Imagine a character grid, six rows by six columns. There are 36 positions. In the first 26 positions are the letters of the alphabet. The numbers 0 through 9 are in the final positions. Now you have a perfectly simple encoding device: identify a row and a column in order to specify any character.

This coding mechanism has the advantage of being simple to use, either to encode or decode a message. (Its disadvantages are many, including the lack of any punctuation.) Every character requires two symbols, from a set of (at least) six symbols. The better you can conceal the meaning of the six symbols, the more difficult it will be for someone who doesn’t understand the code to break it.

In the novel I’m currently writing, the characters use the numbers on a digital clock to create a key for the six symbols. When you look at a digital clock, the time is shown using the numbers 0-9 in four positions. For each digit, there are seven small LED bars used to make up the shape of the number. Four of the bars are vertical: two on the left and two on the right. The other three bars are horizontal: top, middle, and bottom.

The number 8 uses all seven bars. In this code key, 8 is used to represent the first row or first column.

Six bars are used to form three different numbers on a clock: 0, 6, and 9. In our code key, it’s a dealer’s choice which to use to represent the second row or second column. Use a 0, 6, or 9 randomly.

Similarly, the digital clock has three numbers that use five bars apiece: 2, 3, and 5. Use 2, 3, or 5 to represent the third row or column. This “shuffle” element in the second and third positions contributes to the opaque nature of the code. Ten characters are used to represent only six positions.

Using four bars, the number 4 represents the fourth row or fourth column. The number 7 uses three bars; it signifies the fifth row or column. Finally, all that remains is the character that only uses two of the digital clock’s LED bars: the number 1, therefore used to denote the sixth row or sixth column.

87-77-77-91, 88-05-20’46 62-40?