Atbash

Simple substitution cipher. Reversed alphabet is all you need here.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Z Y X W V U T S R Q P O N M L K J I H G F E D C B A

SR NB MZNV RH HI MY NAME IS

Caesar (shift)

You will have to figure out by how many letters the alphabet is shifted. Pick our some short words (2 or 3 letter) in the encoded text and apply different shifts on them until you get a correct word that translates:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Example: PAXG RHN LXM RHNK FBGW MH TVABXOX

LHFXMABGZ, RHN FNLM TEEHP RHNKLXEY

MAX HIIHKMNGBMR MH ZXM BM WHGX.

Try MH: M + 3 = P M + 4 = Q M + 5 = R M + 6 = S M + 7 = T M + 8 = U

H + 3 = K H + 4 = L H + 5 = M H + 6 = N H + 7 = O H + 8 = P

MH + shift 7 => TO

Try shift 7 on another word, like MAX

MAX => shift 7 => THE

We have a match. Decode the rest

Affine

Affine is another shift cipher but uses the formula: (ax + b) mod 26 (mod is typically done with a % sign in code)

• x = number in the alphabet
• mod means divide by 26 and take the remainder
• the remainder is 0 indexed, meaning 0 = A, 1 = B, ...

Example: a = 19, b = 3

D KBWOD IJBH QJA PGDQNB ZAH HCJAH

Step 1: Encode the frequency table with the forumla. Encode common letters ETAOIN to see their mapping (use the frequency table of English letters on the resource sheet)

E => 419+3=79 % 26 = 1 = B T = (1919+3)%26 = 0 = A ...

This means that the letter B in the encoded text becomes E in the unencoded text. A becomes T:

D KBWOD IJBH QJA PGDQNB ZAH HCJAH E T E T T

Go to the next set of common letters and do those (SRHLD).

Once you use the formula for all letters in the frequency table you can then use those letters to substitute back into the cipher text.

Nihilist

• Uses a 5x5 polybius square.
• Enter polybius keyword into squares starting at top left, don't repeat letters.
• I/J should be treated as the same square.
• Fill in the rest of the alphabet, don't repeat letters.
• Take the keyword, not the polybius keyword, and repeat it across the cipher text.
• Substitute the Row/Column values for each letter from the polybius square into the cipher text values for the keyword.
• Subtract that number from the associated number in the cipher text. That gives you new numbers that you can look up in the polybius table.

Porta

• They give the porta cipher chart. It's a 13 row chart with A-M as the columns and letter pairs as the rows (AB, CD, etc)
• Repeat the keyword repeatedly above the cipher text.
• Start with the first letter. The letter above (from the keyword) you look up in the row pairs on the left. The letter below you look up in the column header.
• If there is no matching letter in the column header, find the letter in the row and use the column header value as the letter in the decrypted text.

Example to decrypt: Keyword: BIG Cipher text: UVOYK

Cryptarithm

• It's a mathematical formula question.
• Each letter in the formula can represent a number 0-9 (no duplicates).
• Try to figure out letters->numbers that are in the left most position first.

Baconian

• They're likely to use symbols instead of letters.
• Symbols will map to letters A or B. In the 2025 division B region 1 test they had land animals (rhino, zebra, elephant) and ocean animals (fishes, shark). Land animals were A, ocean animals were B.
• Group letters into groups of 5, compare those groupings to the chart in the resource packet.

Fractionated Morse

• what?

Complete Columnar Transposition

• Important: Decrypting ciphertext encoded in 9 columns or less given a “crib” which is no shorter than one less than the number of columns used.
• The number of columns you should try should be numbers that in which the number of characters in the text is divisible by.
• eg. Phrase of 54 characters. Crib of 6 characters. Possible column numbers: 6, 9
  • Column count should be 6 because 9 - 6 = 3 which is greater than 1 so it can't be 9.
  • For this example make a table with 6 columns, 9 rows (54 characters).
• eg. Phrase of 64 characters. Crib is 7 characters.
  • Possible column counts are 4 and 8.
  • We can't use 4 because 4 is 3 less than 7 and you can only be 1 less than 7 and you have to be less than or equal to 9.
• In complete columnar there can be padding if the columns x rows does not equal the number of characters in the cipher text. In this case you need padding.
  • Padding goes in the LAST squares on the LAST ROW.
  • eg. cipher text 55 characters long, crib is 7 characters.
  • column length could be 5 for divisible rows/columns but it is too small compared to crib (7-5=2, does not work)
  • Use the same number of columns as the crib, in this case 7
  • 55 / 7 = 7 with remained of 6. 7 - 6 = 1 padding
  • Padded square in the table would be the bottom right square. If there were 2 paddings needed they would be bottom right and the square next to it.
• Write the cipher text in the table first down each column, then down column 2, etc.
• The crib should be found completely in a row or split between two rows all in different columns.
• Reorder the rows to put the crib word in the correct order. This should spell out a phrase.

Aristocrats

• Pattern recognition.
• Use the reference sheet for letter frequency of the English letter. Use it as reference when guessing letters in words.
• Initially focus on 1, 2 and 3 letter common words.
  • 1 letter words include: A, I
  • 2 letter words include: TO, ON, BE, IT, WE, etc
  • 3 letter words include: AND, THE, ALL
• Once you fill in letter, check for endings like ING, TION, ION