From the course: Symmetric Cryptography Essential Training

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Polyalphabetic substitution ciphers

Polyalphabetic substitution ciphers

From the course: Symmetric Cryptography Essential Training

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Polyalphabetic substitution ciphers

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- [Narrator] . Now, we're gonna talk about Polyalphabetic Substitution Ciphers. In the previous examples, we were only dealing with the single alphabet, that has every input letter corresponded to a single output letter. It was a pretty easy conversion from one letter to the next. In a Polyalphabetic cipher, we have many alphabets. There are many ways that an input letter can be converted to an output letter. And what we essentially have here is we have a combination of several substitution ciphers. And this can be done in a number of different ways. And the strength of these ciphers is based on the length of the period. And that really just means how often does the pattern of the alphabets repeat? So we'll start with the Vigenere Cipher. And then we choose a key, and each character or number of that key is used to select the alphabets. There's an insecure way of generating a key for the Vigenere Cipher. And that's to use a memorable word. So we might use a key like cipher, C I P H E…

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