csce522-lect4

# csce522-lect4 - Cryptography 2 Substitution Transposition...

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Cryptography 2 Substitution Transposition

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CSCE 522 - Farkas 2 Secret Key Cryptosystem Encryption Decryption Plaintext Plaintext Ciphertext K Sender Recipient C=E(K,M) M=D(K,C) K needs secure channel
CSCE 522 - Farkas 3 Basic Encryption Techniques Substitution (confusion) Permutation (diffusion) Combinations and iterations of these

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CSCE 522 - Farkas 4 Simple Alphabetic Substitution Assign a new symbol to each plain text symbol randomly or by key, e.g., C k, A h , B l M=CAB C =k h l Advantages : large key space 26! Disadvantages : trivially broken for known plaintext attack, repeated pattern, letter frequency distributions unchanged
CSCE 522 - Farkas 5 Polyalphabetic Substitution Frequency distribution : reflects the distribution of the underlying alphabet cryptanalysts find substitutions E.g., English: e – 14 %, t – 9.85%, a – 7.49%, o- 7.37%, … Need: flatten the distribution E.g., combine high and low distributions: t a (odd position), b (even position) x a (even position) , b (odd position)

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Vigenere Tableu Use the Vigenere Tableau to encrypt Plain text: HOPE YOU ARE HAVING FUN With key: I think this cipher is hard to break CSCE 522 - Farkas 6
CSCE 522 - Farkas 7 Cryptanalysis of Polyalphabetic Substitution 1. Determine the number of alphabets used 2. Solve each piece as monoalphabetic substitution. Kasiski Method: Uses regularity of English: letters, letter groupings, full words e.g., endings: -th, -ing, -ed, -ion, -ation, -tion,… beginnings: im-, in-, re-, un-, . .. patterns: -eek-, -oot-, -our-, … words: of, end, to, with, are, is, …

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CSCE 522 - Farkas 8 One-Time Pad Perfect Secrecy! Large, non-repeating set of keys Key is larger than the message Advantages : immune to most attacks Disadvantages : Need total synchronization Need very long, non-repeating key Key cannot be reused Key management: printing, storing, accounting for
CSCE 522 - Farkas 9 Summary of Substitution Advantages: Simple Easy to encrypt Disadvantages: Easy to break!!!

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CSCE 522 - Farkas 10 Transposition Letters of the message are rearranged Break patterns, e.g., columnar transposition Plaintext: this is a test t h i s i s a t tiehssiatst! e s t ! Advantages : easy to implement Disadvantages : Trivially broken for known plaintext attack Easily broken for cipher only attack
CSCE 522 - Farkas 11 Cryptanalysis Rearrange the letters Digrams, Trigrams, Patterns Frequent digrams: -re-, -th-, -en-, -ed-, … Cryptanalysis: 1. Compute letter frequencies subst. or perm. 2. Compare strings of ciphertext to find reasonable patterns (e.g., digrams) 3. Find digram frequencies

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CSCE 522 - Farkas 12 Double Transposition Two columnar transposition with different number of columns First transposition: breaks up adjacent letters Second transposition.: breaks up short patterns
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csce522-lect4 - Cryptography 2 Substitution Transposition...

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