We keep getting asked about
https://rustsec.org/advisories/RUSTSEC-2023-0071.html and how it impacts
kumomta.
The answer to that question is: in the default build configuration, we
use openssl's RSA signing implementation rather than that of the rsa
crate. The reason for this is that OpenSSL's RSA implementation is due
to the performance gap between the two implementations
(https://github.com/RustCrypto/RSA/issues/339). The result of this is
that the problematic code and attack vector described in the security
advisory does not apply to KumoMTA, because it is not used to compute
any signatures.
In the interest of not raising any false alarms as more and more people
perform security analyses on kumomta, this commit removes the `rsa`
crate from the build graph. In order to do so, we need to port
verification over to the openssl RSA implementation which is what this
commit does.
I look forward to a future version of the `rsa` crate being published
that has this issue resolved, and that closes the performance gap!
refs: https://github.com/RustCrypto/RSA/issues/390
This is not on by default, and is not currently enabled
for KumoMTA.
Using openssl is currently about 3x faster than the pure rust
RSA crate, and inline with the perf of the ring crate, which
we could use here instead, but which doesn't support keys of
size 1024 which are currently widely deployed by the industry,
and for which larger sizes do not fit in DNS records.
Wez Furlong