Foundations

Alphabets and symbol values

How a cipher chooses the symbols it can use and gives each one a position.

Also found as alphabet indexing · A equals zero · symbol inventory

An alphabet is an ordered list

In a cipher, the alphabet is the list of symbols the method can change, in a fixed order. The order gives every symbol a position, and the number of symbols tells the arithmetic where to wrap. That makes a 26-letter Latin alphabet and a 25-letter I/J square different systems, even though most of their letters match.

The common 26-letter numbering

Symbols: A B C … X Y Z
Values: 0 1 2 … 23 24 25

Numbering from zero keeps the modular formulas short. A source that numbers A as 1 can describe the same substitution, although the written formula will look different.

Characters outside the alphabet

You also need to know what happens to spaces, punctuation, case, accented letters, digits, and other characters. A cipher may leave them alone, remove them, give them positions of their own, or use them only when displaying the result. This can change how the Key lines up even when the letter formula stays the same.