To finish up
I want to say a few words about implementing other cryptosystems using ALU control language. Care must be taken not to fall into traps. You must always take concern about implementing privacy hand in hand with authentication, meaning that you share keys with only the ones you want or are supposed to. If it seems like there is no exact need for authentication you are falling into trap. Consider following example: you have mobile telephones, where authentication seems to be guaranteed by just hearing other person It is exactly the case. Let us not consider voice synthesisers and other fancy expensive things. So you implement DH key exchange. It seems to work fine and you do not need PK database until. Someone buys similar phone and does like that: When phone A wants to talk with phone B it sends it initiates Key exchange with B. But instead it initiates it with clever phone C. It is done rather easily, as so far everything is random numbers and A has no way of knowing if it talks to B or C before hearing voice response. Now C initiates key exchange with B claming to Be A. Now A is connected to B via C who has not done anything serious like spending 1000000$for braking or whatever. It is just normal phone C bought from the same vendor. BTW, this scheme works OK with fibre networks where it is difficult to switch into line unnoticed. But these phones must share some information beforehand not to initiate key exchange with someone they are not supposed to communicate. It might be one-time-pad, PK of Trusted Key distribution Centre or even you can just dial up other person do DH key exchange using voice. Now you can optimise the use of such phones by first entering speech length and security parameter. Then the phone calculates necessary length exponents for DH and displays them. You then use these numbers by dialling up other and telling these by voice while partner inputs them using keyboard. ( As for block cipher you use usually 128 bits key then the size of these one-time key generation exponents should not exceed 100 decimal digits taking into consideration the discrete logarithm methods and Cray code) . After having done DH like that go to cryption and talk.
It can further be optimised by taking some hash from DH input key and showing it on display for users to verify it, also you could have cache for storing some keys for talking to very special people and some keys could be used for talking to group of people. I think some minute before talking very secret matters is not such a great deal to waste.
To come to conclusion-just be very careful when selecting exponential cryptosystems implementing protocols.