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bug#31946: 27.0.50; The NSM should warn about more TLS problems


From: Jimmy Yuen Ho Wong
Subject: bug#31946: 27.0.50; The NSM should warn about more TLS problems
Date: Tue, 26 Jun 2018 07:26:20 +0100

Sorry I was confused in my last reply about modern browsers not allowing you to accept certs. Chrome just hides that functionality really well, so forget about my proposal earlier. (one should not reply to emails at 5 in the morning) Here's my new proposal:

1. Forget about defining what tests belongs in what levels, there should just be one level which is the default sets of tests, let's call this coarse grain setting.
2. Fine grain settings should only allow you to **add** to the default list of checks, so it will be a defcustom of an alist (there's prior art of this), let's call this `nsm-additional-checks`
3. We can predefine a bunch of check functions that users can add to `nsm-additional-checks` without having to write their own.
4. For dh-small-subgroup and dh-composite, the only way to check this in LISP seems to be to supply `:min-prime-bits 2048` to `gnutls-boot-parameters`. In which case GnuTLS will fail with fatal alert for both counts. A user will not be able to accept dh-small-subgroup and dh-composite certs if checks for them are enabled. This is fine, as a user is not able to accept RC4 certs via NSM now, browsers also do it this way.

On Tue, Jun 26, 2018 at 5:11 AM, Jimmy Yuen Ho Wong <address@hidden> wrote:
`dh-composite` can be mitigated by using the "NORMAL:%PROFILE_MEDIUM" priority string[1], "NORMAL:%PROFILE_HIGH" [2] will pass all 26 badssl test while still allowing connection to ELPA/MELPA without even supplying CRL files (GnuTLS already does OCSP stapling verifcation transparently, and Emacs is using it already minus surfacing `GNUTLS_CERT_MISSING_OCSP_STATUS` when it fails). The exact meaning these levels appears to be spread out among different tables in ENISA's Algorithms, Key Sizes and Parameters Report - 2013 [3].

As a possible way to avoid confusion, I would suggest we consolidate the 2 different meaning of profiles (NSM and GnuTLS) into GnuTLS's. Instead of having users to edit an alist like Lars has done in commit 6584bc67, we could:

1. Append `network-security-level` to `gnutls-algorithm-priority`, i.e. `network-security-level` will be a list of predefined symbols that will be mapped to GnuTLS's `%PROFILE_*` strings, and append to it when setting up `gnutls-boot-parameters`.
2. Forget about letting users decide whether they want to accept problematic certs or not, no modern browsers does it anymore. Doing network security checks in 2 different places also introduces impedance mismatch. Specifically, GnuTLS by default disables a number of cyphers and hashes. The only way to stop it from generating fatal alerts is to enable everything GnuTLS has implemented and reinvent all the wheels in LISP (do you really want to reenable SSL3?). This is insane from both a security and performance perspective, as we don't have reliable NETSEC resources to respond to any security issues that we may introduce during the process. Even if we do, there's a larger problem of Emacs's release process.
3. To solve the problem of letting users fine tune the client's acceptable cyphersuite, MACs and whatnot for emergencies out of Emacs' release cycles, let's introduce a bunch of new defcustoms such as `gnutls-cyphersuite`, `gnutls-key-exchange` etc, see [1] for the table.
4. Normally, the fine tuning defcustoms in 3) will be nil, in which case `gnutls-algorithm-priority` takes precedence, otherwise they are combined into a final priority string supplied to `gnutls-boot-parameters`.
5. Merge nsm into the gnutls group. No more distinction between interactive and non-interactive sessions due to 2).

References:

On Tue, Jun 26, 2018 at 2:23 AM, Noam Postavsky <address@hidden> wrote:
Lars Ingebrigtsen <address@hidden> writes:

> There are also more protocol stuff we should warn about on various
> levels.  These should be on `high':

>>            "https://dh-small-subgroup.badssl.com/"        ;; fail
>>            "https://dh-composite.badssl.com/"             ;; fail

So these ones seem kind of problematic, as alluded to on emacs-devel.
It doesn't look like gnutls has an API to get or check the value of the
DH primes (calc-prime-test bails out when given a 1024 bit prime, so we
definitely need library support for this).

https://lists.gnu.org/archive/html/emacs-devel/2018-06/msg00805.html
https://lists.gnu.org/archive/html/emacs-devel/2018-06/msg00807.html





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