Tutorial

Letter substitution cipher

  • Encodes text by replacing each letter with a different letter according to a fixed pattern.
  • Example
      • Cipher text
      A
      B
      C
      D
      E
      F
      G
      …
      Plain text
      D
      H
      E
      J
      O
      L
      M
      …
      Encoded message BCFFE then decodes to HELLO

Aristocrat and Patristocrat

A letter substitution cipher can be either an Aristocrat or Patristocrat.
In an Aristocrat cipher, spaces and punctuation are preserved so it’s easier to solve.
A Patristocrat cipher removes these hints by running all letters together without spaces.

Random substitution

  • With random substitution, any letter can map to another arbitrary letter, there is no pattern

Caesar cipher

  • The plain text is the cipher text shifted by a certain number.
  • Caesar cipher example:
      • Cipher text
      A
      B
      C
      D
      E
      F
      …
      X
      Y
      Z
      Plain text
      D
      E
      F
      G
      H
      I
      …
      A
      B
      C
      When you see encoded word EF, the plain text is HI, as every letter is shifted by 3 positions in this case.

K1 and K2

  • Similar to Caesar, but in addition to shifting, a key word (known by allies but not outsiders) is inserted in the conversion table.
  • K1: the keyword is inserted in the plain text row of the conversion table. Example:
    • Keyword is FAKE, location is 2
      • Cipher text
      A
      B
      C
      D
      E
      F
      G
      H
      I
      J
      K
      L
      M
      N
      O
      P
      Q
      …
      Plain text
      Y
      Z
      F
      A
      K
      E
      B
      C
      D
      G
      H
      I
      J
      L
      M
      N
      O
      …
      In the original letters row, the keyword FAKE starts at location 2 and is followed by the rest of the letters in A-Z order.
      HELLO encodes to KFNNQ
  • K2: the keyword is inserted in the cipher text row of the conversion table. Example:
    • Keyword is CAT, location is 3
      • Cipher text
      X
      Y
      Z
      C
      A
      T
      B
      D
      E
      F
      G
      H
      I
      J
      K
      …
      Plain text
      A
      B
      C
      D
      E
      F
      G
      H
      I
      J
      K
      L
      M
      N
      O
      …
      In the cipher letter row, the keyword CAT starts at location 3 and is followed by the rest of the letters in A-Z order.
      Hello encodes to DHHAK
  • In all cases, each cipher letter cannot be the same as the original letter.
 
💡
As a code breaker, you don’t know the keyword or the shift position. You only know the encoded text, and your task is to find out the original message.
 
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