List of cryptograms/Episodes

Caesar cipher
The Caesar cipher used in Gravity Falls substitute the original letter for the third letter before it. In the case for letters X, Y, and Z you must cycle through to the beginning of the alphabet.

Atbash cipher
Atbash ciphers are decoded by reversing the letters, an example includes that an A turns into a Z.

A1Z26
The A1Z26 cipher is a simple substitution cipher decoded by substituting the nᵗʰ letter of the alphabet for given number n.

Author's symbol substitution cipher
In journal 3, a symbol substitution cipher is used. The symbols are hidden on many pages of the journals. *Note: The symbols for J, Q, X, and Z are currently unknown.

Combined cipher
A combined cipher is a mix of two or more ciphers seen in the show. The first time such cipher has been used is at the end of "Gideon Rises." It's solved by converting to letters using the A1Z26 cipher, then flipping the letters with the Atbash cipher, and finally by using the Caesar cipher. Season 2's combined ciphers start with the Vigenère cipher.

Number codes
These codes are solvable by taking the number beside a parenthesis as an episode number, and the other numbers beside them represents a letter in that episode's credits cryptogram.

Vigenère cipher
Vigenère cipher is a series of Caesar ciphers where each letter shift depends on a key word. Vigenère ciphers use a Vigenère square, like the one below, to encrypt the message. The square is 26 rows of Caesar ciphers starting with a zero letter shift. On each additional row, the shift of the letters is increased by 1.

As Vigenère ciphers a bit more complicated, consider the following example: we want to encrypt the message MABELEATSSPRINKLES and use GRAVITY as the key. We would repeat the key word until it matched the length of the message (in this case, GRAVITYGRAVITYGRAV). Using the table above, we match the message letter to a row and the corresponding key letter to a column and find the letter at their intersection on the table. For example, the row starting with M and the column starting with G intersect at S. The row starting with A and the column starting with R intersect at R. So using our message and the key, the we'd create the following cipher:

Note that any time the message is A, the cipher letter will be the same as the key letter and vice versa.

Another way to think of Vigenère ciphers is that each letter of the key corresponds to a Caesar shift number (A=0, B=1, C=2, etc.) and for each letter of the message you form a Caesar shift based on the corresponding key letter.

Bill's symbol substitution cipher
In the book Dipper's and Mabel's Guide to Mystery and Nonstop Fun!, Bill tells readers "I bet you can't even figure out the code I've hidden throughout this book." The symbols are similar to the author's symbol substitution cipher.

Shorts

 * The Caesar cipher combined, is a clue on how to solve the number code. Together you get, "FROM THE FIRST UNTIL THE LAST SEARCH THE CODES OF CREDITS PAST ONE MEANS ONE SO SEARCH THEM ALL WELCOME TO GRAVITY FALLS."
 * When the number codes are combined, you get the message: "I WAS SO BLIND. HE LIED TO ME. THE DARKNESS IS NEAR."