Outline

• One-Time Pads
• Venona
• HW Exercise
• Codebook Cipher
• Zimmerman Telegram
• Election of 1876
• 20th Century Crypto
• Taxonomy

Introduction

• Today, we continue to look at basic crypto systems.
• We begin by considering one-time pads.
• In this system, we assume our plaintext message, `m`, is encoded as a binary string. To do this we could use ASCII or UTF-8.
• To encrypt, we bit-wise XOR this binary string with a random binary, `r`, string of the same length to make the ciphertext `c`.
• To decrypt, we bit-wise XOR the `c` with `r`.
• The setting for our next few examples is WWII.

One-Time Pad: Encryption Example

One-Time Pad: Decryption Example

But what was the Random String? - Part 1

Suppose the ciphertext was intercepted by the Americans and the receiver (thought to be on the American side) was a double agent. A double agent might claim the one-time pad was as follows, and so the receiver should be let go!

But what was the Random String? - Part 2

Suppose the ciphertext was intercepted by the Americans and the sender was captured but not the key. The sender might claim the one-time pad was as follows, and so the sender should be let go!

One-Time Pad Summary

• Provably secure...
  • Ciphertext provides no info about plaintext
  • All plaintexts are equally likely
• ...but, only when thee used correctly
  • Pad must be random, used only once
  • Pad is known only to sender and receiver
• Note the pad (key) is same size as message
• If are able to send the pad by secrets means to the recipient, why can't we just distribute the message to the recipient by that means?

A Real-World One-Time Pad --

• Project VENONA was Soviet Spy program in the 30's, 40's and 50's.
• Spies from Russia would take one-time pads along when the came from the Soviet Union.
• These were then used to send messages back to handlers in Moscow.
• Thousands of messages were encrypted.
• These messages carried information about nuclear programs, etc.
• However, repeats within the "one-time" pads made cryptanalysis possible

VENONA Decrypt (1944)

[C% Ruth] learned that her husband [v] was called up by the army
but he was not sent to the front. He is a mechanical engineer
and is now working at the ENORMOUS [ENORMOZ] [vi] plant in 
SANTA FE, New Mexico. 

[45 groups  unrecoverable]

detain VOLOK [vii] who is working in a plant on ENORMOUS. He is a 
FELLOWCOUNTRYMAN  [ZEMLYaK] [viii]. Yesterday  he learned that 
they had dismissed him from his work. His active work 
in progressive organizations in the past was cause of his dismissal. 

In the FELLOWCOUNTRYMAN line LIBERAL is in touch with CHESTER [ix]. 
They meet once a month for the payment of dues. CHESTER is 
interested in whether we are satisfied with the collaboration and
whether there are not any misunderstandings. He does not inquire
about specific items of work [KONKRETNAYa RABOTA]. In  as much 
as CHESTER knows about the role of  LIBERAL's group we beg consent
to ask C. through LIBERAL about leads from among people who are 
working on ENOURMOUS and in other technical fields.

• "Ruth" == Ruth Greenglass
• "Liberal" == Julius Rosenberg
• "Enormous" == the atomic bomb

HW Exercise 1.11

(a) Give an example of biometric-based authentication.

A computer which allows you to log in by using your thumb print in a thumb print scanner would be an example of biometric authentication.

(b) Give an example of authentication based on something you have.

You might be allowed in to a building based on whether or not you have a RFID badge with the correct code in it.

(c) Give an example from everyday life where two-factor authentication is used. Which of "something you know", "something you have", "something you are" are used?

To get into my safety deposit box, I go through a door in which my hand is scanned. I then need a key to open the box. This two-factor authentication involves "something you have" (a deposit box key) and "something you are" (a unique hand print).

Codebook Cipher

• Literally, a book filled with "codewords"
• The Zimmerman Telegram which we'll talk about next was encrypted via codebook:

  Februar           13605
  fest              13732
  finanzielle       13850
  folgender         13918
  Frieden           17142
  Friedenschluss    17149
  ...
  

• Modern block ciphers can be viewed as codebooks!
• More about this later...

Codebook Cipher: Additive

• Codebooks also (usually) use additive
• Here Additive is a book of "random" numbers
• Encrypt message with codebook
• Then choose position in additive book
• Add additives to get ciphertext
• Send ciphertext and additive position (MI -- message indicator) (the latter is sent in clear)
• Recipient subtracts additives before decrypting
• Additive sequences that never overlap are essentially a one-time pad. (In practice will choose MI's that will cause overlapping sequences)
• Messages with overlapping additive sequences can be used to get statistical info.
• However, the amount of statistical info needed to decrypt the whole cipher is dramatically increased over not using.

