RSA default key size of 2048 bits too large for low-spec systems

Brent Roman brent at mbari.org
Sat Jun 24 12:58:29 AWST 2017


Hi Matt,

I like the -R option and am already using it to
eliminate the need to generate keys on the initial system boot.
However, I still generate a public/private key pair for each user the 
first time they log in.
(which could delay that by many tens of minutes for the 2048 bit key)
Maybe I should run the key generation in the background...Hmmm...

ECDSA won't work so well here because of legacy systems (old OpenWRT 
routers) that don't support it.

Thanks again,

- brent


On 06/23/2017 08:02 PM, Matt Johnston wrote:
> Hi Brent,
>
> I'll see about improving the visibility of the default key sizes in options.h and also dropbearkey's printout.
> I changed to 2048 because 1024 is likely to become breakable within the next few years, it's best to have secure defaults if systems are going to remain un-updated for that timeframe.
> The change was never mentioned in the CHANGES for 2013.61test - sorry about that oversight, I've added it now.
>
> For a slow device maybe it'd be better to disable RSA and DSS altogether and just use ECDSA keys? Those are siginificantly faster and supported by all recent SSH implementations I know of (at least for desktops and servers). The newer "-R" option can also be used to generate keys on first connection - that might be fast enough with ECDSA.
>
> Cheers,
> Matt
>
>> On Sat 24/6/2017, at 9:31 am, Brent Roman <brent at mbari.org> wrote:
>>
>> I recently upgraded some low power ARM9 systems from dropbear v0.52 to v2017.75
>>
>> Everything went well until a system system tried to generate server keys on first boot.
>> Then it hung while working to generate the default 2048 bit RSA key.
>>
>> Further investigation determined that it had not really hung.
>> Given many tens of minutes, it would complete the initial boot.
>>
>> Why did we decide to change the default to 2048 bit keys given the cost of generating these on the embedded systems for which it is intended to run?
>>
>> The #define for setting the default key size is currently in a .c file.
>> Could this be moved to the options.h file with a comment recommending reducing the default size when targeting slow systems?
>>
>> Note, I do realize that there is a -s option for dropbearkey, but the appropriate values for that option are dependent on the key algorithm selected.  I believe the defaults should always be usable.
>>
>> Dropbear is a great piece of Open Source software.
>>
>> I hope you'll consider this small change.
>>
>> - brent
>>
>>
>>



More information about the Dropbear mailing list