[RFC] self-signed certificates for LuCI

[Hauke Mehrtens]

On 8/30/20 9:57 AM, Paul Spooren wrote:
> Hi team,
> 
> I recently rewrote px5g[1] to use WolfSSL instead of MbedTLS, as the
> former will be included in OpenWrt 20.x per default.
> 
> Both implementations support the generation of RSA and ECC keys, where
> uhttpd currently defaults to RSA with 2048 keys.
> 
> The question came up if we really want RSA certificates for LuCI or if
> the faster and "more modern" ECC P-256 wouldn't be a better choice.
> 
> If px5g is added to the next release, certificates are generated on
> first boot and most users are unlikely to manually recreate RSA ones, not?
> 
> So the question, shouldn't we drop all crypto options from the new px5g
> implementation and _only_ offer P-256? Whoever wants something else than
> the default may use px5g-mbedtls or some OpenSSL based tool?

I would suggest to use ECC P-256 for new certificates by default. If the
user selects RSA we should create an RSA certificate. If there is
already a certificate like in an OpenWrt sysupgrade we should continue
using that.

The best option would be to generate 2 certificates and let the SSL
implementation select if it should use RSA or ECC for each connection
based on the browser capabilities.

All modern browsers support ECC certificates on the P-256 curve and
using them is faster on low end CPUs like most of our MIPS SoCs. Opening
a TLS connection took me about 1 second with RSA 2048 and mbedtls on a
MIPS 24k CPU, I think ECC on the P-256 curve was about 0.3 seconds. I
measured these numbers about 2 years ago and could have forgotten some
of them. OpenSSL was significant faster than mbedtls on these MIPS CPUs,
I did not check wolfssl. Chrome opens 5 connections in parallel, but we
use the TLS cache of mbedtls now.

If some people use this webserver not to access it with a more or less
modern browser but with some other system, these users could could run
into problems with ECC.

Hauke

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