What is isodiphers?

Isodiphers are a type of cipher where the encryption and decryption processes maintain a consistent relationship between the plaintext and ciphertext characters. Essentially, they use a fixed substitution for each letter, allowing for a straightforward mapping that remains unchanged across the text. This simplicity makes isodiphers easy to implement but also vulnerable to frequency analysis and other cryptographic attacks. They are often considered a basic form of encryption and are primarily of historical interest.