Leave the prevMessage out of Activity serialization to save BW.

Do include it in the data that gets signed, so it can be recovered
by trying each likely (recently seen) Activity as the prevMessage, and
checking the signature.

The UserState and DeveloperState already had the necessary state about
recently seen hashes, so this does not impact data use.

One tricky bit is that relayFromSocket needs to wait for the TMChan
to be empty before calling restorePrevActivityHash. Otherwise, the
hashes of items in the channel that have not been processed yet won't be
tried. The TMChan is not really being used as a channel since only 1
item can be in it. It could be converted to a TMVar, but closeTMChan is
used so I left it as a channel.

Note that the server does not restore hashes of messages that pass
through it; it's just a dumb relay.

Sending a single key press now only needs 94 bytes of data to be sent,
down from 169!

---

Also switched to SHA512, since hashes are no longer being sent over
the wire and so the larger size does not matter. SHA512 is slightly
faster and more secure.

This commit was sponsored by Ewen McNeill.

1 files changed, 4 insertions, 4 deletions

diff --git a/Hash.hs b/Hash.hs
index 3bfb5a2..3cc8d94 100644
--- a/Hash.hs
+++ b/Hash.hs
@@ -12,8 +12,8 @@ class Hashable a where
 	hash :: a -> Hash
 
 instance Hashable B.ByteString where
-	-- Encodes the SHA256 using base16 format
-	hash = Hash SHA256 . Val . C8.pack . show . sha256
+	-- Encodes the SHA512 using base16 format
+	hash = Hash SHA512 . Val . C8.pack . show . sha512
 
 instance Hashable Val where
 	hash (Val v) = hash v
@@ -21,8 +21,8 @@ instance Hashable Val where
 instance Hashable Hash where
 	hash = id
 
-sha256 :: B.ByteString -> H.Digest H.SHA256
-sha256 = H.hash
+sha512 :: B.ByteString -> H.Digest H.SHA512
+sha512 = H.hash
 
 -- | A value tagged with a ByteString describing the type of value.
 -- This is hashed by hashing the concacenation of the hash of the