Decrypting The Vigenère Cipher
Let the key be KEY.
You wants to decrypt NGMNI.
- locate the first letter of the key in the left column, and locates on the row the first letter of the ciphered message.
- Then go up in the column to read the first letter, it is the corresponding plain letter.
- Locate the letter K on the first column, and on the row of it, find the cell of the letter N, the name of its column is D, it is the first letter of the plain message.
- Then continue with the next letters of the message and the next letters of the key, then at the end of the key, go back the the first key of the key.
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Encrypted Text |
N |
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Decrypted Message |
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<
Task
Decrypt the message ZMEZIL using the key 'KEY'
Click on the excel icon below to download the Excel activity sheet!
Extension task
- Decrypt the message K IB PR KJM XSPLOQCHXZ using the key 'CIPHER'
The Vigenère Cipher
The Vigenère cipher, was invented by a Frenchman, Blaise de Vigenère in the 16th century. It is a polyalphabetic cipher because it uses two or more cipher alphabets to encrypt the data. In other words, the letters in the Vigenère cipher are shifted by different amounts, normally done using a word or phrase as the encryption key .
Unlike the monoalphabetic ciphers, polyalphabetic ciphers are not susceptible to frequency analysis, as more than one letter in the plaintext can be represented by a single letter in the encryption.
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The Vigenère cipher uses this table in conjunction with a key to encipher a message.
So, if we were to encode a message using the key COUNTON, we write it as many times as necessary above our message. To find the encryption, we take the letter from the intersection of the Key letter row, and the Plaintext letter column.
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Encryption |
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Task
Encrypt the message 'To be or not to be' using the key 'KNOLE'
Extension task
- Create your own Key and encrypt the message 'To be or not to be' ?
- Create your own Key and encrypt your own message?
Update your objectives