Gronsfeld cipher

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Quick Questions

Shade in the A or B circle for each question.

1. Which one is a valid Gronsfeld cipher key?

A 31415

B PIZZA

2. The key is 31415 and the first message letter is A. What happens to it?

A It shifts 31 places and becomes F

B It shifts 3 places and becomes D

3. The key is 123 and the message has 7 letters. Which shifts are used?

A 1 2 3 1 2 3 1

B 1 2 3 0 0 0 0

4. How many different shifts can one Gronsfeld key digit give?

A 26 (letters A to Z)

B 10 (digits 0 to 9)

5. Key digit 2 was used to encrypt a letter and the cipher letter is F. What was the original letter?

A D

B H

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1key = '31415'		APicks the key digit for position i, repeating the key along a longer message
2key[i % len(key)]		B(n - shift) % 26
3To encrypt, shift the letter number forward by its key digit and wrap round with MOD 26		CThe same digit 3 was used for both letters. The key moves to the next digit, so I is shifted by 1 and becomes J. The correct result is KJ.
4To decrypt, shift the letter number backward by the same key digit and wrap with MOD 26		D(n + shift) % 26
5key = '3A415'		EA Gronsfeld key: a string of digits 0-9 where each digit is the shift for one letter
6HI with key 31 gives KK		FA Gronsfeld key can only use digits 0-9. The letter A makes this a Vigenere-style key and gives no valid numeric shift.

Definitions

Gronsfeld cipher

Fill in the Blanks

Complete each line of code by filling in the missing parts.

shift = int(key[_ % len(key)])

shift = int(key[i _ len(key)])

new = (ord(ch) - 65 _ shift) % 26 + 65

shifts = list(range(0, __))

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A key = "ABCDE"

B text = "HELLO"

C d = int(key[0])

D print(d)

Question 2

A key = "2718"

B text = "CATS"

C for i in range(len(text)):

D d = int(key[i])

E print(d)

Question 3

A key = "31415"

B pos = 0

C d = int(key[pos])

D new = (7 + d) % 10

E print(new)

Question 4

A plain = (cipher + d) % 26

B letter = chr(plain + 65)

C print(letter)

Question 5

A key = "31415"

B d = key[0]

C shifted = (4 + d) % 26

D print(shifted)

Answer Key

Gronsfeld cipher

Section A: Quick Questions

A — 31415

B — It shifts 3 places and becomes D

A — 1 2 3 1 2 3 1

B — 10 (digits 0 to 9)

A — D

Section B: Match the Pairs

key = '31415' → A Gronsfeld key: a string of digits 0-9 where each digit is the shift for one letter

key[i % len(key)] → Picks the key digit for position i, repeating the key along a longer message

To encrypt, shift the letter number forward by its key digit and wrap round with MOD 26 → (n + shift) % 26

To decrypt, shift the letter number backward by the same key digit and wrap with MOD 26 → (n - shift) % 26

key = '3A415' → A Gronsfeld key can only use digits 0-9. The letter A makes this a Vigenere-style key and gives no valid numeric shift.

HI with key 31 gives KK → The same digit 3 was used for both letters. The key moves to the next digit, so I is shifted by 1 and becomes J. The correct result is KJ.

Section C: Definitions

Gronsfeld cipher — A Vigenere variant whose key is a string of digits 0-9, like 31415. Each digit is the shift for one letter, and the key repeats along the message.

Section D: Fill in the Blanks

shift = int(key[i % len(key)])

new = (ord(ch) - 65 + shift) % 26 + 65

new = (ord(ch) - 65 - shift) % 26 + 65

shifts = list(range(0, 10))

Section E: Spot the Error

A — A Gronsfeld key is made only of digits 0-9, because each digit is a shift. Letters like ABCDE make a Vigenere key. It should be key = "31415".

D — The key must repeat along the message. Using key[i] runs off the end of a short key. It should use i % len(key) to wrap back to the first digit.

D — Letter positions wrap around the 26-letter alphabet, so the MOD must be 26. The digits 0-9 are the shifts, not the alphabet size. It should be % 26.

A — Encrypting adds the digit shift, so decrypting must subtract it. It should be plain = (cipher - d) % 26.

B — key[0] is the text character "3", not a number, so it cannot be added to a letter position. It must be converted with int(), giving d = int(key[0]).