Private Key Cryptography

1. Private Key Cryptography also referred as Symmetric Encryption or Conventional Encryption or Single-Key encryption was the only type of encryption in use prior to the development of public-key encryption in the 1970s. It remains by far the most widely used of the two types of encryption.
2. Symmetric Cipher Model:

• Plaintext: This is the original intelligible message or data that is fed into the algorithm as input.
• Encryption Algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext.
• Secret Key: The secret key is also input to the encryption algorithm. The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time. The exact substitutions and transformations performed by the algorithm depend on the key.
• Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the secret key. For a given message, two different keys will produce two different ciphertexts. The ciphertext is an apparently random stream of data and as it stands is unintelligible.
• Decryption Algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext.

$$\text{Figure 5.3 Symmetric Cipher Model}$$

3.There are two requirements for secure use of conventional encryption:

• We need a strong encryption algorithm. At a minimum, we would like the algorithm to be such that an opponent who knows the algorithm and has access to one or more ciphertexts would be unable to decipher the ciphertext or figure out the key. This requirement is usually stated in a stronger form. The opponent should be unable to decrypt ciphertext or discover the key even if he or she is in position of a number of ciphertexts together with the plaintext that produced each ciphertext.
• Sender and 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 communication using this key is readable.
• We assume that 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.
• This feature of symmetric encryption is what makes it feasible for widespread use. The fact that the algorithm need not be kept secret means that manufacturers can and have developed low-cost chip implementations of data encryption algorithms. These chips are widely available and incorporated into number of products.