[Buildroot] [PATCH 1/3] package/mbedtls: add BR2_PACKAGE_MBEDTLS_X509_UNSUPPORTED_CRITICAL_EXTENSION

[Yann E. MORIN]

Nicola, Fabrice, Thomas, All,

On 2020-04-24 01:27 +0200, Nicola Di Lieto spake thusly:
> On Thu, Apr 23, 2020 at 10:09:05PM +0200, Thomas Petazzoni wrote:
> >What is this X509_UNSUPPORTED_CRITICAL_EXTENSION functionality that is
> >so weird that it requires patching the mbedtls config.h file ? Why is
> >uacme absolutely requiring this functionality that no other user of
> >mbedtls requires ?
> 
> There is an explanation at
> https://github.com/ndilieto/uacme/issues/23
> 
> Briefly, tls-alpn-01 validation requires (as per RFC8737 section 6.1) a new
> critical certificate extension. mbedTLS doesn't know about it and refuses to
> parse any certificate with such extension unless that build feature is
> enabled.

So, I think I now wrapped my head around this issue, and I think I got
it. Here's what I understood from the different resources [0] [1]:

  - in X.509, some extensions can be added to certificates
  - an extension can be marked as 'critical' or 'not critical'
  - an X.509 parser that encounters an extension marked 'critical' when
    parsing a certificate, and that does not recognise that extension,
    *must* reject that certificate.

mbedtls does the right thing here: it rejects such certificates.

However, embedtls has an option to treat thoe 'critical' extensions as
if they were 'not critical'.

I think we should refuse to use mbedtls with uacme.

[0] https://en.wikipedia.org/wiki/X.509
[1] https://github.com/ndilieto/uacme/issues/23

Regards,
Yann E. MORIN.

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