Maths and Computing:Polyalphabetic Activity

Polyalphabetic: Polyalphabetic Cipher

Polyalphabetic Cipher

The Polyalphabetic Cipher is a substitution cipher.

With a shift value of n a letter is replaced with the letter that is n positions further forward

Decryption

With a shift value of n a letter is replaced with the letter that is n positions further back. Alternatively, this is the equivalent of replacing each letter with the letter that is (26 - n) positions further back

Example

The grid below shows the substution values for a shift value of 3

plaintext

ciphertext

Using different shift values

The polyalphabetic cipher has different shift values for different letter positions

In this activity, plaintext encrypted with a cipher that has three alphabets, uses shift 1 for the 1st, 4th, 7th ... letters, shift 2 for the 2nd, 5th, 8th ... letters and shift 3 for the 3rd, 6th, 9th ... letters

Polyalphabetic Cipher Question

Shift 1 = 9, Shift 2 = 14

Decode boes

Enter Answer

Shift 1 = 9, Shift 2 = 14

plaintext

alphabet 1

alphabet 2

The answer is: save

Polyalphabetic Cipher Question

Shift 1 = 5, Shift 2 = 22 , Shift 3 = 5

Decode gwhpcwtqsi

Enter Answer

Shift 1 = 5, Shift 2 = 22 , Shift 3 = 5

plaintext

alphabet 1

alphabet 2

alphabet 3

The answer is: background

Polyalphabetic Cipher Question

Shift 1 = 19, Shift 2 = 15 , Shift 3 = 11 , Shift 4 = 14

Decode lizftvp

Enter Answer

Shift 1 = 19, Shift 2 = 15 , Shift 3 = 11 , Shift 4 = 14

plaintext

alphabet 1

alphabet 2

alphabet 3

alphabet 4

Maths and Computing:

Polyalphabetic

Polyalphabetic: Polyalphabetic Cipher

Polyalphabetic Cipher

The Polyalphabetic Cipher is a substitution cipher.

With a shift value of n a letter is replaced with the letter that is n positions further forward

Decryption

With a shift value of n a letter is replaced with the letter that is n positions further back. Alternatively, this is the equivalent of replacing each letter with the letter that is (26 - n) positions further back

Example

The grid below shows the substution values for a shift value of 3

Using different shift values

The polyalphabetic cipher has different shift values for different letter positions

In this activity, plaintext encrypted with a cipher that has three alphabets, uses shift 1 for the 1st, 4th, 7th ... letters, shift 2 for the 2nd, 5th, 8th ... letters and shift 3 for the 3rd, 6th, 9th ... letters

Polyalphabetic Cipher Question

Shift 1 = 9, Shift 2 = 14

Decode boes

Enter Answer

Shift 1 = 9, Shift 2 = 14

The answer is: save

Polyalphabetic Cipher Question

Shift 1 = 5, Shift 2 = 22 , Shift 3 = 5

Decode gwhpcwtqsi

Enter Answer

Shift 1 = 5, Shift 2 = 22 , Shift 3 = 5

The answer is: background

Polyalphabetic Cipher Question

Shift 1 = 19, Shift 2 = 15 , Shift 3 = 11 , Shift 4 = 14

Decode lizftvp

Enter Answer

Shift 1 = 19, Shift 2 = 15 , Shift 3 = 11 , Shift 4 = 14

The answer is: storage

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