convert jwk to pem (EC only)

convert.go

package main

import(
  "fmt"
  "gopkg.in/square/go-jose.v2"
  "encoding/json"
  "io/ioutil"
  "crypto/ecdsa"
  "crypto/x509"
  "encoding/pem"
  "bytes"
)

func writePem(der []byte, fname string, name string) {
  block := &pem.Block{
		Type:  name,
		Bytes: der,
  }
  var out bytes.Buffer
  pem.Encode(&out, block)

  ioutil.WriteFile(fname, out.Bytes(), 0666)
}

func main() {
  fmt.Println("start")

  body, err := ioutil.ReadFile("key.json")
  if err != nil {
    panic(err)
    return
  }

  var key jose.JSONWebKey
  if err := json.Unmarshal(body, &key); err != nil {
    panic(err)
    return
  }

  privateKey := key.Key.(*ecdsa.PrivateKey)
  publicKey := privateKey.PublicKey

  derPublic, err := x509.MarshalPKIXPublicKey(&publicKey)
  if err != nil {
    panic(err)
    return
  }

  writePem(derPublic, "t_public.pem", "PUBLIC")



  derPrivate, err := x509.MarshalECPrivateKey(privateKey)
  if err != nil {
    panic(err)
    return
  }

  writePem(derPrivate, "t_private.pem", "PRIVATE")
}

key.json

 {
   "kty":"EC",
   "crv":"P-256",
   "x":"f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU",
   "y":"x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0",
   "d":"jpsQnnGQmL-YBIffH1136cspYG6-0iY7X1fCE9-E9LI"
}