fix(jwk) return error if exporting private key from public key (#128)

lib/resty/openssl/auxiliary/jwk.lua

@@ -202,6 +202,10 @@ end
 
 function _M.dump_jwk(pkey, is_priv)
   local jwk
+  if is_priv and not pkey:is_private() then
+      return nil, "jwk.dump_jwk: could not dump public key as private key"
+  end
+
   if pkey.key_type == evp_macro.EVP_PKEY_RSA then
     local param_keys = { "n" , "e" }
     if is_priv then

t/openssl/aux/jwk.t

@@ -225,4 +225,39 @@ true
 'pkey.new:load_key: failed to construct OKP key from JWK: at least "x" or "d" parameter is required
 '
 --- no_error_log
-[error]
+[error]
+
+=== TEST 4: Errors if tries to export privkey using pubkey
+--- http_config eval: $::HttpConfig
+--- config
+    location =/t {
+        content_by_lua_block {
+            local privkey, err = require("resty.openssl.pkey").new({ type = 'EC', curve = 'prime256v1'})
+            if err then
+                ngx.log(ngx.ERR, err)
+                return
+            end
+
+            local pem, err = privkey:tostring("public")
+            if err then
+                ngx.log(ngx.ERR, err)
+                return
+            end
+
+            local pubkey, err = require("resty.openssl.pkey").new(pem)
+            if err then
+                ngx.log(ngx.ERR, err)
+                return
+            end
+
+            local _, err = pubkey:tostring("private", "JWK")
+            ngx.say(err)
+        }
+    }
+--- request
+    GET /t
+--- response_body eval
+'jwk.dump_jwk: could not dump public key as private key
+'
+--- no_error_log
+[error]