[389-users] Re: minssf and TLS cipher ordering

Hi Marc,

I was under the impression that it would pick the highest supported, but that doesn't seem to be what is happening based on my previous example.

Instead, it seems to just be picking the first compatible, regardless of strength.

Trevor

On Fri, Apr 23, 2021, 10:03 PM Marc Sauton <msauton@redhat.com> wrote:

about ciphers order and TLS cipher suite discovery, NSS will pick the one with highest strength from the available ciphers, and compatible with the TLS client ( handshake)

you can check the configuration with for example (replace the string m1 with an instance name):

dsconf m1 security get

dsconf m1 security ciphers list

dsconf m1 security ciphers list --supported | less

dsconf m1 security ciphers list --enabled

ldapsearch -o ldif-wrap=no -LLLxD "cn=Directory Manager" -W  -b cn=encryption,cn=config | less

and to set ciphers (can be "delicate"):

/usr/lib64/nss/unsupported-tools/listsuites | grep -B1 --no-group-separator "Enabled" | less

dsconf m1 security ciphers set xxxxx

doc ref:

https://access.redhat.com/documentation/en-us/red_hat_directory_server/11/html/administration_guide/enabling_tls#setting_encryption_ciphers

and NSS source:

./lib/ssl/ssl3con.c

./lib/ssl/sslenum.c

On Fri, Apr 23, 2021 at 4:57 PM Trevor Vaughan <tvaughan@onyxpoint.com> wrote:

William,

I do apologize! I'll keep that in mind in the future.

Thanks again for your help,

Trevor

On Fri, Apr 23, 2021, 7:50 PM William Brown <wbrown@suse.de> wrote:

Sorry to call this out, but my name is "William" not "Bill". I have personal reasons to dislike being called that name.

Regardless, happy to help out :)

> On 23 Apr 2021, at 22:11, Trevor Vaughan <tvaughan@onyxpoint.com> wrote:
>
> Bill and Pierre,
>
> Thanks for the responses!
>
> It sounds like I have to figure out how to configure the NSS library for 389-DS specifically.
>
> In EL8+ I know that I can configure the global crypto policy but I'm hoping that I can do it for the specific application. I haven't found anything in the documentation so far but at least this gets me pointed in the right direction.
>
> Thanks,
>
> Trevor
>
> On Fri, Apr 23, 2021 at 4:42 AM Pierre Rogier <progier@redhat.com> wrote:
> Hi Trevor,
>
> I do not think it is possible to specify the cypher order negotiation:
>      I am not sure whether TLS protocol allow to specify an order when negotiating the cypher,
>      but at 389 level there is no way to specify an order:
> The NSS security layer provides the list of supported cypher and 389 use
> nsSSL3Ciphers config parameter to enable/disable theses cyphers in the list (without changing the order)
>
>     So my feeling is that if there is an order it is up to the different
>      security layer implementations and may differs between the applications,
>
> Regards,
>    Pierre
>
> On Thu, Apr 22, 2021 at 7:28 PM Trevor Vaughan <tvaughan@onyxpoint.com> wrote:
> Hi William,
>
> In terms of the STARTTLS bits (in theory) properly configuring your client software mitigates the password leak risk. But this also happens with pure (non-RFC) LDAPS connections.
>
> The docs note that minssf applies to the crypto required bits as well as the SASL layer.
>
> Ignoring most of that, my issue is that I don't understand why I have to nail my client software to ciphers explicitly known by 389-DS instead of the two negotiating the strongest things possible out of the gate.
>
> For instance, if I use AES256 with a minssf=256, everything works just fine.
>
> But, if I use AES128:AES256:@STRENGTH (which should sort strongest to weakest) then access is denied.
>
> How do I get 389-DS to negotiate the strongest ciphers first (regardless of the method)?
>
> Thanks,
>
> Trevor
>
> On Wed, Apr 21, 2021 at 7:34 PM William Brown <wbrown@suse.de> wrote:
> Hi there,
>
> > On 22 Apr 2021, at 03:52, Trevor Vaughan <tvaughan@onyxpoint.com> wrote:
> >
> > Hi All,
> >
> > OS Version: CentOS 8
> > 389-DS Version: 1.4.3.22 from EPEL
> >
> > I have a server set up with minssf=256 and have been surprised that either 389-DS, or openssl, does not appear to be doing what I would consider a logical TLS negotiation.
> >
> > I had thought that the system would start with the strongest cipher and then negotiate down to something that was acceptable.
> >
> > Instead, I'm finding that I have to nail up the ciphers to something that the 389-DS server both recognizes and is within the expected SSF.
> >
> > Is this expected behavior or do I have something configured incorrectly?
>
> That's not what minssf does.
>
> minssf says "during a bind operation, reject if the encryption strength used is less than 256 bits or equivalent".
>
> The "bit strength" is arbitrary though, because it's a concept from sasl, and generally is very broken.
>
> Remember, minssf does NOT do what you think though! Because bind is the *first* message on the wire, the series of operations is
>
>
>    client                   server
> open plain text conn  ->   
>                       <-   accept connection
> send bind on conn     ->   
>                       <-   reject due to minsff too weak.
>
>
> So you have already leaked the password!
>
>
> The only way to ensure this does not occur is to set "nsslapd-port: 0" which disables plaintext. Then you *only* use ldaps on port 636, which is guarantee encrypted from the start.
>
> It is worth noting that the use of starttls over ldap, does *NOT* mitigate this issue, for a similar reason.
>
>
> Caveat: If you are using kerberos/gssapi you can NOT disable plaintext ldap due to these protocols attempting to install their own encryption layers.
>
>
> Hope that helps,
>
>
> >
> > Thanks,
> >
> > Trevor
> >
> > --
> > Trevor Vaughan
> > Vice President, Onyx Point, Inc
> > (410) 541-6699 x788
> >
> > -- This account not approved for unencrypted proprietary information --
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> —
> Sincerely,
>
> William Brown
>
> Senior Software Engineer, 389 Directory Server
> SUSE Labs, Australia
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>
> --
> Trevor Vaughan
> Vice President, Onyx Point, Inc
> (410) 541-6699 x788
>
> -- This account not approved for unencrypted proprietary information --
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>
> --
> --
>
> 389 Directory Server Development Team
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>
> --
> Trevor Vaughan
> Vice President, Onyx Point, Inc
> (410) 541-6699 x788
>
> -- This account not approved for unencrypted proprietary information --
> _______________________________________________
> 389-users mailing list -- 389-users@lists.fedoraproject.org
> To unsubscribe send an email to 389-users-leave@lists.fedoraproject.org
> Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/
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—
Sincerely,

William Brown

Senior Software Engineer, 389 Directory Server
SUSE Labs, Australia
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