Classical mathematical cipher using linear transformation f(x) = (ax + b) mod 26. Requires coefficient ‘a’ to be coprime with 26 for reversibility. One of the oldest mathematical ciphers based on modular arithmetic. Input domain: every byte is accepted. A-Z and a-z are enciphered in place with their case preserved; every other byte - digit, punctuation, whitespace, control or high-bit - is carried through unchanged, which is the usual pen-and-paper convention and makes the round trip exact for arbitrary input. Nothing is ever discarded.
| Property | Value |
|---|---|
| Category | Classical Ciphers |
| Sub-category | Classical Cipher |
| Security status | 🎓 Educational Only |
| Complexity | Beginner |
| Inventor | Unknown (Ancient) |
| Year | 1929 |
| Origin | 🏛️ Ancient |
| Source | algorithms/classical/affine.js |
Status: 🎓 Educational Only
| Issue | Description | Mitigation |
|---|---|---|
| Frequency Analysis | Letter frequencies preserved, making frequency analysis effective against longer texts | Use only for educational purposes, never for actual security |
| Small Key Space | Only 312 possible keys (12 valid ‘a’ values × 26 ‘b’ values), vulnerable to brute force | Consider as demonstration cipher only |
3 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.
Vector 1 — DCode Reference Test
| Field | Value |
|---|---|
key |
352c33 |
input |
44434f4445 |
expected |
534e565358 |
Vector 2 — GeeksforGeeks Example
| Field | Value |
|---|---|
key |
31372c3230 |
input |
48454c4c4f |
expected |
4a4b5a5a59 |
Vector 3 — Identity Transformation
| Field | Value |
|---|---|
key |
312c30 |
input |
4142434445464748494a4b4c4d4e4f505152535455565758595a |
expected |
4142434445464748494a4b4c4d4e4f505152535455565758595a |