For these ciphers it is now true that the same ciphertext character may represent different plaintext letters. When a polyalphabetic cipher is used, the ciphertext character replacing a particular plaintext letter may vary through the cryptogram and might, for instance, depend on its position in the plaintext message or the content of the plaintext that precedes it. However, it is still true that each ciphertext character represents a unique plaintext character and there is always the danger of an attacker compiling a dictionary of known plaintext and ciphertext pairs for a given key.Īnother way of achieving the objective of flattening the frequency histogram is by the use of a polyalphabetic cipher. This ensures that more than one ciphertext character may represent the same plaintext character.
When a homophonic cipher is used, the frequency histogram of the cryptogram is made flatter by increasing the size of the alphabet. Chapter 9: Cryptography in Everyday Life.Using symmetric algorithms for authentication and data integrity.Using symmetric algorithms for confidentiality.Chapter 3: Historical Algorithms: Simple Examples.
