AY Honors/Cryptography/Answer Key

Cryptography

Skill Level

1

Year

2025

Version

11.09.2025

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 Olleh Dneirf). Other forms include a rail fence ciphers as shown below, which write the words in a zig-zag pattern and then encode across the rows.

A two-line rail fence cipher, to encode "Pathfinders Strong." Once entered into the blocks, we would then send the encoded message as "PTFNESTOGAHIDRSRN"

A three-line rail fence cipher, to encode "pathfinders strong." Once entered into the blocks, we would send the encoded message as "pfetgahidrsrntnso"

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 column and row number.

Using the square above, we would encode the word "Pathfinder" as: "53 11 44 32 12 42 33 41 51 24"

3c

Square

In a square cipher, the letters are placed in a grid, and encoded against another grid. A common square grid is a four square grid, made up (as the name implies) of four 5X5 grids. The upper left and lower right are a regular alphabet, the upper right and lower left may have a code word (in this case Pathfinder Strong is the code word - the code word is written in the squares first, then the grid is finished with the rest of the alphabet).

To encode a message, it is first broken into two-letter segments. So pathfinder becomes pa th fi nd er (if there were an odd number of letters, you would use an x to fill the final pair). The first letter of the pair is found in the upper left grid, the second of the pair in the lower right. To encode, move along the row from the first letter to pair up with the column of the second letter. Then for the second letter, move along its row to align with the column of the first letter.

In the image here, we have also color coded to see. So PA = BN, TH = QC, FI = EG, ND = KR, ER = AU. We would then send the encoded message (Pathfinder) as BNQCEGKRAU

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