On Wed, Jan 27, 2021 at 12:40:33PM +0800, Kevin Shell via Exim-users wrote:
> Is it possible to make Exim smtp client not perform certificate chain
> checks, instead trusting remote SSL/TLS peer certificate by Subject
> Key Identifier or fingerprint?
>
> I want to trust some self signed certificates.
Separately from the how, which I am not the right person to answer, it
is worth noting that the "Subject Key Identifier" is NOT a trustworthy
proxy for the underlying key. The creator of the certificate is free
to set it to any value of their choice, e.g. to the value for some
other unrelated key.
Therefore, you MUST NEVER conclude posession of the private key based on
a subject key identifier match.
Perhaps you were thinking of a DANE-style SPKI digest match, with the
digest computed by the client directly from the public key (rather that
extracted from an extension in the certificate). Those would be fine.
There's precedent for supporting such fingerprint matching in at least
one other MTA.
I also assume that the check in question would be destination domain or
MX host dependent, so that different EE certs or public key digests,
would be trusted for different sites. So you'd need some sort of
lookup table to match the peer cert or key against the policy for
the given nexthop.
If you operate both ends, one approach would just be to use DANE, which
Exim is known to support, and which obviates the need for all those
per-destination policy tables (for the DNSSEC-signed domains that
implement DANE TLSA records).
--
Viktor.
