4.2 Symmetric cryptosystems

To recap from Chapter 2, Secure Channel and the CIA Triad, confidentiality is achieved using functions f_K : ℳ→𝒞 that transform the private information m, also referred to as plaintext, into a scrambled message c = f_K(m), referred to as ciphertext. Here, f_K is an encryption function. It maps a plaintext m from some larger set ℳ, the plaintext space, to a ciphertext c which, in turn, is an element of some larger set 𝒞, the cipher space.

The functions f_K must be bijections, so that we can form the inverse functions, denoted by f_K⁻¹, which transform the ciphertexts back to the original plaintexts. The inverse functions are the decryption functions. The bijectivity of the f_K also means that ℳ and 𝒞 have the same number of elements.

In order to specify a complete symmetric cryptosystem, we need to define all of its ingredients:

• The plaintext space ℳ and the cipher space 𝒞

• The keyspace...