Function private_key 

pub fn private_key(
    rd: &mut dyn BufRead,
) -> Result<Option<PrivateKeyDer<'static>>, Error>

Available on (crate features rustls or boring or acme) and crate feature rustls and crate feature std only.

Return the first private key found in rd.

Yields the first PEM section describing a private key (of any type), or an error if a problem occurs while trying to read PEM sections.