Let's say someone signs a document for long term validation using a certificate issued by a trusted CA. Throughout the following years, the document is timestamped again and again by multiple trusted TSA.
Now we project ourselves in a distant future, ie 40 years from now. The original CA is long gone, out of business. I can certainly validate the integrity of the original signature, by going through and validating the timestamps chain. But how can I be sure that the signer certificate was really issued by the same CA that was trusted many years ago, but has since disappeared ? Should I have kept a copy of the CA certificate from the time it was active, and trusted, and preserve its integrity using timestamps ? Would have this bring any more trust in the validation ?
Edited for clarification:
Lets have 2 docs D1 and D2. Both contains a valid signature S1 and S2, and both were timestamped with Trusted TSAs (could be the same TSAs!). The only difference is S1 certificate was issued from CA1 and S2 certificate was issued from CA2. From an integrity point of view, everything is valid. From an origin point of view, 40 years ago, when both documents were signed, we only trusted CA1. CA1 and CA2 now no longer exists. How can I make sure, without doubt, D1 is a document I trust (because it was signed a a time we trusted CA1), and D2 a document I don't trust.
Maybe this is solved only by timestamping, but I feel I would be trusting the process the document went through, rather than the document itself.