diff --git a/removeads.py b/removeads.py
index 209a76d..6e9ae4f 100755
--- a/removeads.py
+++ b/removeads.py
@@ -294,6 +294,34 @@ def parseMKVTree(mkvinfo, inputFile):
 # 0000 0001  xxxx xxxx  xxxx xxxx  xxxx xxxx  xxxx xxxx  xxxx xxxx  xxxx xxxx  xxxx xxxx - value 0 to 2^56-2
 
 
+def getEBMLLength(length):
+    if (0 <= length) and (length <= 2^7-2):
+        size = 1
+    elif length <= 2^14-2:
+        size = 2
+    elif length <= 2^21-2:
+        size = 3 
+    elif length <= 2^28-2:
+        size = 4
+    elif length <= 2^35-2:
+        size = 5
+    elif length <= 2^42-2:
+        size = 6
+    elif length <= 2^49-2:
+        size = 7
+    elif length <= 2^56-2:
+        size = 8
+    elif length < 0:
+        logger.error('Impossible to encode a negative length with EBML.')
+        return None
+    else:
+        logger.error('Impossible to encode a length larger than 2^56-2 with EBML.')
+        return None
+    
+    encodedLength = length + ((128>>(size-1))<<((size-1)*8))
+    res = (encodedLength).to_bytes(size, byteorder='big')
+    return res
+
 def changeEBMLElementSize(inputFile, position, addendum):
     logger = logging.getLogger(__name__)
     
@@ -350,6 +378,7 @@ def changeEBMLElementSize(inputFile, position, addendum):
     
     newSize = sizeOfData+addendum
     if  newSize > maxSize:
+        # TODO: we can reencode the header with a larger length field ...
         logger.error('New size is too big to be encoded in actual size field.')
         exit(-1)
 
@@ -360,7 +389,8 @@ def changeEBMLElementSize(inputFile, position, addendum):
     lseek(infd, position, SEEK_SET)
     write(infd, newSizeBuf)
     
-    
+    # TODO: we will return the increase of length of the modified element (if its new size does not fit the older one)
+    return 0
 
 def changeCodecPrivateData(mkvinfo, inputFile, codecData):
     logger = logging.getLogger(__name__)
@@ -408,24 +438,37 @@ def changeCodecPrivateData(mkvinfo, inputFile, codecData):
         write(infd, tail)
         lseek(infd, position, SEEK_SET)
         write(infd, codecData)
+    elif currentLength == futureLength:
+        # Almost nothing to do except overwriting old private codec data with new ones.
+        lseek(infd, position, SEEK_SET)
+        write(infd, codecData)
+    else:
+        lseek(infd, position+currentDataLength, SEEK_SET)
+        tail = read(infd, currentLength-(position+currentDataLength))
+        lseek(infd, position+len(codecData), SEEK_SET)
+        write(infd, tail)
+        lseek(infd, position, SEEK_SET)
+        write(infd, codecData)
+        # We reduce the length of file.
+        ftruncate(infd, futureLength)
+
+    # We have to modify the tree elements up to the root that contains the codec private data.
+    keys = key.split('.')
+    logger.info(keys)
         
-        keys = key.split('.')
-        logger.info(keys)
-        
+    delta = futureLength-currentLength
+    # if there is no modification of the private codec data, no need to change anything. 
+    if delta != 0:
         for i in range(0, len(keys)-1):
             keys.pop()
             key=".".join(map(str, keys))
             pos, size = elements[key]
             logger.info('Trying to fix element with key: %s at position: %d with actual size: %d.' % (key, pos, size))
-            changeEBMLElementSize(inputFile, pos, futureLength-currentLength)
-        
-    elif currentLength == futureLength:
-        logger.error("Not yet implemented")
-        exit(-1)
-    else:
-        logger.error("Not yet implemented")
-        exit(-1)
-    
+            # Changing an element can increase its size (in very rare case).
+            # In that case, we update the new delta that will be larger (because the element has been resized).
+            # For now, the function always return 0. 
+            delta+=changeEBMLElementSize(inputFile, pos, delta)
+            
 
 def getFormat(ffprobe, inputFile):
     logger = logging.getLogger(__name__)