Challenge Description

Danish (original)

Thor, den mægtige tordengud, har udført en utrolig bedrift – han har kastet Mjølner, sin legendariske hammer, med så enorm kraft, at den nu kredser om Jorden i en uendelig himmelsk bane. Men Mjølner er jo ikke bare et almindeligt våben. Overladet med elektricitet sender den kraftige lynimpulser ned mod planeten. Disse impulser er ikke kun ødelæggende gnister; de bærer mystiske signaler på tværs af det elektromagnetiske spektrum, krypteret med Mjølners krav til Thor, for at vende tilbage til sin retmæssige herre.

Thor, fast besluttet på at genvinde sin elskede hammer, har tilkaldt FDCA for hjælp. Jeres mission: opfang signalet, afslør dets kryptiske besked, og dekrypter den guddommelige kryptering for at bringe Mjølner tilbage til Thor.

Jordstationen har en en hjemmeside der kan få dig igang her: http://groundstation.jul.fdca.dk

English (from chatgpt)

Thor, the mighty thunder god, has accomplished an incredible feat – he has thrown Mjölnir, his legendary hammer, with such immense force that it now orbits Earth in an endless celestial path. But Mjölnir is not just an ordinary weapon. Charged with electricity, it sends powerful lightning impulses down toward the planet. These impulses are not just destructive sparks; they carry mysterious signals across the electromagnetic spectrum, encrypted with Mjölnir’s demands to Thor, to return to its rightful master.

Thor, determined to reclaim his beloved hammer, has called upon FDCA for help. Your mission: intercept the signal, decipher its cryptic message, and decode the divine encryption to bring Mjölnir back to Thor.

The ground station has a website that can get you started here: http://groundstation.jul.fdca.dk

File

We are also given the following file:

Solution

The given file is a GNU Radio Companion Flow Graph, opening it with GNU Radio Companion we get:

The ZMQ SUB Source address is not set, and we are also getting error because sps (samples per symbol) cannot be calculated since sym_rate (symbol rate) is not set.

Looking at the source for the provided website, we find two comments:

<!--Husk, vi skal bruge jordstationens domæne eller ip addresse, og port 1337 for at behandle signaler fra den.-->
<!--sps == verdener i nordisk mytologi-->

english version (chatgpt):

<!--Remember, we need the ground station's domain or IP address, and port 1337 to process signals from it.-->
<!--sps == worlds in Norse mythology-->

So the source address should be tcp://groundstation.jul.fdca.dk:1337 and the sym_rate should be samp_rate/9 = 4242 (because sps = samp_rate/sym_rate), so that sps becomes 9 (the number of worlds in norse mythology), and since samp_rate = 4242*9 setting sym_rate = 4242 makes sps = 9.

Setting the right parameters and running it via Run > Execute and waiting at most 10 minutes, we get the following output from the builtin console:

***** VERBOSE PDU DEBUG PRINT ******
()
pdu length =         18 bytes
pdu vector contents = 
0000: 5a 58 57 55 7b 6d 67 34 35 62 5f 61 31 34 68 6e 
0010: 35 7d 
************************************

Decoding it from hex and rotating the letters by 6 we get the flag: FDCA{sm45h_g14nt5}