Symmetric encryption, also referred to as conventional encryption or single key encryption was the only type of encryption in use prior to the development of public-key encryption in 1976. The symmetric encryption scheme has five ingredients (see Figure 1): 1. Plaintext: This is the original intelligible message or data that is fed to the algorithm as input. 2. Encryption algorithm: The encryption algorithm performs various substitutions and permutations on the plaintext (see the examples of the substitution and permutation ciphers in Lecture 8). 3. Secret Key: The secret key is also input to the encryption algorithm. The exact substitutions and permutations performed depend on the key used, and the algorithm will produce a different output depending on the specific key being used at the time. 4. Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the key. The ciphertext is an apparently random stream of data, as it stands, is unintelligible. 5. Decryption Algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext. There are two requirements for a symmetric key cryptosystem 1. We assume it is impractical to decrypt a message on the basis of the ciphertext plus knowledge of the encryption/decryption algorithm. In other words, we do not need to keep the algorithm secret; we need to keep only the key secret. 2. Sender and the receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. If someone can discover the key and knows the algorithm, all communications using this key is readable. We will describe how we can use a public-key cryptosystem for a secure key exchange later in this lecture
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