In real-life cryptography, we are not usually concerned with individual encryption and decryption functions, but rather with classes of functions indexed by a key. 'C = E{k}(M)' and 'M = D{k}(C)' denote these. For keyed functions, our corresponding automatic equality is M = D{k}(E{k}(M)). With different key indexes to our function classes, we do not expect equalities like the above (in fact, finding them would usually indicate bad algorithms): M != D{k1}(E{k2}(M)). This inequality works out nicely because all the folks without access to the key K will not know which decryption function to use in deciphering C.

The design of specific cryptographic algorithms has many details, but the basic mathematics are as simple as their portrayal in these panels.