The DER (binary) encoded representation of the certificate's private key, in either PKCS#1 format or unencrypted PKCS#8 format.
This property (or the private_key_pem property) can be set when constructing a key (eg, from a file), but cannot be read.
PKCS#8 format is supported since 2.32; earlier releases only support PKCS#1. You can use the `openssl rsa` tool to convert PKCS#8 keys to PKCS#1.