What is a Simple Substitution Cipher?

A simple substitution cipher (also known as a monoalphabetic substitution cipher) is one of the oldest and simplest encryption techniques. Each letter in the plaintext is replaced by a different letter according to a fixed substitution rule. The entire alphabet is mapped to a shuffled alphabet key.

Plain: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Key: Z E B R A S C D F G H I J K L M N O P Q T U V W X Y

Using this key, "HELLO" becomes "SVOOL" — each letter is substituted with the corresponding letter from the key alphabet. Decryption reverses the process using the same key.

The simple substitution cipher is vulnerable to frequency analysis attacks, as letters like 'E', 'T', and 'A' appear more frequently in English and can be identified by their repeated occurrences .

Key features:

• Custom key — you can create your own substitution alphabet
• Encrypt/Decrypt — both operations supported with the same key
• Random key generation — quickly create a secure random substitution
• Preserves case — maintains uppercase/lowercase formatting
• Non-alphabetic characters — spaces, numbers, and punctuation remain unchanged

What is the Luhn Algorithm?

The Luhn algorithm (also known as the "mod 10" or "modulus 10" algorithm) is a simple checksum formula used to validate various identification numbers, such as credit card numbers, IMEI numbers, and Social Security numbers [citation:1][citation:5].

The algorithm was developed in 1954 by Hans Peter Luhn, an IBM researcher, and is now in the public domain [citation:2][citation:6]. It is widely used by major payment systems including Visa, Mastercard, American Express, and Discover [citation:6].

How it works:

1. Starting from the rightmost digit, double every second digit.
2. If doubling results in a two-digit number, sum those digits (or subtract 9).
3. Sum all digits (doubled and non-doubled).
4. If the total is divisible by 10, the number passes the Luhn check [citation:6].

Key characteristics:

• Detects — most single-digit errors and adjacent digit transpositions
• Does not detect — transposition of 09 to 90, and some double errors [citation:7][citation:8]
• Not cryptographic — it's a checksum, not an encryption method [citation:5][citation:6]

The algorithm is widely applied to bank card numbers, IMEI codes, SIM card ICCIDs, and Canadian Social Insurance Numbers [citation:1][citation:4][citation:5].

What is the Vigenère Cipher?

The Vigenère cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution. It was invented by the French diplomat Blaise de Vigenère in the 16th century, though it was incorrectly attributed to him — the cipher was actually first described by Giovan Battista Bellaso in 1553.

The cipher uses a keyword to shift letters of the plaintext. Each letter of the keyword determines the shift for the corresponding letter of the plaintext:

C_i = (P_i + K_i) mod 26

Where C_i is the ciphertext letter, P_i is the plaintext letter, and K_i is the keyword letter (A=0, B=1, ..., Z=25). The keyword is repeated to match the length of the plaintext.

The Vigenère cipher was considered "le chiffre indéchiffrable" (the unbreakable cipher) for centuries because it resists simple frequency analysis attacks. However, it was eventually broken in the 19th century by Charles Babbage and Friedrich Kasiski using the Kasiski examination and frequency analysis of repeated key patterns.

Key features of this calculator:

• Encryption — converts plaintext to ciphertext using the keyword
• Decryption — converts ciphertext back to plaintext using the same keyword
• Preserves case — uppercase stays uppercase, lowercase stays lowercase
• Non-alphabetic characters — spaces, numbers, and punctuation remain unchanged

The Vigenère cipher is still used today in recreational cryptography and as an educational example of polyalphabetic substitution ciphers.