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I'm looking into increasing my use of signed and encrypted "things" - LibreOffice Documents, Off-the-record chat, PDFs, emails, etc. I'm finding that some things only support x509 format-certificates, and others only support OpenPGP format-certificates.

I also like that OpenPGP is not CA-dependent, and dislike that the CA model is vulnerable to government interference. However, the CA model is convenient in certain use cases (e.g. Emailing a signed PDF to an old, wizened academic who knows nothing about encryption).

I thought about creating a Frankensteinian x509/OpenPGP hybrid from the same key pair, but I'm not sure if it's worth the trouble.

So I think the question is: Is it advisable to own and maintain two key pairs - one in x.509 format, and the other in OpenPGP?

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For all practical purposes, X.509 certificates and OpenPGP key pairs live in separate worlds. You need an X.509 certificate in order to interoperate with people/systems who use X.509 certificates. You need an OpenPGP key pair to exchange secured emails with people who use OpenPGP. If you want to do both then you need both.

One key idea is that such keys make no sense. In a way. An X.509 certificate, or a signed OpenPGP public key (same concept), binds a public key to an identity. What is your identity ? If you look at it closely, you will find that you have several. Legally, you are a citizen (or subject) of some country, and one of the monopolies of central governments is to define identities of people who are under its nominal scope. But that government is not the one who issues your X.509 certificate, let alone signs your OpenPGP key. Furthermore, I strongly doubt that your official ID card / passport / driving license / whatever would bear the name "scuzzy-delta".

In fact, with OpenPGP's Web of Trust, your identity is community-driven: you have as name whatever value other people found convincing. It depends on the community, and you perfectly could have several such identities (many people have a "personal email address" and a "business email address"). As for X.509 certificates, your identity there is procedure-driven: it is whatever is implicitly verified by the process by which the CA "ascertains your identity" (some CA, for some certificates, only check that you control an email address).

Under these conditions, it makes little sense to maintain an X.509 certificate and/or an OpenPGP key pair "generically". You should have keys for a purpose. For instance, you may want an OpenPGP key pair so that people can exchange encrypted emails with your "scuzy-delta" persona. Or an X.509 certificate with the name and address that you would agree to show to old wizened academics (or, for that matter, to your mother). Or maybe a certificate whose purpose is to authenticate your connections with some VPN.

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