Fixing the error "javax.crypto.BadPaddingException: Decryption error"

I have a encrypted private key which I have to decrypt using Shared key,which I did and got byte array. I have converted the byte[] to utf8 string and then converted that string to Private key. Now, I have to use this private key to decrypt one time encrypted public key. While doing that, I am getting the following error.

javax.crypto.BadPaddingException: Decryption error
at sun.security.rsa.RSAPadding.unpadV15(RSAPadding.java:370) ~[na:1.8.0_311]

I am looking for a developer who has experience in working with AES, RSA encryption and decryption.

Backend using: Java Spring boot