https://tools.ietf.org/html/rfc5280#section-4.2.1.2

Extract public key from certificate in to tempfile

openssl x509 -noout -in [cert-file] -pubkey > tmp.pub.der

Locate ”BIT STRING”

openssl asn1parse -in tmp.pub.der

Extract ”clean” public key

openssl asn1parse -in tmp.pub.der -strparse 19 -out pub.der

Calculate hash of the public key

openssl dgst -c -sha1 pub.der

Tribut to xuf for http://certificateerror.blogspot.com/2011/02/how-to-validate-subject-key-identifier.html