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I want to do an asymmetrical file encryption:

openssl genrsa -des3 -out private.key 4096
openssl rsa -in private.key -pubout -out public.key
openssl rsautl -encrypt -pubin -inkey public.key -in a -out b

But I already have a key pair in my ~/.ssh directory, so how do I use this instead of generating new keys? Simply switching filenames gives an error:

$ openssl rsautl -encrypt -pubin -inkey ~/.ssh/id_rsa.pub -in a -out b
unable to load Public Key

public.key starts with BEGIN PUBLIC KEY while ~/.ssh/id_rsa.pub starts with ssh-rsa, so I figure it's some sort of format problem.

How can I generate a ~/.ssh key that openssl understands, or how can I instruct openssl to understand the keys I have in ~/.ssh?

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1 Answer

You can convert your existing ssh key to PKCS#8 with the following:

mv ~/.ssh/id_rsa{,.old}
umask 0077
openssl pkcs8 -topk8 -v2 des3 -in ~/.ssh/id_rsa.old -out ~/.ssh/id_rsa

After testing the converted key, you can then delete the original.


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Tried this (entering a blank password for the converted private key). When I tried to ssh (using the new private key), OS X displayed a GUI dialog asking for password (in parallel with the terminal asking for a password). Further suggestions? –  forthrin May 29 '13 at 15:25
I haven't tried using it with a blank passphrase, but with a passphrase it works great on OS X (10.8.3) and CentOS. –  demure May 29 '13 at 15:29
Tried it again and it worked. But your suggestion is for converting the private key, while the encryption requires the public key. How do I convert the public key? –  forthrin May 29 '13 at 20:15
No, you should be encrypting with your private key (which only you have) and letting the recipient have access to your public key to decrypt the information. –  Dave Mulligan Apr 16 at 4:42
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