are most common
➭
guess a, e, t
•
Keep going like this…
❖
Even if each position in key is fully scrambled (not just shifted), this mechanism
works
Key length and frequency
31
PAMP DOKe SCAO PBSJ VFSV HRGE ASEX BRQR AGMR KOPZ
HBOI KIZH LFSV HRGE ASEM UHQV LGFI KeZE UMAJ AVQe
LODI HGAJ YSEI HFOL PTKS BFDI ZSMV JVSS HZEQ HHOL
AVAe LCRT YCVI JHEJ VFIL PQTM OOHI LLFI YBMP ZIBT
VFFM TOKF LONP LHAe BDBS YHKS BOEE YSEI HFOL HGEM
ZHMR A

❖
Use binary alphabet:
•
C
i
= (
P
i
+
K
i
mod
N
) mod 2
•
C
i
=
P
i
⊕
K
i
mod
N
❖
Use a key as long as P:
•
C
i
=
P
i
⊕
K
i
❖
One-time pad—perfect cipher!
❖
Key is as long as the message
❖
All messages are equally likely
❖
More on this in a bit…
Vigenère simplification
32

Types of attacks
33

❖
Ciphertext-only — how much ciphertext is needed?
Types of attacks
33

❖
Ciphertext-only — how much ciphertext is needed?
❖
Known plaintext — often “guessed plaintext”
Types of attacks
33

❖
Ciphertext-only — how much ciphertext is needed?
❖
Known plaintext — often “guessed plaintext”
❖
Chosen plaintext (get ciphertext)
•
Not as uncommon as it sounds!
Types of attacks
33

❖
Ciphertext-only — how much ciphertext is needed?
❖
Known plaintext — often “guessed plaintext”
❖
Chosen plaintext (get ciphertext)
•
Not as uncommon as it sounds!
❖
Chosen ciphertext (get plaintext)
Types of attacks
33

❖
Ciphertext-only — how much ciphertext is needed?
❖
Known plaintext — often “guessed plaintext”
❖
Chosen plaintext (get ciphertext)
•
Not as uncommon as it sounds!
❖
Chosen ciphertext (get plaintext)
❖
Leave these to the professionals:
•
Dumpster diving
•
Social engineering
•
“Rubber-hose cryptanalysis” (actually an advanced form of social
engineering)
•
Use threats, blackmail, torture, and bribery to get the key
Types of attacks
33

Really brief history: first 4000 years
34
Cryptographers
Cryptanalysts
3000BC
monoalphabetics
900
al-Kindi - frequency analysis
Alberti – first polyalphabetic cipher
1460
Vigenère
1854
Babbage breaks Vigenère;
Kasiski (1863) publishes

Really brief history: last 100 years
35
Cryptographers
Cryptanalysts
1854
1918
Mauborgne – one-time pad
Mechanical ciphers - Enigma
1939
Rejeeski repeated
message-key attack
Turing’s loop attacks,
Colossus
Enigma adds rotors,
stops repeated key
1945
Feistel block cipher, DES
Linear, Differential
Cryptanalysis
?
1973
Public-Key
Quantum crypto?