CS 257 - Fall 2020.

echo hello | openssl base64
aGVsbG8K

Lecture

Chapter 2: Crypto Overview

• Spartan Scytale Transposition cipher
• Substitution Cypher, e.g., Poe's The Gold Bug
• Double transposition
• One-time pad (XOR ⊕ magic!)
• Codebook
• Election of 1876.
  • Not decided by the Supreme Court – commission was 8 republicans and 7 democrats. Guess what?
  • Tilden won 184 votes (and popular vote!)
  • Hayes won 165. 20 votes were in dispute.
  • States included Florida, Louisiana, South Carolina, and one disputed elector in Oregon.
  • Compromise: Democrats awarded 20 votes to Hayes in return for withdrawal of federal troops from the South to end Reconstruction.
  • Tilden was trying to bribe officials. He had accused Hayes of bribery.
• Battle of Midway
  • Allies claimed water shortage at Midway (broken desalination plant)
  • Saved 100,000+ lives and 1+ years
• Post-WWII History
  • Before Shannon, crypto had been a black art
  • DES marked an inflection point – cryptography came out of the shadows.
• Claude Shannon
  • Diffusion does not erase single character frequency counts, but removes bi-grams.
  • Election of 1876 used both confusion (code book) and diffusion (permutation), albeit both were pretty lame.
• Taxonomy of Cryptography
  • Key distribution problem with symmetric keys
  • Digital signatures – use private key to encrypt authenticated documents
• Taxonomy of Cryptanalysis
  • Forward search.
  • Example- get ciphertext that could be “yes” or “no” Use public key to encrypt “yes” and “no” and compare. Either a match or something else.
  • Defense: pad the message with 64 bits. Then have 2^64 versions of yes
  • Can also forward email message: “Please forward the attached to Bob”

Chapter 3: Symmetric Key

• Stream ciphers are more practical version of one time pad.
• Stretch out stream to length of message and xor.

Stream Ciphers

• A5/1 – implemented in hardware – at that time, processor speed was slow.
• RC4 – possible to implement in software
• Used a lot: WEP (Wired Equivalent Privacy IEEE 802.11 wireless networks) often the first choice presented to users by router configuration tools.
• 2003 superceded by WPA – Wi-Fi Protected Access., 802.11i (WPA2) standard – deprecated WEP-40 and WEP-104
• WEP-40 has 40 bit key and 24 bit initialization vector (IV) == 10 hexadecimal digits
• 40 bit key can be 5 ASCII characters of 8 bits each.
• WEP-104 is 26 hex characters, with 26 4 bit characters plus 24 bit IV , TLS

• Total of 64 bits
• Implemented in hardware
• A5/1 wiki - design shrouded in secrecy. We do not know why it was designed this way.
• If bit is in the majority, then that register steps.
• 2 or 3 registers always step
• Generates a single bit for xor

• 256 Byte values
• Table is a permutation of those values
• Invented by Ron Rivest in 1987
• RC4 stands for Rivest Cipher 4, but is trademarked by Rivest’s company, RSA.
• Also, Ron’s Code. (RC2, RC5, RC6)
• Was initially a trade secret, but description was posted in 1994, which was then broken by Bob Jenkins within days. While never acknowledged, the leaked code was found to match the proprietary software.
• How does Kerkhoff’s principal apply here?
• The array keeps changing, makes it harder to break.

• Rivest makes ‘em and Shamir breaks ‘em.
• Stream ciphers, like the old man in Monty Python’s Holy Grail, are not dead yet.
• They can fit on small devices, like IoT.

Block Ciphers

• Feistel worked at IBM.
• Born in Berlin. Came to US in 1934. WWII under house arrest until 1944 when became a US citizen, got a security clearance, and worked for the air force on IFF (Identification of Friend or Foe). BS from MIT and MS from Harvard in Physics. Developed Lucifer which became DES
• For decryption: if F(R,K) == 0, then P == C

• NBS == National Bureau of Standards
• Which became NIST = National Institute of Standards and Technology
• Issue RFP, and got only a handful of responses. Indicating that there was not much going on the cryptography world.
• After NSA involvement, people were suspicious – suspected an NSA backdoor.
• Result – much more interest in cryptography.
• 2⁵⁶ possible codebooks
• The S-boxes are lookup tables.
• It is a Feistel Cipher
• 8 S boxes convert 48 bits to 32 bits

• S box is just a look up table
• Each row is a permutation
• 6 bits in and 4 bits out
• 48 bits becomes 32 bits

• Linear functions are easy to solve
• The S boxes are not linear – make the algorithm stronger

• This is a code book
• Use a different key, get a different code book

• AES Process which, by contrast to A5/1, was transparent.
• Conference among contestants
• Each wants to shoot holes in other ciphers
• Lots of criticism
• Feistel cipher is easy to decrypt. Here you need an invertible function

• DES has 64 bit block
• 56 bit key length
• 16 rounds
• But AES is more complicated

• You will implement this in homework.
• Opposite of AES, which has strong round function and few rounds
• Delta is magic number – bits in the decimal expansion of sqrt(5)
• 32 rounds here are like 64 rounds of DES
• Not a Feistel cipher – does not use XOR

Block Cipher Modes

• 56 bit key per block
• Bad idea to encrypt independently
• Partial blocks addressed with padding.

• Most popular
• XOR magic!
• ADDITIVE analogy
• A single data transmission error can mess up 2 blocks – but not more.

Integrity

• Multiple meanings of MAC
• MIC – message integrity code
• We said that errors do not propagate in CBC That is in decryption, but not encryption.

• US Navy – John Walker would distribute keys-
• Side business selling keys to soviets
• Weak link in the system

• related key attack for WEP using RC4