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TLS security reduces itself to how much one trusts issuer. However it mathematically possible to reduce dependence from root cert issuer if the traffic is double-encrypted: with two certs from two independent issuers (or more).

Is there any practical approach to do this? Is there any demand for it?

  • Have you come across cases where a well known root CA's certificate is forged? – Limit Jan 9 '17 at 2:24
  • Certificate Transparency is meant to police the CAs. If the cert is publicly logged, a second cert from a second CA does not increase the trustworthiness, IMO. – Z.T. Jan 9 '17 at 2:58
  • Are you asking if there can be two signatures on one certificate? In that case the answer is: No, that is not allowed for X.509 certificates. – StackzOfZtuff Jan 9 '17 at 7:38
  • @Limit the word "forgery" is misleading and I did not intend to use it. – Euri Pinhollow Jan 9 '17 at 10:01
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... if the traffic is double-encrypted: with two certs from two independent issuers (or more).

The traffic is not encrypted with the certificates. The certificates are mainly used for authentication, i.e. to verify that the client is talking to the correct server. The key pair of the leaf certificates is also used inside the RSA key exchange but not with DHE or ECDHE key exchange.

While double TLS like you proposed would be a way to not rely too much on a single CA it would need to be supported by all clients. But, since the encryption itself is not the problem but authentication is it would be much better and less overhead to make the authentication part only less dependent on a single CA. And this is the direction where improvements are done in browsers today. While it is not possible to directly sign the same certificate by multiple CA one can use certificate transparency or projects like Convergence for an extended validation of the certificate. There are also standards like DANE as replacement of the current PKI or for additional validation and certificate pinning also helps in this regard.

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