diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -7,7 +7,7 @@ from datetime import datetime
 import config as conf
 import stats
 from util import Progress
-from hashtree import HashTree
+from hashtree import HashTree,hashBlock
 from networkers import NetworkReader,NetworkWriter
 
 
@@ -36,13 +36,19 @@ class Connection:
 
 
 class Client:
-	def __init__(self,filename):
+	def __init__(self,filename,treeFile=""):
 		self._incoming=None
 		self._outcoming=None
 		self._filename=filename
+		self._treeFile=treeFile
 
 		print(datetime.now(), "initializing...")
-		self._localTree=HashTree.fromFile(self._filename)
+		if treeFile:
+			self._tree=HashTree.load(treeFile)
+		else:
+			self._tree=HashTree.fromFile(filename)
+
+		self._newLeaves=dict()
 
 	## Asks server for node hashes to determine which are to be transferred.
 	#
@@ -50,7 +56,7 @@ class Client:
 	#
 	# Requests nodes in order of a batch DFS. Needs stack of size O(treeDepth*batchSize). Nodes in each tree level are accessed in order.
 	def negotiate(self):
-		localTree=self._localTree
+		localTree=self._tree
 		blocksToTransfer=[]
 		nodeStack=collections.deque([0]) # root
 
@@ -135,6 +141,9 @@ class Client:
 				dataFile.seek(i2*HashTree.BLOCK_SIZE)
 			dataFile.write(binData)
 
+			if self._treeFile:
+				self._newLeaves[i2+self._tree.leafStart]=hashBlock(binData)
+
 			log.info("block #{0}: {1}...{2}".format(i2,binData[:5],binData[-5:]))
 
 			stats.logTransferredBlock()
@@ -147,5 +156,11 @@ class Client:
 		log.info("closing session...")
 		dataFile.close()
 
+		if self._treeFile:
+			log.info("updating hash tree...")
+			for (k,v) in self._newLeaves.items():
+				self._tree.updateLeaf(k, v)
+			self._tree.save(self._treeFile)
+
 	def setConnection(self,connection):
 		(self._incoming,self._outcoming)=connection