Enforcing system crypto policies

[Frank Ueberschar]

Here 
https://docs.fedoraproject.org/en-US/packaging-guidelines/CryptoPolicies/ 
is a proposal to use a specific cipher list string for 
SSL_CTX_set_cipher_list(): "PROFILE=SYSTEM".

Especially this citation: "if that call is present and provided a fixed 
string which does not contain PSK or SRP, replace the string with 
"PROFILE=SYSTEM", or remove the call"

We have to rely on PSK. What ist the reason behind the above advice?

Thanks, Frank

[David Kaufmann]

Hi!

On Thu, Apr 04, 2019 at 11:06:07PM +0200, Frank Ueberschar wrote:
> Here
> https://docs.fedoraproject.org/en-US/packaging-guidelines/CryptoPolicies/ is
> a proposal to use a specific cipher list string for
> SSL_CTX_set_cipher_list(): "PROFILE=SYSTEM".
>
> Especially this citation: "if that call is present and provided a fixed
> string which does not contain PSK or SRP, replace the string with
> "PROFILE=SYSTEM", or remove the call"
>
> We have to rely on PSK. What ist the reason behind the above advice?

The crypto policy does not cover those two options currently.
The part covering GnuTLS applications seems to be more informative:

"If it contains PSK or SRP do nothing (these applications are not
currently covered by the default policy)"

So I'd say you're good about using PSK.

My guesses about why this is set like it is:

The reasoning seems to be something like "Don't force applications to
use a specific set of Cipher/MAC/Signature Algorithm/Key Exchange
methods, if there are multiple interchangeable options."

By setting this to PROFILE=SYSTEM as default you allow the system
administrator to disable specific ciphers for all applications. (e.g.
disable 3DES-CBC system-wide)

If you've got PSK or SRP in the string I'd say it is quite likely that
it's not possible to switch to something else without rewriting parts of
the application, so there is nothing left to disable system-wide anyway.

All the best,
David

[Hubert Kario]

On Thursday, 4 April 2019 23:06:07 CEST Frank Ueberschar wrote:
> Here 
> https://docs.fedoraproject.org/en-US/packaging-guidelines/CryptoPolicies/ 
> is a proposal to use a specific cipher list string for 
> SSL_CTX_set_cipher_list(): "PROFILE=SYSTEM".
>
> Especially this citation: "if that call is present and provided a fixed 
> string which does not contain PSK or SRP, replace the string with 
> "PROFILE=SYSTEM", or remove the call"
>
> We have to rely on PSK. What ist the reason behind the above advice?
>
> Thanks, Frank

more or less what David said. PSK and SRP are very specific use cases, ones 
that don't work in open Internet and require close cooperation and 
communication between server administrator and user. Crypto Policies target 
common use cases with typical configurations (i.e. X.509 certificate 
authentication).