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## PowerPoint Slideshow about 'Playfair ciphers' - jemima-fisher

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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

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