For this challenge we just have a service to connect to and a small hint:
Read 10 times. nc adctf2014.katsudon.org 43010
The service returns weird strings like the following:
▌▌██▐▌▌█ █ ▌▐▐▐ █ ▌█ ▐▐▌▐▐ █ ▌▐▐▐▌ ▌█ ▐▐▐▐▌ ▌█▐ ▐▐ █▐▌▌▐ ▌▐▐▐█▌▌
But they look like a barcode! It is possible to notice peculiar patterns in the strings given by the service, in particular the beginning and the end are always the same. So first of all I wrote a small script to interpret those lines as 0 or 1, then looked for a mapping online and finally wrote a script that decodes the barcode. Each barcode was different so they were definitely not the flag. The hint came in handy as we know that we need to read them 10 times: so the solution was to decode the barcode and send the result back, the service will give us another barcode.
After 10 times we get the flag: ADCTF_4R3_y0U_B4rC0d3_R34D3r
Here is the script that solves the chall