Isso funciona para mim:
ssh-keygen -t rsa -N '' -f foo.rsa
Altere o nome do arquivo para o que você deseja. Observe o uso de 'em vez de'
Não tenho certeza do que você quer dizer com o segundo bit ... A chave gerada já não é PEM?
$ ssh-keygen -t rsa -N '' -f foo.rsa
Generating public/private rsa key pair.
Your identification has been saved in foo.rsa.
Your public key has been saved in foo.rsa.pub.
The key fingerprint is:
56:45:5d:54:a6:11:86:63:20:c9:b7:f3:b8:ba:c0:11 ssta@debianscratch
The key's randomart image is:
+---[RSA 2048]----+
| ....ooo++*|
| o...+..+ |
| E ..o .. |
| . .o |
| . S + |
| . o . . |
| o . |
| . . |
| oo |
+-----------------+
$ file foo.rsa
foo.rsa: PEM RSA private key
$ cat foo.rsa
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----