Export RSA private key to PKCS8 format

[akostadinov]

Hi, I don't see any way to export RSA private key to PKCS8 PEM format as for example required by Chrome extension .crx.

There is a discontinued rubygem about this [1] but I think a to_pkcs8 method would nicely fit into the main library. There is a pull request for making it work with recent ruby.

[1] https://github.com/postageapp/openssl_pkcs8
[2] https://github.com/postageapp/openssl_pkcs8/pull/4/files

[ioquatix]

Great, can you make a PR here?

[akostadinov]

I don't expect to have time for this. I found I have another way around for the task at hand.

[ioquatix]

What is the other way?

[bdewater]

#119 covers this among other things. Perhaps the relevant commit(s) could be cherry-picked out of the larger work in there to support x25519.