The ADFGX Cipher is a fractionating transposition cipher which combined a modified Polybius square with a single columnar transposition. It is closely related to the ADFGVX Cipher.

The ADFGX Cipher was a field cipher used by the German Army during World War I. It is a historical cryptographic system that gained prominence during World War I and was used by the German Army to secure their communications. Developed by Lieutenant Fritz Nebel, the ADFGX Cipher was considered one of the most complex and secure encryption methods of its time.

The ADFGX Cipher is named after the 5 possible letters used in the ciphertext: A, D, F, G and X. These letters were chosen deliberately because they sound very different from each other when transmitted via morse code. The intention was to reduce the possibility of operator error.

The ADFGX Cipher is a fractionating transposition cipher, combining elements of both substitution and transposition techniques. It involved first substituting each letter of the plaintext with its corresponding pair of ADFGX letters, and then arranging them in a grid for transmission.

A key component of the ADFGX Cipher was the use of a keyword to generate a mixed alphabet, which determined the order of columns and rows in the grid. This added complexity to the encryption process, making it more challenging for adversaries to decipher the message without the key.

Despite its complexity, the ADFGX Cipher was eventually deciphered by French cryptanalysts, notably Georges Painvin, due to its inherent vulnerabilities and the advancement of cryptanalysis techniques. Nevertheless, its use during World War I underscores its historical significance as a sophisticated cryptographic system employed during a pivotal period in history.

Today, the ADFGX Cipher serves as an important case study in the evolution of cryptography and remains a topic of interest for enthusiasts and historians studying the history of codes and ciphers.