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post.scala

case class Keys(keys: Seq[KeyData])
case class KeyData(kid: String, n: String, e: String)

implicit val keyDataFormat = jsonFormat3(KeyData)
implicit val keysFormat = jsonFormat1(Keys)

lazy val publicKeys: Future[Map[String, PublicKey]] =
  Http().singleRequest(HttpRequest(uri = keycloakDeployment.getJwksUrl)).flatMap(response => {
    Unmarshal(response).to[Keys].map(_.keys.map(k => (k.kid, generateKey(k))).toMap)
  })

private def generateKey(keyData: KeyData): PublicKey = {
  val keyFactory = KeyFactory.getInstance(AlgorithmType.RSA.toString)
  val urlDecoder = Base64.getUrlDecoder
  val modulus = new BigInteger(1, urlDecoder.decode(keyData.n))
  val publicExponent = new BigInteger(1, urlDecoder.decode(keyData.e))
  keyFactory.generatePublic(new RSAPublicKeySpec(modulus, publicExponent))
}