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 cipher exemplifies early 20th-century manual encryption methods aimed at secure and reproducible coding of messages.
To encrypt a message with the D’Agapeyeff Polybius Cipher, a Polybius square is constructed using the keyword. In a 5×5 square, letters of the keyword KEY are placed first, omitting duplicates, followed by the remaining letters of the alphabet (typically combining I and J). Each letter is then represented by its row and column coordinates. For example, the plaintext HELLO is mapped to numeric pairs based on the keyworded square: H=25, E=12, L=32, L=32, O=35, resulting in the ciphertext 25 12 32 32 35. Decryption requires reconstructing the same keyed square and reversing the coordinate mapping to recover the original message.
The strength of the D’Agapeyeff Polybius Cipher lies in its combination of a Polybius square with a keyword. The keyword modifies the standard letter ordering, producing a non-linear numeric substitution that obscures letter frequencies and resists basic frequency analysis. Even short messages benefit from this keyed variation, as repeated letters may map to different positions depending on the square’s arrangement, increasing diffusion. Despite these enhancements, it remains a manually executable cipher, emphasizing clarity and reproducibility over modern cryptographic security.
Historically, the cipher served as both an instructional tool and a practical encryption method in the early 20th century. D’Agapeyeff included it in his 1939 work to illustrate keyed Polybius square encryption, providing a hands-on example for students of cryptography. Its design reflects the experimental phase of classical ciphering where numeric substitution and keyword-based obfuscation were combined to strengthen traditional methods.
Encrypting the word HELLO with the keyword KEY produces the numeric sequence 25 12 32 32 35, showing how each letter is converted to a coordinate pair within the keyed Polybius square. The D’Agapeyeff Polybius Cipher demonstrates the effectiveness of keyworded numeric substitution, illustrating how classical ciphers can incorporate keys to produce more secure and less predictable ciphertext while remaining entirely manual in execution.