After seeing the note on Reddit, amateur cryptographers deduced that the note was written in a code called a bifid cipher. The two rows of letters on the back were the key.
So what does the message say? The first message said (as decoded by several Redditors):
"There's plenty more money to make figure this out and prepare to meet july 19 fifty sixth and sixth hot dog stand outside rue57 cafe ask for mister input."
After the Redditors decoded that message, though, delverofsecrets said another message came, also encrypted, that said:
"YOU HAVE MANAGED TO FIND THE MESSAGE WITH THE HELP OF FRIENDS. YOU CHANGED THE RULES NOW SO WILL I. JULY TWELVE FOUR PM. FIND THE BLUE JAY AT SIX AND A HALF AND FIFTY SIXTH AND TELL HIM YOU ARE THE LAST."
HOWSTUFFWORKS: How Code Breakers Work The second message came from a Redditor called "hoboman" and other Redditors noted that the username led to an IP address at a military base in Arizona -– one that specializes in intelligence training Bifid ciphers were invented around the turn of the 20th century. To make one it's only necessary to make a grid of the alphabet (usually it's 5×5 in English, with the I and J combined). Each letter is represented by a two-number coordinate. So, for instance, "A" would be 11, and "B" would be 21 (assuming you put the column first). The letters of the message are written in a line with the coordinates put below them vertically, like this:
Y O U R C O V E R I S B L O W N 4 4 5 2 3 4 1 5 2 4 3 2 1 4 2 3 5 3 4 4 1 3 5 1 4 2 4 1 3 3 5 3 The numbers are written out in a row, and paired: 44 52 34 15 24 32 14 23 53 44 13 51 42 41 33 53 Each pair is a new letter as per the original square: T K S E I/J H Q M P T... and so on.
To make it harder to crack some people might shift the letters, subsituting a letter for another a certain number of steps down the alphabet (such as putting G in place of A, a seven-step shift) or arrange them randomly in the square.
PHOTOS: Cryptic Codes Yet To Be Cracked
Such a coding scheme isn't all that sophisticated by modern standards and it seems odd that anyone connected with the Department of Defense would approach anyone like this. But that's what makes this kind of puzzle as much fun as it is.
Reddit is rife with speculation in the comments about what this could be -– everything from a job offer from a mysterious company to a marketing scheme.
Only time (and a meeting on a New York street) will tell. Stay tuned.
UPDATE 7/12/12 6 p.m.: At the corner of Six and a Half and 56th street (the avenue is a pedestrian walkway) a crowd of perhaps two hundred gathered. Some were following the Reddit thread in real-time, and the original poster, delverofsecrets, said he would be arriving, even though he was nearly an hour late.
Delverofsecrets turned out to be Ali, who declined to give his last name (although some enterprising Redditors could probably dig it up). He showed the note, proving it was in fact him, and repeated the story of the man handing him the note and the $50 bill on the subway. Asked about the meeting today, he said he didn't know about that either. "It might be a friend of mine trying to mess with me," he said.
Ali added that he didn't expect that many people to show up. "I thought maybe 10 people might be here." As to why he arrived closer to 5 p.m. than four, "I'm just chronically late."
Nobody ever solved the mystery of "Bue Jay." A man dressed in blue, who was raggedy-looking, did appear near the corner at just after 4 p.m. After being briefly mobbed, though, it seemed he wasn't the right person, "Mr. Input."
As for the meeting next week, Ali said he plans to be there, though he might be away as his family plans a vacation then. ("My parents surprised me," he said).
What about all the attention? "Well, I hope I don't become a meme."
via Reddit, Mashable
Image: Reddit / delverofsecrets