# Is this transposition cipher information-theoretically secure for 1 message?

1. Given:

$m = \{0,1\}^{n}$; a plaintext message of length $n$ encoded in binary

$k = randomshuffle([0, 1, ..., 2n-1])$; A secret key consisting out of unique numbers between 0 and $2n-1$ in a true random order. The key is only used once and preshared between sender and receiver.

2. We create $t = m||NOT(m)$; a temporary variable with the original message plus the inverse of the original message. (The intention here is to make the number of 1's and 0's equal)
3. We transpose $t$ using key $k$ as indices to create the ciphertext $c$.

For example, the key 5-4-3-2-1-0 would reverse the order of the bits of $t$.

Full example:

1. Given: m = 010 and k = 5-4-1-3-0-2
2. We append the inverse of 010 (which is 101): t = 010101
3. We tranpose t using k. The resulting ciphertext: c = 101100

To decrypt the ciphertext you would simply reverse the transposition using your key and drop the second half of the message.

Is this scheme unbreakable?

For a scheme to be information-theoretically secure, you need that $$\Pr[M=m\mid C=c]=\Pr[M=m\mid C=c^\prime]$$ for all $c,c^\prime$ (that is, any ciphertext has the same probability $M=m$, so the ciphertext doesn't change the probability $M=m$).
Let's suppose we have a $c$ and a $c^\prime$. Both of them have the same number of ones and zeroes, because both are half ones and half zeroes. That means there's a permutation that takes $c$ to $c^\prime$ (in fact, there are a lot of them, but pick one); call it $\sigma$. We want to show that the number of permutations taking $c$ to some $t_0$ is the same as the number taking $c^\prime$ to $t_0$.
We can convert a permutation $\pi$ taking $c$ to $t_0$ into a permutation $\pi\sigma^{-1}$ taking $c^\prime$ to $t_0$: $\pi\sigma^{-1}(c^\prime)=\pi(c)=t_0$. Likewise, if $\pi^\prime$ takes $c^\prime$ to $t_0$, then $\pi^\prime\sigma$ takes $c$ to $t_0$. Also, $\pi\sigma\sigma^{-1}=\pi\sigma^{-1}\sigma=\pi$, so the functions are inverses. So, there are exactly the same number of permutations taking $c$ to $t_0$ as taking $c^\prime$ to $t_0$, so $$\Pr[T=t_0\mid C=c]=\Pr[T=t_0\mid C=c^\prime]$$ for all $c,c^\prime,t_0$.
Lastly, we know that $\Pr[M=m]$ is the sum of the probability that $T=t$ for all $t$ which have $m$ as their first half. Since all these probabilities are independent of the ciphertext, so is $\Pr[M=m]$. This applies whether or not you consider that $\Pr[T=t]$ is zero if $t$'s first half and second half aren't bitwise inverses; no matter what, it doesn't depend on the ciphertext, and neither does $\Pr[M=m]$. So, this scheme is information-theoretically secure.