How to prove uniform distribution of $m\oplus k$ if $k$ is uniformly distributed? All values $m, k, c$ are $n$-bit strings. $\oplus$ stands for the bitwise modulo-2 addition.
How to prove uniform distribution of $c=m\oplus k$ if $k$ is uniformly distributed? $m$ may be of any distribution and statistically independant of $k$.
For example you have a $m$-bit string that is with probability p=1 always '1111...111'. Adding it bitwise to a random $k$-bit string which is uniformly distributed makes the result also uniformly distributed. Why?
 A: If $X$ is a (vector) random variable that takes on each of the $2^n$ values 
$$\{00\cdots 00, \quad 00\cdots 01,\quad 00\cdots 10,\quad \ldots\ ,
\quad 11\cdots 10,\quad 11\cdots 11\}$$
with equal probability $2^{-n}$, then for any fixed $n$-bit vector $m$,
$Z = X\oplus m$ also has the same distribution
since for all choices of $n$-bit vector $a$ we have that
$$P\{Z = a\} = P\{m\oplus X = a\} =P\{X = m\oplus a\} = 2^{-n}\tag{1}$$
But suppose that $Y$ is another random variable taking on the same $2^n$ values
with arbitrary distribution (including the possibility that $Y$ takes on
some values with probability $0$) and suppose that $Z = X \oplus Y$.
Given that the event $\{Y = m\}$ occurs, suppose that the conditional distribution
of $X$ conditioned on $\{Y = m\}$ is uniform on the $2^n$ values of $X$. Then,
we have that
$$P\{Z = a\mid Y = m\} = P\{X\oplus Y = a\mid Y = m\} = P\{X = m\oplus a \mid Y = m\} = 2^{-n}. \tag{2}$$
Equation $(2)$ shows that $P\{Z = a \mid Y = m\}$,
the conditional probability that $Z = X\oplus Y$ equals $a$
given that $Y = m$  is $2^{-n}$. If $(2)$ holds for all $m$, then
since the unconditional probability
that $Z = a$ is, by the law of total probability, a weighted sum of
the conditional probabilities, we also have that
$$P\{Z = a\} = \sum_m P\{Z=a\mid Y = m\}P\{Y = m\} = 2^{-n}\sum_m P\{Y = m\} = 2^{-n}.$$
Note that we have implicitly assumed that $X$ and $Y$ are independent because
we have assumed that for all choices of $m$, the conditional distribution of
$X$ given that $\{Y = m\}$ is a uniform distribution on the $2^n$ values of $X$, that is the conditional distribution of $X$ is the same regardless of the choice of the value of $Y$.  
Two points are worth noting here.


*

*It is not necessary that $Y$ also be uniformly distributed on
the $2^n$ values as long as $X$ and $Y$ are assumed to be independent.
In fact, as $(1)$ indicates, $Z = X\oplus Y$ is uniformly distributed
on the $2^n$ even if $Y$ is a degenerate random variable taking on value
$m$ with probability $1$. (Note that even in this case, $X$ and $Y$ are
independent random variables nonetheless).

*While $X$ and $Y$ being independent is sufficient for $Z$ to
inherit the uniform distribution on the $2^n$ values that $X$ enjoys,
independence of $X$ and $Y$ is not necessary either. For example,
with $n=2$ and $X$ uniformly distributed on $\{00, 01, 10, 11\}$,
consider the joint distribution given below:
$$P\{X = 00, Y = 00\} = \frac{1}{4}, \quad P\{X = 11, Y = 00\}= \frac{1}{4},\\
P\{X = 01, Y = 11\} = \frac{1}{4}, \quad P\{X = 10, Y = 11\}= \frac{1}{4},$$
where $X$ and $Y$ are clearly not independent, $Y$ is not uniformly
distributed on $\{00, 01, 10, 11\}$ (though it is uniformly
distributed on $\{00, 11\}$), and yet $Z$ is uniformly distributed on
$\{00, 01, 10, 11\}$ just as $X$ is.
A: This is not true. For example, if $m = k$, $c$ is not uniformly distributed.
A: I don't think the answer is correct because it relies on you making the assumption and then using that assumption to prove the point.
i.e.
Given that the event {Y=m} occurs, suppose that the conditional distribution of X conditioned on {Y=m} is uniform on the 2n values of X. 

I'm not sure if this is allowed because this is the part you are trying to prove and it creates a circularity problem. It is because you are effectively using two constraints, that it is conditional and the other is uniform. Then you are stating that it works even when there is unconditional probability, but you have already "assumed it is uniform" previously, so you can't just use that again...
The uniformality part just appears from nowhere without justification.
I would just use the empirical proof such as the last two points mentioned in Dilip's answer.
