NWRUG Quiz: The Cryptographic Challenge

Throughout history, cryptography has had dramatic effects on the outcome of wars, monarchies and the lives of individuals. One example of this is the successful cracking of the German Enigma cipher during World War II. This ultimately proved to be instrumental in the Allied forces victory.

In this puzzle you will have the opportunity to try your hand at the ancient art of code-cracking by decrypting a text message that uses two distinct ciphers to render its contents unintelligible.

The Ciphers

The Ceasar Cipher

The earliest and simplest form of ciphers is known as "monoalphabetic substitutions". This is a system of encryption wherein every occurrence of a particular plaintext letter in the alphabet is replaced by a cyphertext letter. This will generate a second alphabet where the 26 letters are in different positions. Juxtaposing the two alphabets will yield a one-to-one mapping of the plaintext letter to its encrypted counterpart.

The Caesar Cipher is an example of such a substitution cipher. It is attributed to Julius Caesar who used it to communicate with his generals. Also known as a shift cipher, it replaces each plaintext letter with a letter that is a fixed number of places down the alphabet. For example, with a shift of three, a B in the plaintext alphabet becomes an E in the ciphertext one.

Here is an example of a full cipher alphabet with a shift parameter of 3:

Plain alphabet:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher alphabet:   DEFGHIJKLMNOPQRSTUVWXYZABC

Placing the two alphabets side by side makes it easy to decrypt a message like:

FRZDUGV GLH PDQB WLPHV EHIRUH WKHLU GHDWKV; WKH YDOLDQW QHYHU WDVWH RI GHDWK
EXW RQFH.

Z. VKDNHVSHDUH, MXOLXV FDHVDU

Which in its plain text form is:

COWARDS DIE MANY TIMES BEFORE THEIR DEATHS; THE VALIANT NEVER TASTE OF DEATH
BUT ONCE.

W. SHAKESPEARE, JULIUS CAESAR

The tricky part of the Caesar cipher is that the shift parameter is often unknown to the code cracker.

The Vigenère Cipher

This cipher was long thought to be unbreakable because, unlike the Caesar cipher, there is no simple one-to-one mapping of the plaintext to the cipher alphabet.

The Vigenère cipher is applied by utilizing a series of different Caesar ciphers based on the letters of a chosen keyword. The ordinal position of each letter in the keyword is what determines the shift parameter values of the various Caesar ciphers used. The resulting cipher alphabets are then applied in sequence and then repeated to encode the letters in the message.

This is perhaps easier to understand with an example.

If we use the word ABLE as the keyword, we can see that the shift values for each letter will be 0, 1, 11, and 4. Note that the letter A produces no shift since its value is 0.

Let's use the Vigenère Cipher to encrypt the following message with the keyword ABLE:

ATTACK AT DAWN

The following four cipher alphabets come into play:

Shift 0 (Letter A):

Plain alphabet:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher alphabet:   ABCDEFGHIJKLMNOPQRSTUVWXYZ

Shift 1 (Letter B):

Plain alphabet:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher alphabet:   BCDEFGHIJKLMNOPQRSTUVWXYZA

Shift 11 (Letter L):

Plain alphabet:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher alphabet:   LMNOPQRSTUVWXYZABCDEFGHIJK

Shift 4 (Letter E):

Plain alphabet:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher alphabet:   EFGHIJKLMNOPQRSTUVWXYZABCD

One trick you can use to facilitate knowing when to apply which cipher alphabet is to repeat the keyword above the encoded message, like so:

Keyword: ABLEABLEABLE
Message: ATTACKATDAWN

Using these four cipher alphabets in circular sequence will produce the following encrypted message:

AUEECL LX DBHR

On to the quiz!

Stage 1. Crack the cipher and decrypt the message

Below you will see a message that has been encrypted with the Vigenère cipher:

EFMZRNQMWOBEBUIXDDMTRDGAXGJUBKNEIVWATHLHJDZUOAOENVIBROCXCSLZUIFUDCSPHSGMDTQTQAUNVSWTVOAKXUPZVNGATAQJQLRVGSIYROSMWSJUBKGATAITHFNVIIZKEOSMWSJUBKUTHWVIYUQXSHPKBNVHCOBSLRRJJSAZFOLHJFMRVKRPDKBNVSOHDYKUZEFPXHPGAOABDBMBRNVYNCCJBNGIPFBOPUYTGZGRVKRHCWWTFIZLJFMEBUPTCOXVEEPBPHMZUEYHVWAZVCFHUGPOCPVGVOVEFOEMDTXXBDHVTRQYPRRXIZGOASVWTCNGAAYETUMJCRBZGOUSVNTFPBCGY

Your job is to decrypt this. The problem is, the keyword has been encrypted with the Caesar cipher! The encrypted keyword is JICAHUHN. It is a common English word. Once you have cracked the encryption and discover the keyword, you should be able to write some code to decrypt the main message.

Regarding the format of the message, note that both the keyword, as well as the message will be in uppercase letters. Also, all spaces and punctuation marks have been removed from the message body to make it harder to crack the encryption.

Stage 2. Identify the source and encrypt the title

Once you have decrypted the message, you should be able to identify its source. The next stage is to write some code that can encrypt a message using the Vigenère cipher and encrypt the title of the source using the same keyword as before.

Stretch Goal: A tougher nut to crack

If you manage to complete both of the tasks above and want a more challenging problem to crack, this is for you. Below is another message encoded with the Vigenère cipher for you to crack:

SOMZAQLLIWSZKBBOGKLBBQJNNVODSKIPVSSQWUIDLGWTBTALGPVSHPETIDAJUVNULYOHVMJRBVZYALWUIQKRDLBUUQAUAQLFAJPUWDCVIXGDIFEAJIWZIZWBQJIFGPWULMMRDVZUKRKOMXHNAVXXWJAHZZZMSAWIJGPLJQSSPPNGDNNVODSKOTGRWCHPVSAQPOIFOFAUEQHPAWIDWYLYWSJYIAPQWVLLZUWLYLKMFZAQCELJERMOGKLVAUFELVMFJWKYUGKGYZWYWNNVODSKOHZQWJANIZLQKTMMJCAEYGAQEAMEGKAHZQKNWYSXALCTGODYETQELFWAQFAQLVAEAZHLBAOPEAMSJYJKXDGENHUEFMXSMBJMCYIYKRNBTKEYCUQRAAAUBAFCOJWYHSPLZBJMCYIYEGJNQESLWYBNWAWBAQARWWXXACOHKOMKQSIFWBGUWIDCZNMFGRDLEAJJZIMOSSOLQFJCMBQDWQORQXDYJKQZYCJBQFQYJKMEHCYPIXDWXLKMMQAPBBJMZBKQKMXQMOLQKMJQSSPFXDGENHUYWPODPAKSXJWZKAEVCEDWRPMILFATAQDTAZIESPPPAFKUESTQFHKFETSRPOMKVMWULIAJHKWULZAABQJ

The problem is, this time you do not know the keyword! The final stage of the challenge is to write some code that can crack the cipher and reveal the message.

(This quiz was adapted from the Secret Messages puzzle on the Puzzle Node website.)