Deck of Every Card spades puzzle
Getting an X of Spades card from Deck of Every Card yields a string of letters (letter count equals X).
For new players, the string is a subset of the common string "UJLWTTLEJCCQYOIYMWESZWPWFOBUNKWGULYKDQNADDBWXUWKRRRTBIHXBUMWAIZGEDTHJGGUGEWCCCICOPJTNM". It is coded with the Vigenère cipher, the key is "ABCDEFGHIJKLMNOPQRSTUVWXYZ". The deciphered string reads "THIS ONE WAS RELATIVELY EASY GO ASK GRANDPA SEA MONKEE ABOUT STAGE TWO FOR YOUR PERSONALIZED CHALLENGE", without spaces.
After a player asks Grandpa about "stage two" the cards start giving subsets of some different string. The new string is different for each player, but the same for the same player. It is coded with the Playfair cipher, the key is the player's name. The deciphered string reads "GREAT WORK YOU ARE READY FOR YOUR FINAL CHALLENGE ASK ABOUT ??????????? FOR THE LAST STAGE BE SURE TO DESTROY WHEN YOU ARE DONE READING", without spaces, where ??????????? is replaced by a user-specific password. The password is usually (but not always) an 11-letter long English word (such as "copywriters").
After the player asks Grandpa about the player-specific stage two password the cards start giving subsets of the final string, common for all players. The string is "KQAESGDOJDBQCBYQVRSEFATCBMJORWXKCKBVSUYNLICNYBGKUYDIIOOGSHXLEKWROPXYKYSURVCJGYVZCH". It is coded with the Vigenère autokey cipher, the key is the second half of the plaintext. The deciphered string reads "CONGRATULATIONS YOU HAVE COMPLETED THIS PUZZLE GO ASK GRANDPA ABOUT DEEP SPADING FOR YOUR REWARD", without spaces.
- The stage one was decoded on Jul 6, 2015, at that time only the first 3 letters of the ciphertext were unknown.
- The stage two was publicly decoded on Jul 13, 2015.
- The stage three was first publicly decoded on Jul 16, 2015.
- You need to unlock the stage two only once.
- You don't need to draw any cards or own the deck to unlock the stage two.
- HotStuff confirmed that the stage two password is not always 11 letters long, and that due to an error, some passwords that start with T will actually start with "a weird XT".
- Known passwords are as short as 5 letters  and as long as ?.
- It's possible to find the stage two password without discovering the whole ciphertext, by encrypting the cleartext with the password part replaced with some characters, then finding out and decrypting known sequences which are not parts of the resulting ciphertext (it's also possible to guess the password knowing only a part of it). Note that the ciphertext part after the password differs for even-length and odd-length passwords. There's an even clearer explanation of that HERE.
- You can't skip steps by just entering the stage one or three passwords, you need to first unlock them properly.