How to read Android platform key signature programmatically

Question

I need to read the signature of the platform key, which signed the whole build on the device. Currently, I can read the signature of an arbitrary package like com.android.systemui etc. However, due to some custom ROMs, no package is guaranteed to be installed as a part of the system.

So, my question: is there a way to read the signing signature of the system on Android devices programmatically?

---

Note: I am not trying to read the key itself. Signature, or its hash is sufficient for me.