1 files changed, 43 insertions, 0 deletions
diff --git a/PrevActivity.hs b/PrevActivity.hs
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+++ b/PrevActivity.hs
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+module PrevActivity where
+
+import Types
+import Crypto
+
+import Control.Concurrent.STM
+
+-- | Remove the prevActivity from a message. Doing this before sending
+-- it over the wire saves transmitting that data, without weakening
+-- security at all.
+removePrevActivityHash :: AnyMessage -> AnyMessage
+removePrevActivityHash msg = case msg of
+	User (ActivityMessage a) -> User (go a)
+	Developer (ActivityMessage a) -> Developer (go a)
+	_ -> msg
+  where
+	go a = ActivityMessage $ a { prevActivity = Nothing }
+
+type RecentActivity = STM (SigVerifier, [Hash])
+
+noRecentActivity :: RecentActivity
+noRecentActivity = return (mempty, [])
+
+-- | Restore the prevActivity to a message received without one.
+-- This needs a RecentActivity cache, and it tries hashes from that cache
+-- as the prevActivity until it finds one that makes the message's
+-- signature verify.
+restorePrevActivityHash :: RecentActivity -> AnyMessage -> STM AnyMessage
+restorePrevActivityHash ra msg = case msg of
+	User (ActivityMessage act) -> 
+		User . ActivityMessage <$> (go act =<< ra)
+	Developer (ActivityMessage act) ->
+		Developer . ActivityMessage <$> (go act =<< ra)
+	User (ControlMessage {}) -> return msg
+	Developer (ControlMessage {}) -> return msg
+
+  where
+	go act (_, []) = return act
+	go act (sigverifier, (h:hs)) = do
+		let act' = act { prevActivity = Just h }
+		if verifySigned sigverifier act'
+			then return act'
+			else go act (sigverifier, hs)