AY Honors/Cryptography/Answer Key

Cryptography

Skill Level

1

Year

2025

Version

12.09.2025

Approval authority

North American Division

1

Define cryptography and list its major applications.

Cryptography is the science or process of encoding and decoding messages to allow more secure transfer of information.

2

Define the following terms:

2a

Code

In a code, words or phrases are encrypted (as opposed to individual letters)

2b

Cipher

A Cipher encrypts a message at the level of individual or small groups of letters

2c

Steganography

Steganography is the process of hiding data/information inside other data, for example hiding textual information in an image file

2d

Plain text

Plain Text refers to the unecrypted information prior to encoding

2e

Key

A key is the information necessary to encrypt and decrypt a particular message

2f

Public-key cryptography

A public key is part of a two-key encryption/decryption model, where one key is private, and another is publicly available. Public Key Cryptography is also called Asymmetric Cryptography

2g

One-way function

A one way function is a model of encrypting information where it is relatively easy to convert the data one way, but much more difficult to reverse the process. In some computer applications, this one way function results in a hash value, allowing secure storage of sensitive information or passwords.

3

Define each of the following categories of ciphers, give an example of each, and encipher your name using each technique.

3a

Transposition

Transposition is a classic cipher that moves the position of letters in a word or phrase, rather than replacing them. At its simplest, this could involve just shifting things around (Hello Friend might be encoded as Dnier Folleh). Other forms include a rail fence ciphers (where the message is first written in two roes, and the encoding takes a letter from each row in order, jumbling them (if the top row was Hello_ and the bottom row were Friend, our message would be Hferli Leon_d - note we added a "_" to ensure both rows had an equal number of letters). Other forms include writing the text in a set number of letters at a time, and placing these in a grid, and then encoding by writing them in as they appear in columns rather than rows.

3b

Polybius Square

A traditional Polybius Square is a 5X5 grid with the alphabet filling in the square (English alphabet needs a larger square, or combines two letters, such as I and J or U and V of C and K). Each square is then given a 2 digit code as per its row and column number. So if the top of our square were ABCDE, and the next row was FGHI/JK, then when writing our coded message, anytime we wanted to use "A" we would instead write "11" - row 1 column 1. H would be 23 - wow 2 column 3.

3c

Square

A Square cipher is one (like the Polybius Square) where the letters are placed in a grid, and their encoding represents where in the grid they were. Several squares may be combined to make a more complex cipher. A common one is a four square cipher, where four 5X5 grids are placed together to make a bigger square, the upper left and lower right 5X5 being standard alphabet layout, the other two usually having keywords to jumble the letters. Words to are broken into two-letter groups, and the substitution from each comes by moving across the row of the chosen letter and the along the column of the second letter to find the replacement value.

3d

Substitution

A substitution cipher is a simple cipher, where each letter/number is replaced by an alternate letter/number/symbol. The characters are changed for the encoded message, but not the order of characters.

3e

Polyalphabetic

In a polyalphabetic cipher, the substituted letter is determined both by the key and the placement in a word. So it may be that every third letter, the cipher shifts, or there is a different cipher for first letter, second letter, third letter, etc. In this style, the encoded message letters may represent different letters in the unencoded message, as different ciphers are used for different positions. It is more complicated to decode than a simple substitution cipher.

4

Investigate whether each item in the list below is a code, a cipher, steganography, or a plain text message.

5

Classical cryptology is the study of codes and ciphers used in the past. Discover how the following cryptographic forms were encoded in antiquity, and try to duplicate their processes to record a message:

6

Prepare a message containing the Pathfinder Law enciphered in a Caesar cipher and indicate the key used.

7

Use steganography to write the Pathfinder Pledge.

8

Choose a cipher to encrypt a favorite Bible text and let another person decrypt it. Be sure to give that person the type of cipher and the key used.

9

Decrypt a message made by another person using the key and cipher type they used.

References