Obfuscation

The D’Agapeyeff Polybius Cipher is a classical cipher introduced by British cryptographer Alexander D’Agapeyeff in his 1939 book Codes and Ciphers. It combines the principles of a Polybius square with a keyword to create a keyed numeric substitution system. Each letter of the plaintext is converted into numeric coordinates within a square grid based on the keyword, and this keyed approach modifies the standard Polybius mapping, adding complexity and obfuscation.

The Columnar Transposition cipher is a classical transposition cipher widely used in the 16th and 17th centuries, though its origins are often attributed to various European cryptographers experimenting with letter rearrangement methods. Unlike substitution ciphers, the Columnar Transposition cipher does not alter the letters themselves but instead rearranges their order according to a predetermined key. The cipher’s security relies on the secrecy of the key, which determines the column order used to read off the message after writing it in rows.

The Zigzag Cipher is a classical transposition cipher that rearranges the letters of a message by writing them in a back-and-forth, diagonal pattern across multiple rows, then reading them off row by row to produce the ciphertext. It is most commonly known today through its rail-based variant, which was popularized in the 19th century and is often associated with early military and telegraph use.

The Straddling Checkerboard cipher is a classical cipher system invented by the American cryptographer George Washington Bazeries in the late 19th century and later popularized in manual cipher systems throughout the 20th century. It is a type of substitution cipher that converts letters into single or double-digit numbers using a configurable grid, combining both simplicity and efficiency.

The Semaphore cipher is a visual signaling system that encodes letters as distinct physical positions rather than transforming text through mathematical substitution. Although often grouped with ciphers for convenience, Semaphore is more accurately described as a manual encoding system, where meaning is conveyed by the position of flags, arms, or mechanical indicators.

The M-209 cipher is a mechanical rotor-based cipher device developed by the American engineer Boris Hagelin and adopted by the U.S. military in 1941. It was a portable, hand-operated machine designed for tactical battlefield communications, encrypting messages character by character using a combination of rotors, a rotating drum with movable pins, and a set of lugs to generate a polyalphabetic substitution.

The Knapsack cipher, also known as the Merkle–Hellman knapsack cryptosystem, is an early public-key cryptosystem introduced by Ralph Merkle and Martin Hellman in 1978. It is based on the subset-sum problem, where the goal is to select numbers from a set that sum to a target value. The cipher uses a private superincreasing sequence, then applies a multiplier m and a modulus n to produce a public key.

The Keyed Caesar cipher is a variation of the classical Caesar cipher, adapted to increase security by combining a keyword with a shift-based substitution. The original Caesar cipher, attributed to Julius Caesar around 58–50 BCE, shifts each letter of the alphabet by a fixed number of positions. In the Keyed Caesar cipher, a keyword is first used to create a modified alphabet: the letters of the keyword are placed at the start of the alphabet (omitting duplicates), followed by the remaining unused letters in standard order.

The Kangaroo cipher, sometimes referred to as a jumping key cipher, is a classical polyalphabetic cipher whose exact origins are uncertain, but which appeared in cryptographic literature and practice in the late 19th century to early 20th century. Unlike fixed-key polyalphabetic systems such as the Vigenère cipher, the defining feature of the Kangaroo cipher is that the key does not advance in a simple, linear fashion. Instead, it “jumps” through the key according to a predefined rule, much like a kangaroo hopping unevenly forward.

The Blowfish cipher is a symmetric-key block cipher designed by Bruce Schneier in 1993 as a fast, secure alternative to existing encryption algorithms like DES. It operates on 64-bit blocks of plaintext and uses variable-length keys ranging from 32 to 448 bits. Blowfish employs a Feistel network structure consisting of 16 rounds of substitution and permutation.