> On Jul 10, 2017, at 3:45 AM, Niklas Keller <m...@kelunik.com> wrote:
>
>
> What's the best way / a working way to reject weak signature schemes in
> OpenSSL 1.0.{1,2}?
Most CAs have stopped issuing SHA-1 certificates. Any old ones will expire
over the
next year or two. While Google has demonstrated a SHA-1 collision, that proof
of
concept is far from a practical attack.
The simplest solution is to let the CAs solve the problem as SHA-1 certificates
fade
out of the picture. You can if you wish leave out from the set of trusted
roots any
CAs that have not yet stopped issuing SHA-1 certificates.
You can of course implement a verify callback that inspects each certificate in
the
chain, and triggers an error when its signature is SHA-1 and it is not the last
one
in the chain. This requires keeping some state attached to the X509 store
context,
and I don't think is worth the effort.
See code involving "TLScontext_index" in:
https://github.com/vdukhovni/postfix/blob/master/postfix/src/tls/tls_client.c#L318
https://github.com/vdukhovni/postfix/blob/master/postfix/src/tls/tls_client.c#L942
https://github.com/vdukhovni/postfix/blob/master/postfix/src/tls/tls_verify.c#L163
With such a context, you can keep track of the maximum depth seen by the
callback,
and reject SHA-1 at lower depths. I do not recommend doing this.
--
Viktor.
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