SSH rejects RSA passphrase for keys created with GPG/Enigmail

Question

Last year I created an 4096 bit RSA key pair with GPG / Enigmail in Thunderbird. Enigmail lists me a primary key used to sign and authenticate e-mails, and a subkey used to encrypt e-mails. I exported the key pair to an .asc file.

Now I wanted to use these keys for a SSH connection and exported the keys like this:

gpg2 --armor --export-secret-keys [key] > sec_key.txt
ssh-add sec_key.txt

When ssh-add asks me for a passphrase, I enter the right one (that works in Enigmail) but it is rejected:

Bad passphrase, try again

Using the public key (exported with --export) leads to the same error, as well as using the subkey. "ssh -v -i sec_key.txt localhost" gives this error:

key_parse_private_pem: PEM_read_PrivateKey failed

But the passphrase must be correct because I'm able to change the passphrase with GPG:

gpg --edit-key [key]

I also tried to remove the passphrase completely but then I cannot log into ssh. "ssh-add sec_key.txt" then gives no output but returns 1.

What can I do to use this keys for ssh connections?

Answer 1

You cannot use a PGP key directly with SSH; see How do SSH keys differ from keys used for other purposes? for details on differences and Are GPG and SSH keys interchangable? for how to make it work.