Symmetric Encryption (eK = dK )

Definition: Symmetric Encryption - A type of encryption where the same algorithm and key is used to encrypt and decrypt the message.

Two Symmetric Encryption Technologies

Symmetric Nibble Permutations:

ASCII      Some Code

The Method:

  1. Consider a string of bytes as a string of nibbles (4 bits).
  2. Permute the nibbles with a permutation p such that p∘p=id.

The goal is to mitigate frequency attacks.

A Simple Example (using just 6 bytes):

p=    [ 3,10, 8, 0,11, 6, 5, 9, 2, 7, 1, 4]

p∘p=[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11]

Encode "Codeit":

CodeIt    to nibbles → 436F64656974   p → F76446496536    to "ASCII" → ÷dFIe6

4 is in the 0th position so it goes to the 3rd position, and F is in the 3rd position so it goes to the 0th position

XOR (exclusive or) Bit Masking ⊻

Some Code

The Table:

 ⊻   0   1 
 0   0   1 
 1   1   0 

The Equation:

X⊻(Y⊻Y) = (X⊻Y)⊻Y = X

For our purposes (X⊻Y)⊻Y = X is the relevent part of the equation.

The Method:

  1. Choose a random string m of bits of length divisible by 8, called the key.
  2. For a given string s of bytes of the same bit length as m XOR each bit of s with the corresponding bit of m.

An Example:

The Mask:
m= 1011001011001110

The String:

Co =436F= 0100001101101111

Mask

The Result:
  1111000110100001

Mask Again

The Result:
  0100001101101111 = Co