Classical Encryption Techniques Quiz

Classical Encryption Techniques

• Two main types: substitution and transposition
• Substitution replaces plaintext characters with other letters/numbers/symbols
• If plaintext is viewed as a bit sequence, substitution replaces plaintext bit patterns with ciphertext bit patterns
• Transposition rearranges plaintext letters without changing the characters

Substitution Techniques

• Replaces plaintext letters with other letters, numbers or symbols
• If plaintext is in bits, substitution involves replacing plaintext bit patterns with cipher text bit patterns

Caesar Cipher

• Earliest and simplest substitution method
• Each letter is replaced by a letter a fixed number of positions up or down the alphabet
• Example: with a shift of 1, A becomes B, B becomes C, etc.
• Named after Julius Caesar
• To cipher a given text, a shift value (integer) is needed to indicate how many positions the letters are shifted.
• Encryption can use modular arithmetic: A=0, B=1, ..., Z=25
• Encryption formula: E(p) = (p + k) mod 26
• Decryption formula: D(c) = (c - k) mod 26
• The shift 'k' value in the range of 1 to 25
• The alphabet is wrapped around (z+1 = a)
• Example: "amp" (shifted by 3) becomes "dps"

Playfair Cipher

• Best-known digraph substitution cipher.
• It treats plaintext pairs of two letters as units (digraphs)
• Uses a 5x5 matrix built using a keyword.
• Letters I and J are treated as one letter
• Encrypts two letters at a time following specific rules:
  • If both letters are in the same column, take the letter below each in the column (wrapping from bottom to top)
  • If both letters are in the same row, take the letter to the right of each (wrapping from right to left)
  • If the letters are not in the same row or column, form a rectangle around them, taking the horizontal opposite letters for the cipher
• Example keyword "monarchy"

Transposition Techniques

• Rearranges plaintext letters, but doesn't change the letters themselves
• Simple arrangement change of letters
• Less secure than substitution, and easily recognizable
• Can be made more secure by multiple stages of transposition (more complex permutation, harder to decrypt)

Rail Fence Cipher

• A basic transposition cipher
• The plaintext is written in a zig-zag pattern across a series of rows, then read out (row by row).
• The number of rows is called the key.
• Example: (2 rail fence) "This is a secret message" Encrypted result: "TIHSSAESCRERTMSGSAME"
• More complex ciphers use more rows (rails).