diff --git a/hashtree.py b/hashtree.py
new file mode 100644
--- /dev/null
+++ b/hashtree.py
@@ -0,0 +1,62 @@
+import hashlib
+import os
+import collections
+
+
+class HashTree:
+  HASH_LEN=16 # bytes
+  BLOCK_SIZE=4096 # bytes
+  
+  ## Prepares a tree containing leafCount leaves.
+  def __init__(self,leafCount):
+    self.store=[None]*(leafCount*2-1)
+    self.leafStart=leafCount-1
+    self.index=self.leafStart
+    
+  @classmethod
+  def fromFile(cls,fd):
+    stat=os.fstat(fd.fileno())
+    size=stat.st_size # !! symlinks
+    leafCount=(size-1)//HashTree.BLOCK_SIZE+1 # number of leaf blocks
+    res=cls(leafCount)
+    
+    for i in range(leafCount):
+      data=fd.read(HashTree.BLOCK_SIZE)
+      res.insertLeaf(hashlib.sha256(data).digest()[HashTree.HASH_LEN:])
+    res.buildTree()
+    
+    return res
+    
+    
+  ## Inserts a leaf at the first empty position.
+  #  
+  #  Useful and used only during the tree construction.
+  def insertLeaf(self,h):
+    self.store[self.index]=h
+    self.index+=1
+    
+  ## Updates a hash stored in the leaf.
+  def updateLeaf(self,index,h):
+    if index<self.leafStart: raise IndexError()
+    
+    self.store[index]=h
+    self.updateNode((index-1)//2)
+  
+  ## Updates the node at index and all its ancestors.
+  def updateNode(self,index):
+    while index>=0:
+      self.store[index]=hashlib.sha256(self.store[index*2+1]+self.store[index*2+2]).digest()[HashTree.HASH_LEN:]
+      index=(index-1)//2
+      
+  ## Fast construction of the tree over the leaves. O(n).
+  def buildTree(self):
+    for i in range(self.leafStart-1,-1,-1):
+      self.store[i]=hashlib.sha256(self.store[i*2+1]+self.store[i*2+2]).digest()[HashTree.HASH_LEN:]
+
+
+if __name__=="__main__":
+  f1=HashTree.fromFile(open("serverFile.txt",mode='rb'))
+  f2=HashTree.fromFile(open("clientFile.txt",mode='rb'))
+
+  for i,(h1,h2) in enumerate(zip(f1.store,f2.store)):
+    print("{0:2}".format(i),h1.hex(),h2.hex(),h1==h2)