Ok, so I'm gonna try to copy an rename it.

Here was the error message I get when trying to convert my key file :

[root@master certificate_manager.d]# openssl x509 -text -outform der -in GFBienne.key -out GFBienne-key.pem
unable to load certificate
140612166498192:error:0906D06C:PEM routines:PEM_read_bio:no start lineem_lib.c:707:Expecting: TRUSTED CERTIFICATE
[root@master certificate_manager.d]# ls -l