Prepare L23 flipped note

lectures/flipped/L23.md

@@ -18,43 +18,40 @@ passwords.
 
 ## Creating rainbow tables [25 minutes]
 
-I've created Python file "hash-and-reduce.py" which does almost the
-thing in "[How Rainbow Tables
-Work](https://kestas.kuliukas.com/RainbowTables)" by Kuliukas; it
-operates on 7-digit numbers instead of 6-digit numbers, but otherwise
-it's the same. That is, it takes a 7-digit number (e.g. a phone
-number) as command-line input and outputs the MD5sum and the
-reduction, which is another 7-digit number obtained from the MD5sum.
-A real rainbow table also uses different reduction functions 
-at different positions on chains, but we won't do that here for simplicity.
+I've created Python file "hash-and-reduce.py" which does almost the thing in
+"[How Rainbow Tables Work](https://kestas.kuliukas.com/RainbowTables)" by
+Kuliukas; it operates on 7-digit numbers instead of 6-digit numbers, but
+otherwise it's the same. That is, it takes a 7-digit number (e.g., to give it a
+real world context, a phone number) as command-line input and outputs the MD5sum
+and the reduction, which is another 7-digit number obtained from the MD5sum. A
+real rainbow table also uses different reduction functions at different
+positions on chains, but we won't do that here for simplicity.
 
 * Your task: run "hash-and-reduce.py" on an input of your choice through 10
 cycles and report the initial plaintext and final hash. Imagine how a GPU can do
 this efficiently. Come up and write your plaintext and hash on the board.
 
 ## Using rainbow tables [20 minutes]
 
-OK, now we have information about some precomputed hash chains
-(a simpler version of rainbow tables). Let's use it to reverse the hash 
-for an input. I will do an example and then you can try one.
+OK, now we have information about some precomputed hash chains (a simpler
+version of rainbow tables). Let's use it to reverse the hash for an input. I
+will do an example and then you can try one.
 
-Let's say that you have a hash that you want to lookup the plaintext
-for. We'll assume that it's one of the plaintexts that is in our
-table. How do you get the hash you want to lookup? For the purpose
-of this exercise, take one of your plaintexts and hash it.
+Let's say that you have a hash that you want to lookup the plaintext for. We'll
+assume that it's one of the plaintexts that is in our table. How do you get the
+hash you want to lookup? For the purpose of this exercise, take one of your
+plaintexts and hash it.
 
-OK, so now you have a hash that you want to reverse. But the hash
-function is one-way. So you repeatedly reduce it and hash it
-and (given your choice of hash) you'll eventually encounter one of the
-final hashes.
+OK, so now you have a hash that you want to reverse. But the hash function is
+one-way. So you repeatedly reduce it and hash it and (given your choice of hash)
+you'll eventually encounter one of the final hashes.
 
 * How many times do you need to do this before you can give up?
 
-OK, now you have a final hash that matches. Start with the initial
-plaintext and run through until you hit the hash that you're
-trying to reverse, to check that you didn't have a false positive collision.
-The thing just before that is the plaintext
-you're looking for.
+OK, now you have a final hash that matches. Start with the initial plaintext and
+run through until you hit the hash that you're trying to reverse, to check that
+you didn't have a false positive collision. The thing just before that is the
+plaintext you're looking for.
 
 ## Rant 2: Bitcoin mining [5 minutes]
 
@@ -70,6 +67,6 @@ hardware to mine Bitcoin.
 # After-action report, plam, 27 Feb 2023
 
 I did the rainbow table exercise and realized that it wasn't as clear as it
-could be. After lecture, I rewrote the rainbow table exercise to clarify it.
-I did talk about Bitcoins, although the popularity of NFTs has waned
-now at least, and Ethereum has moved to proof-of-stake.
+could be. After lecture, I rewrote the rainbow table exercise to clarify it. I
+did talk about Bitcoins, although the popularity of NFTs has waned now at least,
+and Ethereum has moved to proof-of-stake.

lectures/live-coding/L23/hash-and-reduce.py

@@ -1,19 +1,30 @@
+"""Rainbow table operates on 7-digit numbers.
+
+$ python hash-and-reduce.py 1234567
+Plaintext:  1234567
+MD5sum:     fcea920f7412b5da7be0cf42b8c93759
+Reduced:    9207412
+
+"""
+
 from hashlib import md5
 from sys import argv
 
+
 def reduce(digest):
-  reduced = ""
-  for c in digest:
-    if c.isdigit():
-      reduced = reduced + c
-      if len(reduced) == 7:
-        return reduced
-
-plaintext=argv[1]
+    """Reduce a hash into a new 7-digit number."""
+    reduced = ""
+    for c in digest:
+        if c.isdigit():
+            reduced = reduced + c
+            if len(reduced) == 7:
+                return reduced
+
+
+plaintext = argv[1]
 md5_digest = md5(plaintext.encode()).hexdigest()
 reduced = reduce(md5_digest)
 
-print ("Plaintext: ", plaintext)
-print ("MD5sum:    ", md5_digest)
-print ("Reduced:   ", reduced)
-
+print("Plaintext: ", plaintext)
+print("MD5sum:    ", md5_digest)
+print("Reduced:   ", reduced)