Add rationale section

pep-0247.txt

@@ -115,6 +115,24 @@ Specification
         '\x90\x01P\x98<\xd2O\xb0\xd6\x96?}(\xe1\x7fr'    
 
 
+Rationale
+
+    The digest size is measured in bytes, not bits, even though hash
+    algorithm sizes are usually quoted in bits; MD5 is a 128-bit
+    algorithm and not a 16-byte one, for example.  This is because, in
+    the sample code I looked at, the length in bytes is often needed
+    (to seek ahead or behind in a file; to compute the length of an
+    output string) while the length in bits is rarely used.
+    Therefore, the burden will fall on the few people actually needing
+    the size in bits, who will have to multiply digest_size by 8.
+
+    It's been suggested that the update() method would be better named
+    append().  However, that method is really causing the current
+    state of the hashing object to be updated, and update() is already
+    used by the md5 and sha modules included with Python, so it seems
+    simplest to leave the name update() alone.
+
+
 Changes
 
     2001-09-17: Renamed clear() to reset(); added digest_size attribute