (which does not contain the private key).

Apart from that even private key does not help to simply decrypt sniffing traffic if the key exchange is done with Diffie-Hellmann (DHE, ECDHE). In this case the attacker would need to mount a real man-in-the-middle attack to actively intercept and modify traffic.

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