hsign

--theinsektAPIs/hsign
--experiment, not tested much
--will use a hash algorithm + shared salt to sign a message
--will verify the message by checking that not to much time has passed
--(to prevent attackers from sending a copy of the message)
--will verify the message doing hash + salt to see that it's the same hash value

os.loadAPI("theinsektAPIs/sha256")


function send(receiverID,message,protocol, secret)
 --prepare the box that holds the data, and a time stamp
 local box={time0=os.time(),day0=os.day(), data=message,}
 --turn the box into a string
 local serializedBox=textutils.serialize(box)
 --put it in a table, and put hash of it+secret
 local message={
   ["box"]=serializedBox,
   ["hash"]=hash(serializedBox..secret),
 }
 --send the prepared message table, return true or false
 return rednet.send(receiverID,message,protocol)
end




--if timed out returns false,nil, you should nil check the first value
--returns ok,senderID, message, protocol,hash
--ok is true if the message was correctly signed
--the user of this function could blacklist old hash values
function receive(protocol, timeout,secret, timeLimit)
 --receive from rednet
 local senderID, message, protocol = rednet.receive(protocol,timeout)

 --get the time that the message was received
 local time2=os.time()
 local day2=os.day()

 --check that rednet receive didn't have a timeout
 if senderID==nil then
  return false, nil
 end

 --type check message
 if type(message)~="table" or type(message["box"])~="string" or type(message["hash"])~="string" then
  return false, senderID, "", protocol,nil
 end

 --get and unserialize box
 local serializedBox=message["box"]
 local box=textutils.unserialize(serializedBox)

 --type check box
 if type(box)~="table" or type(box["time0"])~="number" or type(box["day0"])~="number" then
  return false, senderID, "", protocol,nil
 end

  --check that time stamps aren't to old
  if timeLimit==nil then timeLimit=10 end
  if math.abs(timeDiff(time2,day2,box["time0"],box["day0"]))>timeLimit then
   return false, senderID, "", protocol,nil
  end

  --check that the hash is correct
  local hashValue=hash(serializedBox..secret)
  if hashValue~=message["hash"] then
   return false, senderID, "", protocol,nil
  end

  --passed all tests
  return true,senderID, box["data"], protocol,hashValue
end




--returns the time difference in seconds
function timeDiff(time1,day1,time2,day2)
  --converts to minecraft ingame hours
  local minecraftDiff=(day1-day2)*24+(time1-time2)
  --converts to real seconds
  local secondsDiff=minecraftDiff*((20/24)*60)
  return secondsDiff
end

function hash(msg)
 return sha256.sha256(msg)
end