less than {days} days will be treat as exired.

set regen=true to force gen new cert.

write signing Certkey with `CN: {prefix}-ca@{time.Now().Unix()}` to certFile if keyFile valid, it will be use, or generate.

if client/server certificate generate by this package, return[0] is client/server certificate, return[1] is CA certificate.

validate if cert is valid to this ca.

less than {days} days will be treat as exired.

verifies that the signature on cert is a valid signature from issuer.

check if certificate and key are one pair.