Back to Student Worksheet
Math Grade 6-8 Answer Key

Cryptography: Caesar and Vigenere Ciphers

Encoding and decoding messages with alphabet shifts

Answer Key
Name:
Date:
Score: / 12

Cryptography: Caesar and Vigenere Ciphers

Encoding and decoding messages with alphabet shifts

Math - Grade 6-8

Instructions: Read each problem carefully. Use A = 0, B = 1, C = 2, and so on through Z = 25. For Caesar ciphers, shift each letter by the given number. For Vigenere ciphers, repeat the key as needed and shift by each key letter. Ignore spaces and punctuation unless the problem says otherwise.
  1. 1

    Use a Caesar cipher with a shift of 3 to encrypt the word MATH.

    Move each letter 3 places forward in the alphabet.

    The encrypted word is PDWK because M shifts to P, A shifts to D, T shifts to W, and H shifts to K.
  2. 2

    Use a Caesar cipher with a shift of 3 to decrypt the message KHOOR.

    The decrypted message is HELLO because each letter is shifted 3 places backward in the alphabet.
  3. 3

    Use a Caesar cipher with a shift of 5 to encrypt the word CODE.

    When encrypting with a positive Caesar shift, count forward in the alphabet.

    The encrypted word is HTIJ because C shifts to H, O shifts to T, D shifts to I, and E shifts to J.
  4. 4

    Use a Caesar cipher with a shift of 3 to decrypt the message FUBSWR.

    The decrypted message is CRYPTO because F shifts back to C, U shifts back to R, B shifts back to Y, S shifts back to P, W shifts back to T, and R shifts back to O.
  5. 5

    A Caesar cipher message is BQQMF. Test possible shifts and decide which shift makes a common English word. What is the plaintext and the shift?

    Try shifting each letter backward by 1 first.

    The plaintext is APPLE, and the shift is 1 because each encrypted letter moves one place backward to make a readable word.
  6. 6

    In a Caesar cipher, the most common plaintext letter E was encrypted as J. What shift was used, and what is the plaintext of YJXY?

    The shift was 5 because E moved forward to J. The plaintext of YJXY is TEST because each letter is shifted 5 places backward.
  7. 7

    Use a Vigenere cipher to encrypt DOG with the key CAT. Use A = 0, so C means shift 2, A means shift 0, and T means shift 19.

    Match each plaintext letter with one key letter before shifting.

    The encrypted message is FOZ because D shifts by 2 to F, O shifts by 0 to O, and G shifts by 19 to Z.
  8. 8

    Use a Vigenere cipher to encrypt MATH with the key KEY. Repeat the key as KEYK.

    The encrypted message is WERR because M shifts by K to W, A shifts by E to E, T shifts by Y to R, and H shifts by K to R.
  9. 9

    Use a Vigenere cipher to decrypt RIJVS with the key KEY. Repeat the key as KEYKE.

    For decryption, subtract the key shift instead of adding it.

    The decrypted message is HELLO because R shifts back by K to H, I shifts back by E to E, J shifts back by Y to L, V shifts back by K to L, and S shifts back by E to O.
  10. 10

    Write the repeated Vigenere key for the plaintext SCIENCE if the key is MATH. Then give the shift value for each key letter.

    The repeated key is MATHMAT. The shift values are 12, 0, 19, 7, 12, 0, and 19.
  11. 11

    Explain one way a Vigenere cipher is different from a Caesar cipher.

    Think about whether the shift stays the same or changes from letter to letter.

    A Caesar cipher uses the same shift for every letter, but a Vigenere cipher uses a repeating key that can give different shifts to different letters.
  12. 12

    Use a Vigenere cipher to decrypt LXFOPV with the key LEMON. Repeat the key as LEMONL.

    Use the repeated key L E M O N L and subtract each key shift from the cipher letter.

    The decrypted message is ATTACK because each cipher letter is shifted backward by the matching key letter in LEMONL.
LivePhysics™.com Math - Grade 6-8 - Answer Key