Affine Cipher Decoder

The tool executes affine decryption by applying the modular inverse of a to recover the original message. It accepts ciphertext, a, b, and m, with optional alphabet selection and case handling. It validates that a and m are coprime before computing the inverse. Conceptually, each character is mapped to an index in 0..m-1, the inverse key a_inv is computed, and the plaintext index is P = a_inv*(C - b) mod m. The result is converted back to characters, honoring the chosen alphabet and case policy. Non-letter characters can be preserved or mapped according to configuration, but are not transformed by the arithmetic. Advanced options allow custom alphabets whose length equals m, enabling decoding for nonstandard Latin alphabets or custom symbol sets. This tool is suited for education, puzzle solving, and rapid verification of affine-keyed messages. It emphasizes correctness, mathematical validation (gcd(a,m) = 1), and clear error reporting when the key is non-invertible or inputs are out of range.