> Alice sent over a couple of images with sensitive information to
> Bob, encrypted with a pre-shared key. It is the most secure
> encryption scheme, theoretically...

The challenge consists of two monochrome PNG images of what seems to
be visual snow.  The encryption scheme alluded to is the One-Time Pad,
with the well-known weakness that key streams may not be reused.  If
that property is violated, then `E(A, K) ^ E(B, K) = A ^ K ^ B ^ K = A
^ B`.  Interpreted in terms of an image you'd get both unencrypted
images superimposed.

I wrote some code using [ChunkyPNG](https://chunkypng.com/) performing
this operation.  The resulting image shows a base64-encoded flag:

    $ base64 -d <<< ZmxhZ3swbjNfdDFtM19QQGQhfQ==
    flag{0n3_t1m3_P@d!}