Had to update whole ubuntu system, to fetch this version. (Ubuntu 14.04 goes until dfu-util 0.5)
Unfortunately the error still shows up:
oliver@oliver-lenmt:~/workplace/RedBear$ dfu-util -d 2b04:d058 -a 1 -s 34 -D device_private_key.der
Copyright 2005-2009 Weston Schmidt, Harald Welte and OpenMoko Inc.
Copyright 2010-2014 Tormod Volden and Stefan Schmidt
This program is Free Software and has ABSOLUTELY NO WARRANTY
Please report bugs to email@example.com
dfu-util: Invalid DFU suffix signature
dfu-util: A valid DFU suffix will be required in a future dfu-util release!!!
Opening DFU capable USB device...
Run-time device DFU version 011a
Claiming USB DFU Interface...
Setting Alternate Setting #1 ...
Determining device status: state = dfuIDLE, status = 0
DFU mode device DFU version 011a
Device returned transfer size 4096
DfuSe interface name: "DCT Flash "
Downloading to address = 0x00000022, size = 16384
dfu-util: Last page at 0x00004021 is not writeable
However retrieving again the private key with 0.8, I have seen different sizes compared to the keys I retrieved with 0.5.
-rw-rw-r-- 1 oliver oliver 16350 dic 14 18:08 device_private_key3.der
-rw-r--r-- 1 oliver oliver 16384 oct 29 23:55 device_private_key.der
First was retrieved today with dfu-util 0.8, second, at some point of time, with dfu 0.5.
The additional bytes retrieved with 0.5 are the reason of the problem to fail to perform the restoration.
1.- Ensure when retrieving & restoring the private key you are using dfu-util is 0.8
2.- If (as me) your key was retrieved with dfu-util 0.5 (or size is 16384) there is still an option, because in my case the additional bytes were filed with 0xff so make a copy of your keyfile and truncate the file:
$ cp original_0_5_retrieved_key.der original_0_5_retrieved_key_reduced.der
$ truncate original_0_5_retrieved_key_reduced.der -s 16350
Restore it with dfu-util, and back to cloud!
Thanks again for your hint @guohui !