CRYPTOGRAPHIC TOOLS

CRYPTOGRAPHIC TOOLS
An important element in many computer security services and applications is the use of cryptographic algorithms.
We begin with symmetric encryption, which is used in the widest variety of contexts, primarily to provide confidentiality.
2.1 CONFIDENTIALITY WITH SYMMETRIC ENCRYPTION
The universal technique for providing confidentiality for transmitted or stored data is symmetric encryption. This Lecture introduces the basic concept of symmetric encryption. This is followed by an overview of the two most important symmetric encryption algorithms: the Data Encryption Standard (DES) and the Advanced Encryption Standard (AES), which are block encryption algorithms. Finally, this section introduces the concept of symmetric stream encryption algorithms.
Symmetric Encryption
Symmetric encryption, also referred to as conventional encryption or single-key encryption, was the only type of encryption in use prior to the introduction of public-key encryption in the late 1970s.

A symmetric encryption scheme has five ingredients ( Figure 2.1 ):
• Plaintext: This is the original 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 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.
• 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 secure use of symmetric encryption:
1. We need a strong encryption algorithm. At a minimum, we would like the algorithm to be such that an adversary 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 adversary should be unable to decrypt ciphertext or discover the key even if he or she is in possession حوزة of a number of ciphertexts together with the plaintext that produced