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This outlines the gronsfeld cipher, which is a modification of the vigenere cipher. I like and find this cipher interesting because of it using numbers instead of letters, and I like cryptography which uses numbers.

Interesting. Gronsfeld ciphers then form a subset of Vigenere ciphers. That is, there's a one-to-one correspondence between Gronsfeld ciphers and Vigenere ciphers where keywords are formed from the letters A through J.

Here's the Prezi I shared today on the Vigenere cipher.

This post is about cracking the vigenere cipher. It also uses computer programming to solve it which I found interesting.

Interesting to read a little more about the history of the Vigenere Cipher. Also explains how it is cracked.

Good link from Khan Academy that has good practice questions/explanation of the Vigenere cipher.

For longer ciphers, Kasiski analysis is useful for determining possible keywords using a modified frequency analysis and repetition analysis. This is an online tool to find repeat sequences in a ciphertext, and can be used to quickly decrypt a Vigenere ciphertext.