Understanding Vignere Cipher

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Vignere Cipher includes a twist with Caesar Cipher algorithm used for encryption and decryption. Vignere Cipher works similar to Caesar Cipher algorithm with only one major distinction: Caesar Cipher includes algorithm for one-character shift, whereas Vignere Cipher includes key with multiple alphabets shift.

Mathematical Equation

For encryption the mathematical equation is as follows −

Ek(Mi)=(Mi+Ki)mod26$$E_k\left(M{}_i\right)=\left(M_i+K_i\right)mod26$$

For decryption the mathematical equation is as follows −

Dk(Ci)=(Ci−Ki)mod26$$D_k\left(C{}_i\right)=\left(C_i-K_i\right)mod26$$

Vignere cipher uses more than one set of substitutions, and hence it is also referred as polyalphabetic cipher. Vignere Cipher will use a letter key instead of a numeric key representation: Letter A will be used for key 0, letter B for key 1 and so on. Numbers of the letters before and after encryption process is shown below −

The possible combination of number of possible keys based on Vignere key length is given as follows, which gives the result of how secure is Vignere Cipher Algorithm −

Vignere Tableau

The tableau used for Vignere cipher is as shown below −