Is there a particular reason why you want to store these values instead of computing them on-the-fly?

The trend in RSA private key format design has been to store fewer precomputed values rather than more. Modern key formats store only d and can compute everything else from that and the public key.

Also: are you actually using multiprime RSA? (re: crt_values)

I need to store a custom format that I have no control over, which requires these values to be stored

Do you actually need to support multiprime RSA?

It would probably be worth keeping that out of the public API, especially as it currently has an odd definition owing to PKCS#1 also needing the CRT coefficient for two primes, so the current vector stores "additional CRT coefficients" beyond that one IIUC.

I don't think we should make the fields public. If you only need to read pre-computed values, then a bunch of getters should do the job. If you need to restore the values from serialized data, then we would need to think about validation.

Also: are you actually using multiprime RSA? (re: crt_values)

No, just dp, dq and qinv

I can modify this PR to have getters for those 3 values in a few days

In that case it sounds like you should just add read-only getter methods for those three