private static String getStringItem(JsonObject obj, String name) throws GeneralSecurityException {
    if (!obj.has(name)) {
      throw new GeneralSecurityException(name + " not found");
    }
    if (!obj.get(name).isJsonPrimitive() || !obj.get(name).getAsJsonPrimitive().isString()) {
      throw new GeneralSecurityException(name + " is not a string");
    }
    return obj.get(name).getAsString();
  }
Converts a Json Web Key (JWK) set with public keys into a Tink KeysetHandle.

<p>It requires that all keys in the set have the "alg" field set. The currently supported
algorithms are ES256, ES384, ES512, RS256, RS384, RS512, PS256, PS384 and PS512. JWK is defined
in https://www.rfc-editor.org/rfc/rfc7517.txt.
java_src/src/main/java/com/google/crypto/tink/jwt/JwkSetConverter.java