Letter Number Substitution

The Letter Number Substitution cipher is a simple substitution system in which each letter of the alphabet is replaced by its corresponding numeric position. For example, A=1, B=2, …, Z=26. This cipher is sometimes called the A1Z26 Cipher and is one of the most straightforward methods to convert letters to numbers for encoding messages.

The cipher operates by directly mapping each letter to its numeric equivalent. It is a monoalphabetic substitution cipher, which means each letter always maps to the same number. Spaces and punctuation are typically ignored or preserved in their original form to simplify encoding and decoding.

Letter Number Substitution: Encoding

To encode a message, replace each letter with its numerical position in the alphabet. For example, encoding the message “HELLO”:

Plaintext: H  E  L  L  O
Numeric:   8  5 12 12 15

Ciphertext: 8 5 12 12 15

Notice that each letter is replaced with its corresponding number and spaces are preserved or ignored depending on implementation.

Letter Number Substitution: Decoding

Decoding reverses the process by converting numbers back to their alphabetic equivalents:

Ciphertext: 8 5 12 12 15
Mapping back: 8 → H, 5 → E, 12 → L, 12 → L, 15 → O

Plaintext: HELLO

Letter Number Substitution: Notes

While extremely simple, the Letter Number Substitution cipher demonstrates the basic principle of monoalphabetic substitution and is often used in educational contexts or as a component in more complex encryption schemes. It is highly susceptible to frequency analysis and provides no real security for serious applications.