In a Caesar cipher, also called a Caesar shift, Caesar substitution cipher, or shift cipher, you shift the values of the letters in the message by some fixed amount. In the original Caesar cipher, Julius Caesar reportedly used a shift of three to send secret messages to his commanders, so each letter was replaced by the letter that comes three positions later in the alphabet. The letter A was encrypted as D, B was encrypted as E, and so forth. Letters at the end of the alphabet wrap around to the beginning so, for example, X becomes A, Y becomes B, and Z becomes C. In this example, the shift value, 3, was the cipher's key.

Write a program that uses a Caesar cipher to encrypt and decrypt messages. Let the user enter some text and a shift and then click a button to encrypt the message. Let the user then enter a new key and click another button to decrypt the message. Verify that decryption works only when the shift is correct.

To make this easier, write extension methods that encrypt and decrypt strings, strip punctuation and spaces from strings, and break strings into five-grams.