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Four-Square Cipher

Classical polygraphic cipher using four 5x5 squares for digraph encryption, offering enhanced security over simple substitution ciphers.

Properties

Property	Value
Category	Classical Ciphers
Sub-category	Polygraphic Cipher
Security status	🎓 Educational Only
Complexity	Intermediate
Inventor	Felix Marie Delastelle
Year	1902
Origin	🇫🇷 France
Source	algorithms/classical/foursquare.js

Security

Status: 🎓 Educational Only

No vulnerabilities are recorded for this implementation.

Documentation

• Four-Square Cipher Wikipedia
• Practical Cryptography Tutorial

References

• Classical Cryptography Guide
• Delastelle Cipher Systems

Test vectors

3 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.

Vector 1 — Keywords EXAMPLE and KEYWORD over one digraph pair, J folded onto I. No published source carries this value

Field	Value
key	4558414d504c452c4b4559574f5244
input	48454c50
expected	46594e46

Vector 2 — Keywords with repeated letters - FORTIFICATION and BATTLE both dedupe. No published source carries this value

Field	Value
key	464f5254494649434154494f4e2c424154544c45
input	41545441434b41544441574e
expected	54504d4c49465450464c584b

Vector 3 — Odd-length message padded to an even number of digraphs with X. No published source carries this value

Field	Value
key	4a4f484e2c5041554c
input	424541544c4553
expected	41414e4f50505358

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