get the digest of the public key per rfc5280 https://tools.ietf.org/html/rfc5280#section-4.2.1.2

keyid.go

package main

import (
	"crypto/rsa"
	"crypto/sha1"
	"crypto/tls"
	"encoding/asn1"
	"fmt"
	"log"
	"os"
)

// Usage: go run keyid.go ./my.crt ./my.key
// > f1f6a4a0cd859c01a5458d8bc39190a6435793bd
func main() {
	cert, err := tls.LoadX509KeyPair(os.Args[1], os.Args[2])
	if err != nil {
		log.Fatal(err)
	}
	key, ok := cert.PrivateKey.(*rsa.PrivateKey)
	if !ok {
		log.Fatal("does not look like an RSA key")
	}
	mdata, err := asn1.Marshal(key.PublicKey)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%x\n", sha1.Sum(mdata))
}