What is the key difference between monoalphabetic and polyalphabetic ciphers?

The key difference between monoalphabetic and polyalphabetic ciphers lies in their substitution methods. Monoalphabetic ciphers utilize a single, fixed letter-to-letter substitution throughout the entire encryption process. This means that each letter in the plaintext is consistently replaced with the same letter in the ciphertext, no matter where it appears. This predictability makes monoalphabetic ciphers relatively simple to analyze and break, especially through frequency analysis, as each letter appears the same number of times in the ciphertext as it does in the plaintext.

In contrast, polyalphabetic ciphers employ multiple shifting alphabets or substitutions which vary over the course of the encryption. This means that the same letter can be encrypted to different letters depending on its position in the text and the specific alphabet being used at that moment. This complexity significantly increases the security of the cipher as it distributes the frequency of characters more evenly, making patterns harder to detect and thus more resistant to cryptanalysis.

Understanding this distinction helps in grasping fundamental concepts of cryptography and the evolution of cipher techniques from simple to more complex methods for securing information.