used symmetric encryption for secre

used symmetric encryption for secret communication. It remains the more widely used
of the two types of encryption.
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 substitu tions
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 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 possession of a number of ciphertexts together with the plaintext
that produced each ciphertext.
2. 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.
There are two general approaches to attacking a symmetric encryption
scheme. The first attack is known as cryptanalysis. Cryptanalytic attacks rely on