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Date: Sun, 27 Nov 2016 15:22:18 +0100
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To: Jakub Zelenka <bukka@php.net>
Cc: PHP Internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=001a1141cf7a2ec96a0542491575
Subject: Re: [PHP-DEV] [RFC] Distrust SHA-1 Certificates
From: me@kelunik.com (Niklas Keller)

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2016-11-27 14:09 GMT+01:00 Jakub Zelenka <bukka@php.net>:

>
>
> On Sun, Nov 27, 2016 at 1:06 PM, Jakub Zelenka <bukka@php.net> wrote:
>
>>
>>
>> On Sat, Nov 26, 2016 at 3:49 PM, Niklas Keller <me@kelunik.com> wrote:
>>
>>> Morning Internals,
>>>
>>> I plan to distrust SHA-1 certificates by default in PHP 7.2. All major
>>> browsers will no longer trust SHA-1 certificates starting already
>>> 2017-01-01.
>>>
>>> Unfortunately, PHP doesn't even provide a way yet to limit the accepted
>>> algorithms for certificates. The RFC fixes that and introduces new
>>> defaults
>>> for PHP 7.2. The "signature_algorithms" context option will also be
>>> backported to PHP 5.6, which is only supported until the end of 2016 with
>>> regular releases, but after that there will be two more years of
>>> security-only updates. Therefore I'd like to get this done before the end
>>> of 2016.
>>>
>>> Currently the RFC aims for BC and doesn't restrict the algorithms on
>>> older
>>> versions. As all major browsers start distrusting those certificates on
>>> 2017-01-01 I'm not sure whether that's the correct choice. I'd like to go
>>> secure-by-default there and disable SHA-1 also on older versions. People
>>> which really need longer can always opt-out and add the needed algorithms
>>> again. Unfortunately, we didn't announce any plans regarding SHA-1 yet,
>>> so
>>> this might be a bit last-minute.
>>>
>>> You can read the full RFC in the wiki:
>>> https://wiki.php.net/rfc/distrust-sha1-certificates
>>>
>>>
>> I think you should change the format to match the one supported by
>> OpenSSL [1] which is also simpler.
>>
>
It is exactly in OpenSSL's format for SSL_CTX_set1_sigalgs_list.


> In general I'm not a big fan of such defaults especially when new values
>> can be added later (e.g. EdDSA that is specified in TLS 1.3) so we have to
>> keep it up to date which was kind of issue in the past.
>>
>
That's true, but I don't see a way in OpenSSL do have a blacklist instead,
but we could do that in our own verify callback.


> However I see the point that we should make it easier for users to have it
>> secure by default so it's probably a good choice. It's not actually just
>> about SHA
>>
>
> Ah sent before I finished it. :) I wanted to say that it's not just about
> SHA-1 but also MD5 that I think we might still support but would have to
> double check that...
>

I wanted to double-check that, too, but didn't do that yet. If MD5 is still
supported, we should definitely put it into PHP 5.6 as a security update.

As far as I can see, MD5 support is the case if it has not been disabled by
the OpenSSL version in use, as all supported algorithms are allowed by
default.

SSL_CTX_set1_sigalgs is anyway only supported starting in OpenSSL 1.0.2, so
we need a custom verify callback for older OpenSSL versions. In our own
verify callback we can use a blacklist instead of the suggested whitelist
by default.

Regards, Niklas

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