Fourth cipher challenge:
Plaintext:
If you can read this, well done! This sort of cipher is called a polyalphabetic cipher. For examples like this one with a keyword of length two, it’s possible to solve it by hand just trying possibilities. However, with a longer keyword this gets very hard. There are more sophisticated techniques, one of which is called Kasiski examination. This involves looking for repeated letters in the ciphertext. It’s likely that the number of letters in between these repeats is a multiple of the keyword length. Look at wikipedia for more information! By the way, if you didn’t work it out, the previous cipher was a keyword substitution cipher with keyword keyword. So far, we’ve only considered substitution ciphers, but there are other alternatives. We could for example reorder the ciphertext.
Encryption method: Vigenere cipher with keyword "PI".
Method of deciphering:
I used the toolkit by nrich to decipher this message. By the hint form the last dcipher challenge, the keyword is of length 2. A useful table of conversion in Vigenere cipher can be found in the Wikipedia page of Vigenere cipher. Like substitution cpher, we find the repeating 3-letter word, and assume it to be 'the', which is the most common word in English.
Actually, I found the solution by another much more faster way. Since many ciphers by nrich have the phrase 'well done', I compared it wtih 'lmat swcm', and found the keyword immediately. It is not a practical way to decipher ciphers, but it may be useful for a ciphers in a series, like this one.