Substitution

The Simple Substitution cipher is one of the oldest and most straightforward encryption methods, where each letter of the plaintext is replaced with a unique corresponding letter or symbol from a fixed ciphertext alphabet. Its origins trace back to classical antiquity, with early examples appearing in the works of Julius Caesar around 58–50 BCE for basic shift substitution, and more formalized systems appearing in Europe during the Renaissance.

The Running Key cipher is a classical polyalphabetic substitution cipher that extends the core idea behind the Vigenère cipher by replacing a short, repeating keyword with a long, non-repeating key text. Instead of cycling a small key like KEY, the cipher uses an entire passage of natural language, such as a book, newspaper, or letter, as the encryption key.

The ROT Cipher, also known as the Caesar Cipher or Caesar Shift, is one of the simplest and earliest known encryption techniques used for encoding messages. It is named after Julius Caesar, the Roman military leader who reportedly used this method to protect confidential communications during his time.

The Rosicrucian Cipher is a mysterious cryptographic method often associated with the Rosicrucian Order, an enigmatic and esoteric secret society dating back to the early 17th century. While the existence of the Rosicrucian Cipher is historically uncertain, it has been the subject of intriguing legends and myths.

Polygraphia is a historical treatise on cryptography and steganography written by Johannes Trithemius, a German abbot and scholar, in the late 15th century. The word polygraphia is derived from Greek, where poly means many and graphia means writing reflecting the treatise's focus on various methods of secret writing and communication.

The Polybius Square cipher is a classical substitution cipher invented by the ancient Greek historian and scholar Polybius around 200 BCE. It converts letters into pairs of numbers based on a 5×5 square grid, allowing plaintext to be transmitted using numbers instead of letters. Traditionally, the letters I and J are combined to fit the 25-cell grid.

The Polyalphabetic cipher is a classical encryption method that uses multiple substitution alphabets to encode plaintext, significantly increasing security compared to monoalphabetic ciphers. It was first formalized by the Italian cryptographer Giovan Battista Bellaso in 1553 and later popularized by Vigenère in 1586 through what is now called the Vigenère cipher. In a polyalphabetic system, a keyword determines which substitution alphabet is applied to each plaintext letter.

The Playfair Cipher is a manual symmetric encryption technique that encrypts pairs of letters (bigrams) instead of single letters. It was invented by Charles Wheatstone in 1854 but became known as the Playfair Cipher after it was promoted by Lord Playfair. This cipher was used extensively during the World War I era for secure military communications, as it provided better security than simple substitution ciphers by addressing frequency analysis vulnerabilities.

The Pigpen Cipher, also known as the Masonic Cipher or Freemason's Cipher, is a simple substitution cipher used for encoding secret messages. It has its roots in the ancient practice of symbolic writing and is often associated with Freemasonry, a fraternal organization known for its use of secret symbols and rituals.

Ottendorf Cipher is a classical book cipher technique that encodes a secret message by referencing the positions of words or letters within a pre-agreed text, typically a book, newspaper, or any shared document. Each element of the ciphertext specifies a page, line, and word (or sometimes letter), allowing the recipient to reconstruct the message by locating the indicated elements. The strength of the Ottendorf Cipher relies entirely on the secrecy of the chosen text and the agreed-upon indexing scheme.