RSA PrivateKey to PublicKey conversion in Java

The Private-Public Key Infrastructure is a foundation of modern security and cryptography. We use private keys to sign or encrypt messages and public keys to decrypt or verify messages. Your app probably will have to use or store both types. But do you now you can convert a Java security RSA PrivateKey object to a PublicKey object? And it’s quite simple too
Java code how to convert RSA Private Key to a RSA Public Key github.com/ps-after-hours/pri...
How read a Private Key from PEM PKCS#8 file in Java • How read a Private Key...
0:00 Public and Private Key, what is what
1:40 How to convert RSA Private Key to Public Key in Java
2:35 Java code walkthrough
3:26 RSA PrivateKey to PublicKey converter
4:18 Summary

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does anyone know of an arduino library rom/rtc.h ?
I find rtc libraries but I'm stuck on the rom/ part, loading other RTC libraries doesn't satisfy.