Fix to GenerateChallenge
Fix to GenerateChallenge

crypto/x509 now enforces that rsa public keys must have NULL parameters.
The old fix no longer solves the issue and instead will silently fail when
parsing the key, ultimately causing a null pointer dereference at
(pubkey := cert.PublicKey.(*rsa.PublicKey)).
Currently working with crypto/x509 to add support for RSAES-OAEP keys
golang/go#30416

certificate-transparency-go have accepted a temporary fix to /x509
google/certificate-transparency-go#470
so we will be building against them for the time being.

coreos/go-tspi,coreos/go-tspi,google/go-tspi,google/go-tspi