[1] RFC 5280 - Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile () <https://tools.ietf.org/html/rfc5280#section-4.2.1.2>
[2] Certificate Error: How to validate the Subject Key Identifier (SKI) from a X509 certificate () <http://certificateerror.blogspot.com/2011/02/how-to-validate-subject-key-identifier.html>
[3]
basic constraints の cA が真なら、必須です。