Symmetric encryption

Question 1

What is the symmetric cipher model? or fundamentals of symmetric key encryption?

Answer 1

1. An encryption scheme {Ee | e ∈ K} and {Dd | d ∈ K} is
   symmetric-key if for each associated pair (e, d) it is
   computationally “easy” to determine d knowing only e and to determine e from d. In practice e = d.
2. Also known as: secret-key, single-key, one-key, shared-key and conventional encryption.
3. Sender and recipient share a common key

Question 2

what are the 2 requirements for the secure use of symmetric key encryption?

Answer 2

1. STRONG ENCRYPTION ALGORITHM
   a. MINIMUM: attacker who knows algorithm and has access to one or more ciphertexts should be unable to decipher ciphertext or figure out key.

b. STRONG: attacker should be unable to decrypt ciphertext or discover key even if he/she is in possession of a number of ciphertexts together with plaintext that produced each ciphertext
2. SENDER AND RECEIVER MUST OBTAIN THE COPIES OF THE SECRET KEY IN A SECURE FASHION AND MUST KEEP IT SECURE
3. ONLY THE KEY IS SECRET, WE ASSUME THAT EVERYONE KNOWS THE ALGORITHM

Question 3

Explain the detailed model of symmetric key cryptosystem

Answer 3

1. The message source produces a message in plaintext: X = [X1, X2, . . . , Xi]. The 1 -> i elements of X are letters of some finite alphabets.
2. An encryption key of the form K = [K1, K2, . . . , Kj] is generated.
   a. If the key is generated at the message source, then it must also be provided to the destination by means of some secure channel.
   b. Alternatively, a third party could generate the key and securely deliver it to both message source and destination.
3. Encryption algorithm forms ciphertext Y = E(K, X) = [Y1, Y2, . . . , Yn].
4. The intended receiver, in possession of the key K, is able to invert the transformation: X = D(K, Y) to obtain the plaintext X.
5. Attacker:
   a. knows the encryption (E) and decryption (D) algorithms,
   b. observing Y but not having access to K or X, may attempt to recover X or K or both X and K, by generating Xˆand/or Kˆ.