This summer EFF unveiled the sixth limited edition member's t-shirt for the 23rd annual DEF CON, the premier world hacker conference in Las Vegas. This year’s design, like the shirts we produced in 2013 and 2014, includes a puzzle that involves the use of encryption.

The front of this year’s shirt features a long cipher text, displayed in a 1940s typeface:

Here's the string for those following along at home:

5c91dd90f2958c976a73a54aac97f2559eab74b3fd72e7695fd77ba0994d0772bd41510e2b1c61a5a8215ba5b88b617c

In keeping with DEF CON's “film noir” theme for this year, the back of the shirt features an illustration of a fatale-istic detective surrounded by clues. Unpack the clues correctly, and you can decode the cipher text to uncover a secret message!

The detective is seated in front of a device that may be familiar to history buffs. It’s not a typewriter, but rather an Enigma machine, used by the Nazis in WWII to encrypt military communications. The Allies eventually broke the Enigma code with the help of Alan Turing, father of modern computing — a story told recently in the the film The Imitation Game. (Which side is our detective working for? A hint is provided by the inclusion of the Croix de Lorraine on her lapel.)

The original Enigma machine had three rotors with changeable settings, each with three digits. Not coincidentally, a trio of three-digit clues printed in glow-in-the-dark ink becomes apparent when the garment is viewed under a blacklight or in a darkened room:

EFF
DES
II - III - IV

The glow also reveals another cipher text, (WUTZABRN), floating in the steam emerging from a coffee cup marked with a key.

Using an online Enigma emulator, a clever detective can use the three Enigma settings to descramble the key, which reveals the following:

BACKDOOR

The “DES” on the wall was also a clue for the front cipher text, which decrypted (using the key in Electronic Codebook mode) and the “BACKDOOR" key to “Join the resistance. VHTWCMZTYOGZIJRDAUB.” Running this new cipher text back through the Enigma yielded “ENCRYPTALLTHETHINGS” — Encrypt all the things.

Congratulations to our three winners CryptoK, pseudoku, Elegin who successfully solved the puzzle with only minutes left of the conference! Notably, they found that if you swap rotor 2 and 3 in our puzzle you get ISRTPNHW which is an anagram for PWNSHIRT. This seasoned team also solved EFF's 2013 shirt, and won the DEF CON badge challenge for three consecutive years. Nothing brings people together quite like encryption and digital freedom. Join the resistance—encrypt all the things.

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