The Letter Number Substitution cipher is a classical substitution method in which each letter of the alphabet is replaced by a corresponding number. Its origins are difficult to trace precisely, as variations of letter-to-number mapping have existed since ancient times, but it became widely referenced in cryptographic manuals during the 19th century for educational and puzzle purposes. The method provides a simple way to transform text into a numeric sequence while remaining reversible, making it suitable for manual encryption and early secret messaging.
In the Letter Number Substitution cipher, letters are assigned numbers according to a predefined scheme, most commonly A=1, B=2, …, Z=26. To encrypt a message, each letter of the plaintext is replaced by its numeric equivalent. For example, to encrypt the word HELLO: H=8, E=5, L=12, L=12, O=15. The resulting ciphertext is the numeric sequence 8 5 12 12 15. Spaces or delimiters may be used between numbers to aid readability. Decryption reverses this process by mapping each number back to its corresponding letter.
The Letter Number Substitution cipher is simple to use but offers very limited security. Since each number directly represents a letter, the frequency of letters in the ciphertext mirrors that of the plaintext, making it vulnerable to frequency analysis. Despite this, it is valuable for educational purposes, illustrating the basic principle of substitution and numeric encoding, and can be combined with other ciphers—such as transposition or polyalphabetic systems—to enhance security.
Variants of the cipher include using a shifted number sequence (e.g., A=3, B=4…) or grouping numbers to represent multiple letters, which introduces a basic form of obfuscation. Historically, the system was employed in puzzle books, telegraph shorthand, and simple encoded correspondence, highlighting early attempts to use numeric representation as a method for concealing messages without complex machinery.
Encrypting a simple word like HELLO into 8 5 12 12 15 demonstrates the cipher’s core concept: each plaintext letter is represented by a number, preserving the order while transforming the message into a numeric format. The Letter Number Substitution cipher remains a clear example of classical substitution, teaching the foundations of encoding letters into alternative representations and providing insight into the historical development of cryptographic methods that preceded modern digital encryption.
See A1Z26
Letter Number Substitution Cipher