diff --git a/hanafuda-APILanguage.cabal b/hanafuda-APILanguage.cabal
index 9cded08..3530d35 100644
--- a/hanafuda-APILanguage.cabal
+++ b/hanafuda-APILanguage.cabal
@@ -23,6 +23,7 @@ library
   -- other-extensions:
   build-depends:       aeson
                      , base >=4.9 && <4.13
+                     , base64-bytestring
                      , bytestring
                      , containers
                      , hanafuda >= 0.3.3
diff --git a/src/Hanafuda/Message.hs b/src/Hanafuda/Message.hs
index c95114a..08f561c 100644
--- a/src/Hanafuda/Message.hs
+++ b/src/Hanafuda/Message.hs
@@ -22,6 +22,7 @@ import Data.Aeson (
   )
 import Data.Aeson.Types (toJSONKeyText)
 import Data.ByteString (ByteString)
+import qualified Data.ByteString.Base64 as B64 (decode, encode)
 import Data.Map (Map)
 import Data.Monoid ((<>))
 import Data.Text (Text)
@@ -182,10 +183,10 @@ instance ToJSON PublicState where
   toEncoding = genericToEncoding defaultOptions
 
 instance FromJSON ByteString where
-  parseJSON = withText "ByteString" (return . encodeUtf8)
+  parseJSON = withText "ByteString" (either fail return . B64.decode . encodeUtf8)
 instance ToJSON ByteString where
-  toJSON = String . decodeUtf8
-  toEncoding = toEncoding . decodeUtf8
+  toJSON = String . decodeUtf8 . B64.encode
+  toEncoding = toEncoding . decodeUtf8 . B64.encode
 
 data PublicGame = PublicGame {
       nonce :: ByteString