RSA key configuration limitations

[Dmitry Belyavskiy]

Any feedback please? The CI failures on Github seem irrelevant.

On Thu, Jun 23, 2022 at 2:12 PM Dmitry Belyavskiy <dbelyavs at redhat.com>
wrote:

> I've fixed the known failure and kindly ask to rerun the GitHub CI for
> https://github.com/openssh/openssh-portable/pull/325 and approve the PR
> in general.
>
> On Thu, Jun 16, 2022 at 9:31 AM Dmitry Belyavskiy <dbelyavs at redhat.com>
> wrote:
>
>>
>>
>> On Wed, Jun 15, 2022 at 5:33 AM James Ralston <ralston at pobox.com> wrote:
>>
>>> On Mon, Jun 13, 2022 at 4:34 AM Jochen Bern <Jochen.Bern at binect.de>
>>> wrote:
>>>
>>> > On 10.06.22 16:50, Dmitry Belyavskiy wrote:
>>> >
>>> > > There is a need to increase RSA key requirements to make the
>>> > > installations more secure. Just updating the default compiled-in
>>> > > value isn't an option because it may significantly break legacy
>>> > > systems compatibility. This PR [1] introduces a new configuration
>>> > > option MinRSABits to be managed for security's sake.
>>> > >
>>> > > If this approach is OK for upstream, please let me know and I will
>>> > > improve this PR according to the feedback.
>>> >
>>> > I realize that with the *current* selection of algorithms available
>>> > in OpenSSH, fine-grained control of minimum key size almost(!) is an
>>> > RSA-only topic, but nonetheless I wonder whether newly-defined
>>> > config syntax thereto should be aimed at extensibility to other
>>> > cryptalgorithms ...
>>>
>>> That ship sailed long ago:
>>>
>>> $ grep SSH_RSA_MINIMUM_MODULUS_SIZE sshkey.h
>>> #define SSH_RSA_MINIMUM_MODULUS_SIZE    1024
>>>
>>> It’s not worth it to attempt to refactor this approach, as with both
>>> the ecdsa family and ed25519, the cipher name specifies the security
>>> strength.
>>>
>>> Dmitry’s merge request both defaults MinRSABits to
>>> SSH_RSA_MINIMUM_MODULUS_SIZE, and prohibits setting MinRSABits to
>>> anything less than SSH_RSA_MINIMUM_MODULUS_SIZE.  So unless the
>>> administrator specifically sets MinRSABits to something greater than
>>> 1024, it will not change the behavior of OpenSSH.  It also documents
>>> MinRSABits in the man pages, and includes MinRSABits in “ssh -G”
>>> output.  All of this seems perfectly reasonable.
>>>
>>
>> I also need to adjust tests.
>>
>>
>>> NIST Special Publication 800-131A (1) prohibits the use of RSA keys
>>> with len(n) < 2048 for all uses but legacy digital signature
>>> verification, and an increasing number of sites (including ours) must
>>> comply with NIST SP 800-131A.  Having the MinRSABits option would make
>>> our lives easier with respect to compliance.
>>>
>>> (1) https://doi.org/10.6028/NIST.SP.800-131Ar2
>>
>>
>> Yes, and this was a part of my (unwritten) rationale :)
>>
>> --
>> Dmitry Belyavskiy
>>
>
>
> --
> Dmitry Belyavskiy
>


-- 
Dmitry Belyavskiy