Encryption operation is done by substitution, transposition and product. For a system to be computationally secure,  the cost of breaking the cipher exceeds the value of the encrypted information and the time required to break the cipher exceeds the useful lifetime of the information.

Symmetric Encryption:

Mathematically, Y = Ek(X) Encryption Model, X = Dk(Y) Decryption model

Data Encryption Standard(DES) developed by National Bureau of Standards in 1970, which transforms a 64-bit binary value into a unique 64-bit binary value based on a 56-bit variable.

DES is simple repetition of primitives of transposition, substitution, spit & concatenation & bit wise operation.