can-you-hear-me-now
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- [aaaa/spacebook](aaaa/spacebook)
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- [comms/56k](comms/56k)
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- [comms/phasors-to-stun](comms/phasors-to-stun)
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- [ground-segment/can-you-hear-me-now](ground-segment/can-you-hear-me-now)
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- [ground-segment/i-see-what-you-did-there](ground-segment/i-see-what-you-did-there)
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- [payload/calendar](payload/calendar)
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- [payload/leakycrypto](payload/leakycrypto)
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\DeclareTextFontCommand{\textsymbol}{\symbolfont}
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\newunicodechar{⬡}{\textsymbol{⬡}}
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\newunicodechar{⊕}{\textsymbol{⊕}}
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\newunicodechar{↩}{\textsymbol{↩}}
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\setmonofont{Noto Sans Mono}[Scale=0.9]
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# Can you hear me now?
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**Category**: Ground Segment
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**Points (final)**: 59 points
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**Solves**: 75
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> LaunchDotCom's ground station is streaming telemetry data from its Carnac 1.0 satellite on a TCP port. Implement a decoder from the XTCE definition.
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**Given files**: `telemetry.zip`
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## Write-up
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by [erin (`barzamin`)](https://imer.in).
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The provided zip file contains `telemetry.xcte`, an [XTCE](https://www.omg.org/xt) file defining the telemetry protocol streaming from the challenge server.
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XTCE is a XML-based protocol description format, used to provide a machine-readable definition of the bit layout in a telemetry stream. I could use COSMOS to load this XTCE definition, but instead I just figured out what the XTCE file meant (without really reading the XTCE specification, because nobody has time for that) and wrote a quick decoder by hand.
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I captured some telmetry data from the server by running
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```
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(cat THE_TICKET) | nc hearmenow.satellitesabove.me 5032 > data
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```
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`telemetry.xtce` describes every packet in the payload is headed by a header of the form (apparently, "abstract" things in XTCE are an instanceable template for a description of parameters; this one gets instanced in every packet as the header):
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```{.xml}
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<xtce:SequenceContainer name="AbstractTM Packet Header"
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shortDescription="CCSDS TM Packet Header"
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abstract="true">
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<xtce:EntryList>
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<xtce:ParameterRefEntry parameterRef="CCSDS_VERSION"/>
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<xtce:ParameterRefEntry parameterRef="CCSDS_TYPE"/>
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<xtce:ParameterRefEntry parameterRef="CCSDS_SEC_HD"/>
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<xtce:ParameterRefEntry parameterRef="CCSDS_APID"/>
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<xtce:ParameterRefEntry parameterRef="CCSDS_GP_FLAGS"/>
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<xtce:ParameterRefEntry parameterRef="CCSDS_SSC"/>
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<xtce:ParameterRefEntry parameterRef="CCSDS_PLENGTH"/>
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</xtce:EntryList>
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</xtce:SequenceContainer>
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```
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The `parameterRef`s point to `xtce:Parameter`s in the `xtce:ParameterSet` nearer the top of the file; the parameters in the header are defined there as
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```{.xml}
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<!-- Parameters used by space packet primary header -->
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<xtce:Parameter parameterTypeRef="3BitInteger" name="CCSDS_VERSION"/>
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<xtce:Parameter parameterTypeRef="1BitInteger" name="CCSDS_TYPE"/>
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<xtce:Parameter parameterTypeRef="1BitInteger" name="CCSDS_SEC_HD"/>
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<xtce:Parameter parameterTypeRef="11BitInteger" name="CCSDS_APID"/>
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<xtce:Parameter parameterTypeRef="2BitInteger" name="CCSDS_GP_FLAGS"/>
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<xtce:Parameter parameterTypeRef="14BitInteger" name="CCSDS_SSC"/>
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<xtce:Parameter parameterTypeRef="2ByteInteger" name="CCSDS_PLENGTH"/>
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```
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The `{n}BitInteger` parameter types are defined further up in the file as exactly what you'd expect them to be. We now know what packet headers look like; let's look for something flag related. A `Flag Packet` is defined in several places in the file (once as an "abstract" packet, which I don't really understand the significance of); it contains a body of parameters `FLAG1` through `FLAG120`, all defined upfile as 7-bit integers
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```{.xml}
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<xtce:Parameter parameterTypeRef="7BitInteger" name="FLAGxxx"/>
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```
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The header associated with the flag packet is
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```{.xml}
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<xtce:BaseContainer containerRef="AbstractTM Packet Header">
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<xtce:RestrictionCriteria>
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<xtce:ComparisonList>
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<xtce:Comparison parameterRef="CCSDS_VERSION" value="0"/>
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<xtce:Comparison parameterRef="CCSDS_TYPE" value="0"/>
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<xtce:Comparison parameterRef="CCSDS_SEC_HD" value="0"/>
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<xtce:Comparison parameterRef="CCSDS_APID" value="102"/>
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</xtce:ComparisonList>
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</xtce:RestrictionCriteria>
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</xtce:BaseContainer>
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```
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The APID is specific to the flag packet; we can just search for it in the stream and decode from there. I threw together some python (using `bitflags`) to decode the flag from the data I recorded:
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```{.python}
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from bitstring import Bits, BitArray, ConstBitStream
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b = ConstBitStream(filename='./data')
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packetlocs = list(b.findall('0x0066'))
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print(f"found packets: {packetlocs}")
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for loc in packetlocs:
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b.pos = loc
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ver = b.read(3).uint
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ty = b.read(1).bin
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sec_hd = b.read(1).bin
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apid = b.read(11).uint
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gp_flags = b.read(2).bin
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ssc = b.read(14).uint
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plength = b.read(16).uint
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print(ver, ty, sec_hd, hex(apid), gp_flags, ssc, plength)
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flag = []
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for i in range(120):
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flag.append(chr(b.read(7).uint))
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print(''.join(flag))
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```
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Which produced the flag:
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```
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λ ~/has/cyhmn
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» python decode.py
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found packets: [600, 1904, 3208]
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0 0 0 0x66 11 1919 94
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flag{delta98823mike:GAFbfoYquKzWaSFdWeYHGMDosGaBTnMbwD_kqwuj↩
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MhhNPaA9t7Iay8GY6CdGUwrYVa_AetBJEqJ6XO1XHl0kbHA}OP`P<
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```
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## Resources and other writeups
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- https://www.omg.org/xt
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- https://bitstring.readthedocs.io/
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@ -128,3 +128,4 @@ This gives `3.781315936823621`; pasting this into the contest, we got the flag.
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- https://www.ibiblio.org/apollo/CMC_data_cards_15_Fabrizio_Bernardini.pdf
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- https://www.ibiblio.org/apollo/index.html#Playing_with_Colossus_
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- https://www.ibiblio.org/apollo/Documents/Apollo15_Colossus3_CMC_Data_Cards.pdf
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- https://bitstring.readthedocs.io/
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