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Explore the world of cipher techniques in this interactive session! Understand LFSR ciphers, substitution ciphers, Playfair ciphers, and ADFGX ciphers. Learn to determine key length, solve recurrences, and break ciphertexts using linguistic structures. Delve into encryption strategies and weaknesses of Playfair ciphers. Discover the history behind ADFGX ciphers and how they combine cryptography with error-correction. Enhance your knowledge of matrix-based block ciphers and decryption methodologies. Gain practical insights and decryption ideas for ADFGX and ADFGVX ciphers. Join us for a deep dive into the fascinating world of ciphers!
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DTTF/NB479: Dszquphsbqiz Day 8 • Announcements: • Homework 1 coming Monday… • Practical quiz during week 4 on putting your code to work • Questions? • Today: • Wrap up LFSR ciphers • Substitution, Playfair, and ADFGX ciphers
LFSR Demo • Determine key length by computing determinants • Use the key length to solve for the recurrence. • Verify your solution by using the solution to generate the whole key
Substitution ciphers • Each letter in the alphabet is replaced by another one. • Which ciphers have we seen are substitution ciphers? • Which aren’t and why? • Breaking ciphertext only uses linguistic structure. Frequencies of: • Single letters • Digrams (2-letter combinations) • Trigrams • Where do T&W get their rules like “80% of letters preceding n are vowels”? (p. 26) • See http://keithbriggs.info/documents/english_latin.pdf • Lots of trial and error when done by hand. • Could automate with a dictionary.
Playfair ciphers • Matrix-based block cipher used in WWI • In a 5x5 matrix, write the letters of the word “playfair” (for example) without dups, and fill in with other letters of the alphabet, except I,J used interchangeably.
Playfair encryption • Break plaintext into letter pairs • If a pair would contain double letters, split with x • Pad end with x • hellothere becomes… • he lx lo th er ex • For each pair, • If they are in the same row, replace each with the letter to its right (mod 5) • he KG • If they are in the same column, replace each with the letter below it (mod 5) • lo RV • Otherwise, replace each with letter we’d get if we swapped their column indices • lx YV He lx lo th er ex KG YV RV QM GI KU To decrypt, just reverse!
Weaknesses • P is unknown, but structure of its bottom is predictable • Can break using digram frequencies • Example: If a digram and its reverse both appear often, it’s probably ER and RE. • Each plaintext letter maps to how many possible ciphertext letters? He lx lo th er ex KG YV RV QM GI KU
Playfair ciphers • Used in Dorothy L. Sayers’ 1932 mystery novel Have His Carcase • Marketing beats technology? • Invented by Charles Wheatstone • Lyon Playfair, a Scottish Baron, promoted it • Who got the glory?
ADFGX ciphers • Why ADFGX? • Morse code for these are very different • Combined cryptography with error-correction • Matrix 1: • 25 letters (i and j merged again) randomly placed • Each plaintext letter replaced by its row and column labels • hello there • XA FA AA AA FF DG XA FA DF FA
ADFGX ciphers (2) • XA FA AA AA FF DG XA FA DF FA • Matrix 2: pick a random keyword and write the previous result under it in scanline order. • Shuffle the columns into alphabetical order • Then read down the columns
ADFGX ciphers (3) • XA FA AA AA DG FF XA FA DF FA • Matrix 2: pick a random keyword and write the previous result under it in scanline order. • Shuffle the columns into alphabetical order • Then read down the columns to get ciphertext: • XAXFAFAAAAAADDAGFFFF
ADFGX Decryption easy… • …if you know the original matrix and the keyword. • Example? • Read about decryption ideas in text • Variation: ADFGVX cipher allows 26 letters + 10 digits
