[Cryptography] Is it mathematically provably impossible to construct a mechanism to test for back doors in programs?

[Ben Laurie]

On 2 June 2014 18:57, Nemo <nemo at self-evident.org> wrote:
>> The real answer is that while no algorithm with the properties given
>> above can exist, there is nevertheless the possibility of detecting a
>> great number of backdoors in a practical way.  Also, the contortions
>> that program C above went through are rather easy to detect themselves
>> and would lead one to reject program C.
>
> This is a generally under-appreciated point worth repeating (and
> rephrasing).
>
> Although it is impossible to build a function that correctly answers
> "yes" or "no" to "is this program safe?", it is very possible to build
> functions that answer "yes" or "I don't know".
>
> The same principle applies during code review. Your code needs to be not
> only correct, but simple enough to be "obviously correct". Otherwise it
> will fail my review and I will ask you to rewrite it.
>
> To build secure systems, we do not need to be able to detect all back
> doors; we just need to be able to write code that provably does not have
> back doors. The former is impossible; the latter is not.

This is absolutely the key point.