[openpgp] Secure and interoperable secret key transfer

[Vincent Breitmoser <[e-mail]>]

Hi,

there should be a standardized format to transfer secret keys in a way
which provides confidentiality and integrity protection. At the moment,
most tools leave the user completely on their own to perform this not
too common but centrally important operation.

While symmetric file encryption is a technically usable mechanism, the
de facto state of the art is that no implementation enforces (or even
encourages) its use for secret key transfer, or provides support for
decryption on the import side. Which very likely means that many users
move their keys around unprotected.

The passphrase is not a suitable mechanism for this: It provides only
partial confidentiality, no integrity protection, and most importantly
its use for this purpose confuses users about what the actual purpose of
the passphrase is - and how strong it should be for each use case.

The tl;dr of this post is that I would really like to get rid of these
two FAQ points of ours:
https://www.openkeychain.org/faq/#how-to-import-an-openkeychain-backup-with-gpg
https://www.openkeychain.org/faq/#what-is-the-best-way-to-transfer-my-own-key-to-openkeychain

All this said, I'm not entirely sure what the best way to do this could
be.  Since symmetrically encrypted data packets exist, it's quite
possible that just a paragraph encouraging its use for this purpose
somewhere near the transferable secret key section would suffice.

 - V