Hawkynt

Bifid Cipher

Fractionating cipher invented by Félix Delastelle in 1901. Combines Polybius square with transposition, replacing each letter with two coordinates then rearranging them in blocks. Significantly stronger than simple substitution ciphers.

Properties

Property	Value
Category	Classical Ciphers
Sub-category	Classical Cipher
Security status	🎓 Educational Only
Complexity	Intermediate
Inventor	Félix Delastelle
Year	1901
Origin	🇫🇷 France
Source	algorithms/classical/bifid.js

Security

Status: 🎓 Educational Only

Known vulnerabilities

Issue	Description	Mitigation
While more resistant than monoalphabetic ciphers, still vulnerable to frequency analysis with sufficient text	—	Use variable block sizes and longer keywords
Custom keyword grids can sometimes be recovered through cryptanalysis	—	Educational use only - not suitable for actual security

Documentation

• Wikipedia Article
• Historical Context
• Educational Tutorial

References

• dCode Implementation
• Practical Cryptography
• CrypTool Portal

Test vectors

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

Vector 1 — Wikipedia worked example - square BGWKZ/QPNDS/IOAXE/FCLUM/THYVR, FLEEATONCE taken as one block

Field	Value
key	4247574b5a51504e4453494f41584546434c554d54485956522c3130
input	464c454541544f4e4345
expected	5541454f4c5752494e53

Vector 2 — Plain A-Z square, period 5. The Wikipedia article carries no value for this input; the vector covers the unkeyed square

Field	Value
key	35
input	48454c4c4f
expected	464e4e5644

Vector 3 — Keyword CIPHER, period 3. dCode carries no value for this input; the vector covers a keyed square and a block shorter than the message

Field	Value
key	4349504845522c33
input	41545441434b
expected	445154524b49

Vector 4 — Period 1 - a single-letter block is its own coordinate pair, so one letter passes through unchanged

Field	Value
key	31
input	41
expected	41

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