In a substitution cipher, one letter of the alphabet is substituted for another one.
Using this basic code, HELLO becomes URYYB.
In a telephone keypad cipher, letters of the alphabet are used to represent numbers as they appear on a common telephone keypad.
The letters Q and Z can represent either 0 or 1, which typically don't have any letters on a phone keypad. With this code, the telephone number (202) 324-5678 can be written any of the following ways:
B Q B D A G K M R V
C Q A F B I J N P X
A Q B E C H L O S T
The pigpen cipher or masonic cipher turns letters into fragments of a grid.
With this code, the letter U becomes the symbol > based on its position in the grid.
The four square cipher is a little more complex. It involves matching up coded letters to their counterparts in a series of grids.
This code breaks down messages two letters at a time.
Take the message "HELLO," for example: the first two letters are HE. Someone using this code would find H in the upper left grid and E in the lower right grid. Then, they would draw an imaginary rectangle connecting the two letters, and substitute the original letters with whichever other two letters the corners of the rectangle land on.
In this case, HE would be converted to FA. Using the same grid, the message "PLEASE HELP" would be converted to GL EO TY FA NF.
Not every code is so easily solvable. In fact, the following code has confounded the FBI for 19 years.
Authorities found a note containing the above code on the body of murder victim Ricky McCormick in 1999. The FBI considers it the only clue regarding the homicide, but it has been unable to crack the code since then, and has even enlisted help from the American Cryptogram Association and members of the public.