## 5/01/2014

### cryptography2

Cryptography 2

Symmetric Encryption

Symmetric encryption or symmetric key or single-key encryption is an encryption system which the encryption and the decryption processes are preforms using the same key.

Symmetric Encryption model:

1- Plaintext (P) : The original message or data , the plaintext is an input to the encryption algorithm .
2- Encryption algorithm (E) : This algorithm preforms various substitution and transposition on the plaintext and produces the ciphertext.
3- Ciphertext (C) : Is the output produced by the encryption algorithm , the ciphertext is scrambled message and it is appears like a random stream of data.
4- Encryption key (K) : Encryption key or secret key is a value that independent of the plaintext.
Encryption key is an input to the encryption algorithm.
The encryption algorithm will produces a different with different keys.
5- Decryption algorithm (D) : This algorithm is a reverse of the the encryption algorithm , and it takes ciphertext and the encryption key as input and produces the plaintext as output.

How Symmetric Encryption Work :

Alice want to send encrypted message to Bob and both have the secret key which generated by the encryption algorithm or by a third-party software, this process will work as the following steps

1- Encryption algorithm E (on Alice's computer) takes the plaintext P ,and the secret key K and generates a ciphertext C.
C = E(P,k)
2- The ciphertext C will be transferred via internet
3- Decryption algorithm (on Bob's computer) takes the ciphertext and the secret key K (the same key) and regenerate the original plaintext P again
P = D(C,K)

Hint #1 : The encryption key is independent of the plaintext or the encryption algorithm.
Hint #2 : The sender and receiver must have copies of the encryption key and they must secure that key.
Hint #3 : We do not need to keep the encryption algorithm secure because it is impractical to decrypt a ciphertext by only using the decryption algorithm , and that's the same for the encryption algorithm.
Hint #4 : Key exchange must be done via a secured channel.