Finding sum of two encrypted numbers Let's consider such process:


Two emitents emit two (integer) secret numbers independently
They encrypt (encode) these number in such a way that no-one (except emitent) can decode these numbers. Emitents also cannot decode/know numbers of each other.
Now having two block of decoded numbers we need to find the sum of these numbers


So, I need algorithms for achieving above


Number decoding (point 2 above)
Sum calculation (point 3 above)
Let $a$ and $b$ be the numbers emited rsp. by person A and person B. $E(x)$ means encoded form of $x$. 

$E(a)$ and $E(b)$ are publicly known, right?

Note that if person A knows $a$, $E(a)$, $E(b)$ and person B knows $b$, $E(a)$, $E(b)$ and it is possible to calculate $a+b$ from $E(a)$ and $E(b)$ (that is what you want to do, right? So it must be possible) person A can calculate $a+b$ from the data he has. He also knows $a$ and calculating $b$ given $a+b$ and $a$ is trivial. Now person A knows the secret number of person B and person B can calculate the secret number of A same way.

I don't know what is this system going to be used for, but even if A and B trust each other there are still some security risks here.