Security STARTTLS and DANE

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1

Can someone pls raise my learning curve.
I test my certificate with About the email test and I just read this (translated from Dutch)
Certificate generated thru dehydrated-0.7.2_1

-- quote --
The public key of at least one of your mail server certificates is based on a signing algorithm with parameters that should be phased out because they are weaker, although you can still use them if absolutely necessary. They will likely be insufficient in a future update.

Mail server (MX)|Deprecated signature parameter:
devrijegeest.nl | YR1: RSAPublicKey-2048 | ...to be phased out

Test explanation:

We test whether the public key of your mail server certificate and of every intermediate certificate is based on a secure signing algorithm with secure parameters. Digital signatures are used during certificate verification. The public key of a certificate can be used to verify that a signature was created using the corresponding private key.

Signing algorithm:

The signing algorithm is determined when the certificate is generated. A certificate can contain only one signing algorithm. It is possible to support more than one algorithm by configuring multiple certificates on a single server. Only the signing algorithms ECDSA, RSA, and EdDSA are considered sufficiently secure. In practice, EdDSA is rarely or never used. All algorithms are vulnerable to the quantum threat. For the time being, they offer sufficient protection. Quantum-safe alternatives are not yet part of the TLS standards.

--- end quote

Appreciate any reply and evt solution to this.
Thanks.

2

The only real solution is to ignore the overly-alarmist warning this test tool is generating. RSA2048 is still a decent key length at this time. In due course it will no doubt be deprecated, but that's true of every sort of algorithm and key.

3

I agree with @danb35 the best option is to just ignore that.

The "leaf" cert for that domain uses an RSA 4096 bit signature algorithm.

But, the Let's Encrypt "intermediate" that signed it uses a 2048 bit RSA. It looks like this intermediate is what the testing tool is warning about. You cannot do anything about the intermediate as it is controlled by LE. You could switch to an ECDSA leaf which then uses an ECDSA intermediate and perhaps that test tool would not warn about that. But, as I understand it some mail clients still don't support ECDSA certs.

This is confusing. It says RSA is secure but yet warns that the 2048 bit LE intermediate is not secure.

The industry is working on quantum-safe options but these are not commonly available. LE recently posted this blog about that: A Post-Quantum Future for Let's Encrypt - Let's Encrypt

4

It's warning about the key length. At the moment NCSC (the dutch one) guidance is that 2048-3071 bit RSA keys should be phased out (TLS Security Guidelines | NCSC-NL). At the moment, domain still gets 100% even with the warning.