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Alice and Bob have TLS certificate authorities. My device trusts Alice's CA, and connects to servers that present a certificate rooted at Alice's CA. It does not explicitly trust Bob's CA, or the servers that use his certificates.

Alice trusts Bob, but Bob already has his CA, and his certificates are already deployed on servers. Can Alice create a signature of some kind, such that my device would trust Bob's certificates because it trusts Alice's CA?

This could be useful if the devices that trust Alice's CA cannot be patched to also trust Bob's CA explicitly.

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    In short: you cannot add a signature to an existing certificate, but you can create a certificate with the same subject and key as Bob CA, but signed by Alice CA. This cross-signed Bob CA can be used to validate certificates issued by the original Bob CA because it has the same properties (key, subject, ...). And one only need to trust Alice CA for this since it has issued this cross-signed Bob CA. Note though that the cross-signed Bob CA must be known during validation, the original Bob CA is not sufficient. Aug 20, 2023 at 6:39

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