Zimmerman Telegram

• Perhaps most famous codebook ciphertext ever
• It was a major factor in U.S. entry into World War I
• In 1917, the German Foreign Minister, Arthur Zimmerman, sends a telegram to the German Ambassador in Mexico City.
• At the time, the U.S. was not yet involved in WWI. Remember the War started 1914 ends 1918 (11th hour, 11 day, 11 month), shortly after the U.S entered the war.
• The Germans were planning to begin unrestricted submarine activity in the war, and the telegram asks the German Ambassador to try to recruit Mexico as an ally in a war against the U.S.
• In return, Mexico could reconquer Texas, New Mexico, and Arizona.

Zimmerman Telegram Decrypted

• The British were able to decrypt the Zimmerman Telegram as seen above.
• To do this, they used a recovered partial codebook from the Russians...
• Then filled in missing parts.
• Turns out the German's didn't know their system was broken because via different channel the same information had been sent unencrypted.

Random Historical Items

• Spartan Scytale (5th century BCE)-- a rod that could be used to make a transposition cipher
  
• Caesar's cipher (1st century BCE)
• Poe's short story: The Gold Bug (1840s) uses letter frequency to decrypt a substitution cipher
• Election of 1876

Election of 1876

• "Rutherfraud" Hayes versus "Swindling" Tilden
• Popular vote was virtual tie
• Electoral college delegations for four states (including Florida) in dispute
• A commission was established to determine which state delegations were legitimate.
• Commission gave all four states to Hayes
• Vote on straight party lines
• Tilden accused Hayes of bribery
• Was it true?

Election of 1876 and Crypto

• Encrypted messages by Tilden supporters to officials in disputed states later emerged
• Cipher: Partial codebook, plus transposition
• Codebook substitution for important words

  ciphertext    plaintext
  ==========    =========
  Copenhagen    Greenbacks
  Greece        Hayes
  Rochester     votes
  Russia        Tilden
  Warsaw        telegram
  .
  .
  .
  

1876 Encryption Algorithm/Cryptanalysis

• Encryption:
  • Apply codebook to original message
  • Pad message to multiple of 5 words (total length, 10,15,20,25 or 30 words)
  • For each length, a fixed permutation applied to resulting message
• Cryptanalysis:
  • Permutations found by comparing several messages of same length
  • This works as the same key is applied to all messages of a given length
  • Guess codebook words from many messages

Example of 1876 Encryption/Decryption

• Ciphertext: Warsaw they read all unchanged last are idiots can't situation
• Codebook: Warsaw == telegram
• Transposition: 9,3,6,1,10,5,2,7,4,8
• Plaintext: Can't read last telegram. Situation unchanged. They are all idiots.
• A weak cipher made worse by reuse of key
• Lesson? Don't overuse keys!
• It turned out from decrypting these messages that it was Tilden who had attempted to bribe officials.

Early 20th Century

• WWI -- Zimmerman Telegram
• "Gentlemen do not read each other's mail" -- Henry L. Stimson, Secretary of State, 1929. Between the wars, the US stopped its official cryptanalyses programs
• WWII -- golden age of cryptanalysis
  • Midway/Coral Sea
  • Japanese Purple (codename MAGIC)
  • German Enigma (codename ULTRA)

Post-WWII History

• Claude Shannon -- father of the science of information theory
• Computer revolution -- lots of data to protect
• Data Encryption Standard (DES), 70's
• Public Key cryptography, 70's
• CRYPTO conferences, 80's
• Advanced Encryption Standard (AES), 90's
• The crypto genie is out of the bottle...

Claude Shannon

• The founder of Information Theory
• 1949 paper: Information Theory of Secrecy Systems
• Fundamental concepts
  • Confusion -- obscure relationship between plaintext and ciphertext
  • Diffusion -- spread plaintext statistics through the ciphertext
• Proved one-time pad is secure
• One-time pad is confusion-only, while double transposition is diffusion-only

Taxonomy of Cryptography

• Symmetric Key
  • Same key for encryption and decryption
  • Two types: Stream ciphers, Block ciphers
• Public Key (or asymmetric crypto)
  • Two keys, one for encryption (public), and one for decryption (private)
  • And digital signatures -- nothing comparable in symmetric key crypto
• Hash algorithms
  • Can be viewed as "one way" crypto

Taxonomy of Cryptanalysis

From perspective of info available to Trudy

• Ciphertext only
• Known plaintext
• Chosen plaintext
  • "Lunchtime attack" -- if someone leaves for lunch can start using the key to encrypt things and see what they encrypt to.
  • Protocols might encrypt chosen data
• Adaptively chosen plaintext
• Related key
• Forward search (public key crypto)
• And others...