In the 1930s, a US newspaper introduced an interesting way of making their advertisements more attractive. They published two Playfair cryptograms, the keys of which were hidden in an ad on the same page.
The Cipher Ads from Asbury Park
In the advertisement section of a US newspaper from the 1930s I have now found a number of cryptograms that are used for advertisement in a very interesting way. In order to win a prize, the reader needs to solve a double cryptogram, with the key used being hidden somewhere in the advertisements on the same page – certainly a good way to make readers take a close look at the advertisements.
Interestingly, the encryption method used here is not a simple substitution, like it is used in most other advertisment campaigns involving crypto, but a Playfair cipher.
The newspaper in question is the Asbury Park Evening Press, a former daily newspaper published in the town of Asbury Park, NJ. So far, Asbury Park’s claim to faim was that rock singer Bruce Springsteen spent a few years of his life there and that his debut album is titled Greetings from Asbury Park, NJ. The column the cryptograms were published in was named “Cipher Ads”
The “Cipher Ads” issue I am going to introduce in the following was published on March 27, 1935. Here is a scan (for a very high resolution version check here):
There are two cryptograms to be solved:
ET TA GG DC VC WF GM HC MD RE OH GP LZ SS GM(G)
PR UR NQ PR HT LC PG GL HU NE TB EG JD WF OJ AL GM(E)
Here are the rules the solver needs to follow:
What is written here means that the two cryptograms have been created with a variant of the Playfair cipher. The rules describe the decryption process, not the encryption process. If you know how the Playfair cipher works (for instance, because you have read yesterdays’s blog post), you can skip the next paragraph.
The Playfair cipher
Usually, the Playfair cipher is defined as follows:
The Playfair cipher substitutes letter pairs. So, the cleartext needs to be written as a sequence of letter pairs (the following cleartext is a Shakespeare quote taken from Robert Thouless’ life-after-death experiment):
BA LM OF HU RT MI ND SG RE AT NA TU RE SS EC ON DC OU RS EC HI EF NO UR IS HE RI NL IF ES FE AS T
The Playfair cipher requires that no letter pair consist of two equal letters. Therefore, we add an X between the two Ss:
BA LM OF HU RT MI ND SG RE AT NA TU RE SX SE CO ND CO UR SE CH IE FN OU RI SH ER IN LI FE SF EA ST
If the number of letters in the cleartext was odd, another X would have to be added at the last position, but this is not necessary here. Next, we choose a keyword: SURPRISE. Now, we set up a 5×5 matrix, which starts with the keyword (repeating letters are omitted), followed by the remaining alphabet (I and J are considered equal in order to get an alphabet of 25 letters):
S U R P I E A B C D F G H K L M N O Q T V W X Y Z
Now, we replace the cleartext letter pairs (BA, LM, OF, HU, …) according to the three Playfair rules. Here are the rules in a diagram (not that these are the encryption rules, while the newspaper ad describes the decryption, which is the opposite process):
Here are the same rules as a text (I refer to the letter pair to be replaced as XY):
- If X and Y are not in the same column and not in the same row (this is the most frequent case), form a rectangle and replace the two letters by the other two corner letters (the upper cleartext letter is replaced by the other upper letter in the rectangle, the lower cleartext letter by the lower one). For instance, LM becomes FT.
- If the two letters stand in the same row, each one is replaced by its right neighbor. Here, BA becomes CB.
- If the two letters stand in the same column, each one is replaced by its lower neighbor. In our example, AN becomes GW.
When we apply the Playfair rules on our cleartext with our 5×5 matrix, we get the following ciphertext:
CB FT MH GR IO TS TA UF SB DN WG NI SB RV EF BQ TA BQ RP EF BK SD GM NR PS RF BS UT TD MF EM AB IM
Let’s come back to the Cipher Ads challenge. The decrypting process described above refers to standard Playfair, with one exception. When two letters are in the same line they are decrypted by taking the right neighbors, while standard Playfair requires that the left neighbor is taken for decryption.
As in mentioned in the rules, the keyword used has eight letters and starts with a P. This word appears somewhere in one of the advertisements below the cryptograms.
Can a reader solve this challenge?
If so, you can try your luck at another Cipher Ads challenge. The following one was published in the Asbury Park Evening Press on March 27th in the same year (click here for a high resolution version):