Vital?

Have you managed to solve the entire Stage 5 Cipher Challenge? Solutions are now closed, but perhaps you want to take up the full challenge.

Successful solvers of this part were

Josip from Australia

Joseph from Hong Kong

(and probably also Patrick from Woodbridge School and maybe others - I accidentally forgot to leave the submit a solution tab open!)

Josip give the following nice advice

Ok, so I recently started going some cryptography in my spare time after

entering a YouTube competition. After a few hours, I finally caught on the

way to step-by step solve the puzzles. For this particular puzzle, I got

the hint to use frequency analysis from the previous puzzle. Instead of

taking a trip down the tedious path and going through and looking at the

repetition of each letter and word pattern, I used this handy little

website. http://www.richkni.co.uk/php/crypta/freq.php To compliment me with

switching letters back and forth, I used

http://25yearsofprogramming.com/fun/ciphers.htm which also contains a

"FA", but makes it much easier to replace letters and see

immediate results. (I always compare frequency in the text to frequency in

theory and try to match them.)

The following is an example I use to step my way to more efficiently

cracking the code; efficiency is everything. This gave me the vital letter

frequency, as well as 2,3,4,5 and 6 letter repetitions! I immediately

looked at the 3 letter repetition section to notice "qao". I

automatically assumed this to be the word "the", as it is many

times. Now that I have those three letters, I apply this to the six letter

section in hope to get something out. The "goqqon" word gets my

attention because it contains two pairs of like letters. Also because we

know that o=e, it significantly helps. Now, I thought to myself in the

context of the previous two puzzles, 'what could _eXXe_' be? (with XX being

a double letter). It didn't take long for me to think of 'letter'!! I also

noticed that it was able to be plurals, so I added an 's'. From here,

things got slightly easier. With the help of frequency analysis, I tried to

strip the less common letters and replace them with more popular ones, and

sometimes with luck. Research in word patterns I thought benefited me

hugely, and so I recommend them too;

http://scottbryce.com/cryptograms/stats.htm.

I started strongly from around the centre of the puzzle, with "ALL THE

LETTERS IN A MESSAGE IS THAT" and then slowly edged my way to find

smaller words in between that made things clearer. As I write my

'confirmed' (used) letters down on paper, I have a remaining list which can

still be rearranged anywhere without affecting the already made words. This

is really good for times when you are stuck, (as I was). You can simply

change a few letters and hopefully see a pattern that makes you think.

Sometimes you can try to fill in the sentence with your own words also. e.g

"THIS IS CALLED A _____ _____". I knew from the context that the

puzzle was referring to another cipher (which also gave away the word

'cipher') but quickly researched a few cipher names to reveal a nice fit of

'vigenere'. The other tricky part was arranging the less common letters.

However, sometimes the surrounding letters gives it away, such as

"freQuency". Also, when you have words already made, spacing them

helps significantly.

The solution is:

The last message was encrypted using an affine cipher, 'a' to 'q', 'b' to 'z', 'c' to 'i', i.e. moving on nine letters each time. The problem with just using one cipher to encrypt all the letters in a message is that we maintain the frequency of letters in the plaintext, so we're able to use frequency analysis to help decipher it. We could instead use different alphabets for different parts of a message. For example we could encrypt the odd letters with one cipher and the even letters with another. This is called a Vigenere cipher with keyword length two. Good luck!

keyword cipher:

abcdefghijklmnopqrstuvwxyz

KEYWORDABCFGHIJLMNPQSTUVXZ