are most common guess a e t Keep going like this Even if each position in key

Are most common guess a e t keep going like this even

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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?
Arms race between cryptographers and cryptanalysts Often, disconnect between two (e.g., Mary Queen of Scots used monoalphabetic cipher long after known breakable) It resulted in her losing her head Multi-disciplinary field Linguists, classicists, mathematicians, computer scientists, physicists Secrecy often means advances rediscovered and miscredited Public-key cryptography first done by British security agency, rediscovered by Diffie & Hellman Dominated by needs of government: war is the great catalyst Cryptanalysis advances led by most threatened countries: France (1800s), Poland (1930s), England/US (WW2), Israel? (today) How does cryptology advance?

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