+"""Enciphering and deciphering using the [affine cipher](https://en.wikipedia.org/wiki/Affine_cipher).
+Also attempts to break messages that use an affine cipher.
+
+The affine cipher operates one letter at a time. It converts each letter to a
+number, then enciphers that number using a multiplier and a number. The result
+is taken mod 26 and converted back into a letter.
+
+For a multiplier _m_ and adder _a_, a letter converted to number _x_ is
+enciphered as _E(x)_ = (_m_ _x_ + _a_) mod 26. Deciphering uses the modular
+inverse of _m_, _m_⁻¹, so that _D(x)_ = _m_⁻¹ (_x_ - _a_) mod 26.
+
+If `one_based` is `True`, the conversion between letters and numbers maps
+'a' → 1, 'b' → 2, … 'z'→ 26 and the `mod` function is adjusted to keep
+numbers in this range during enciphering and deciphering. If `one_based` is
+`False`, the conversion maps 'a' → 0, 'b' → 1, … 'z'→ 25 and `mod` behaves
+normally.
+"""
+