Which of the following is/are not perfectly secure encryption scheme(s):  Playfair Cipher Vernam Cipher Affine Cipher Vigenère Cipher

The Playfair Cipher, Affine Cipher, and Vigenère Cipher are not perfectly secure encryption schemes. The Vernam Cipher, also known as the One-Time Pad, is the only perfectly secure encryption scheme among the ones listed.

Here's why:

1. Playfair Cipher: This is a manual symmetric encryption technique and was the first literal digraph substitution cipher. The scheme was invented in 1854 by Charles Wheatstone but was named after Lord Playfair who promoted the use of the cipher. It is not perfectly secure because it doesn't change the key for each piece of new information, making it vulnerable to frequency analysis.

2. Vernam Cipher: This is a type of one-time pad cipher that is perfectly secure, provided the key material is truly random, never reused, kept secret from all possible attackers, and of equal or greater length than the message. Most other ciphers do not meet these requirements and thus are not perfectly secure.

3. Affine Cipher: This is a type of monoalphabetic substitution cipher, where each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. It is not perfectly secure because it is vulnerable to known-plaintext attacks.

4. Vigenère Cipher: This is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It is not perfectly secure because, like the Playfair cipher, it doesn't change the key for each piece of new information, making it vulnerable to frequency analysis.