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The linked PDF you provided speculates that "object X" might be a self-obscured star, obscured by its own ejecta.

If we assume that this is indeed a dyson swarm, then the purpose might not be exclusively for collecting energy.

A category II or III civilization would be doing asto-architecture, and would need tremendous amounts of raw materials. Heavy atoms are only produced naturally in one kind of environment: in the hearts of stars. If this star is regularly expelling large quantities of cosmic dust, as the linked article postulates, then it would make an excellent "Factory". Energy would be in copious abundance, and the star itself would be churning out millions of tons of heavy atoms every minute. Even with a short (compared to other stars) lifespan, it would make an excellent factory site for other large astro-engineering projects.

Yes, that was sort of along the lines of my thought. This seems like a poor location for a long-term agricultural project, but conceivably an excellent site for astroengineering. In that case there might be detectable byproducts (for example,as you suggest, the distributions of various elements might be depleted or rearranged). Also, they might use nuclear fusion to synthesize missing very heavy elements or for some other purpose requiring high energies (there is, after all, lots of gas as well), and that might have a detectable signature at high energies.

Finally, Luc Arnold has an interesting paper about detection of artificial structures in transit about their star, and concludes that "multiple artificial objects would produce light curves easily distinguishable from natural transits." If the purpose of this megaengineering is to build structures (or coherent swarms) on a suitable scale, they might be detected in transit about the central star, which might provide the most conclusive proof of all.

A Dyson Sphere would block all light but a massive structure like in Ringworld would cause a dim band around a star rather than the brief dimming that means simply there's a planet. There's no known natural object that would block a region of a star for an extended period. A tight asteroid belt would cause dimming but not block most of the light. Look for a star with a significantly dimmed region or one that appears to be in two parts. Current telescopes would more than likely see most stars are solid even if there was a dim band so when higher powered ones become available it could be another thing to look for.

Luc Arnold looked into this and concluded that

Multiple artificial objects would produce light curves easily distinguishable from natural transits.

He is including structures like the Ring World (or the Culture's Orbitals) in such "multiple artificial objects."

What is more worrisome is that the planet gets heated up due to its proximity to the star and is evaporated. But again, planets have an awful amount of mass, so this shouldn't be too much of a problem either. For example, there is a 4.4 jupiter masses planet around Tau Bootis, in a 3.3d orbit (http://www.exoplaneten.de/tauboo/english.html), but the general estimate for objects of this kind (dubbed "hot jupiters") is that they will survive for billions of years. The reason for this is that the mass loss rate caused by the proximity of the star is still negligible compared to the mass of the planet. Take a look at the article by Ferlet et al., on p. 226 of a recent conference on explanets, the proceedings of which are at http://www.obs-hp.fr/www/pubs/Coll51Peg/proceeding s.html.

The length of the "one time pad" is large, but the number of them available? I mean the number of quasars that are good enough receivable to use for this purpose. I have no idea, but I doubt if its more than 2^32. In that case, brute force would be quite easy: just try each of the available quasar signals.:
Record the signal of each of them at time T, also record the encrypted message at time T, and try them all out in a fast computer.[...]

Well, you have a big problem with your time T. How do you know it? If you do not know the source (which quasar is used), it is also unlikely that you know the exact time T used for the start of the random stream. It is unlikely that you know it with a better precision than a few seconds. If the two parties do not exchange messages frequently or do not re-negociate the start of the random stream frequently, then you may not even know T with a precision of a day.

The NewScientistTech article does not give details about the amount of data available from the quasars, but other articles mention that quasars are typically observed in relatively high frequencies (20-40 GHz). Even if the signal strength is sampled with a low resolution and only a few truly random bits are extracted from the stream, you would still have a stream of bits that is in the Gbps range. This is a reasonably large amount of random data.

So even if the number of usable quasars is rather low (say, a few thousands: 2^10 instead of 2^32 as you mentioned), you would need a lot of antennas and petabytes of storage to record all these random streams. You would have to store something in the order of 2^40 bits per second for several seconds or even days (the uncertainty on T). This is not impossible if you have a large budget, but this is difficult and expensive.

It could even be much worse than 2^40: a recent catalogue of quasars from March 2006 mentions 85221 of them, with new findings doubling this number every second year: 48921 in 2003, 23760 in 2001, etc. Let's say that 2^15 of them are usable (and that you have 2^15 antennas at your disposal). If the signal strength is sampled with a medium resolution of 8 bits at a frequency of 30GHz and your uncertainty interval on T is about one hour, you would need to store 2^15 * 2^3 * 2^35 * 2^12 = 2^65 bits of data before starting your brute force attack. Good luck!

Once you have all this data, you still have to do the brute force attack. You wrote "just try each of the available quasar signals." This is correct but you ignore the fact that the random stream is unlikely to be used as is. It will probably be used to seed a stream cypher. In the simplest case, the random stream would be hashed a couple of times before being xor'ed with the data. You will need a huge amount of computing power to perform all these operations and try each of the available signals at each possible time offset.

Note: it is unlikely that both parties can get the signal and be synchronized with a nanosecond or picosecond resolution. So they would probably negociate a time window (say, with a resolution of one second or so) and some kind of unique marker within that time window in order to know exactly when to start. If you are the attacker and you cannot know which source is used, you probably do not know the time window nor the marker. But even in the unlikely case that you would have a way to obtain one or both of these, you would still have the problem of storing the huge amount of data from all quasars until you know which part of it should be analyzed.

So although a brute force attack based on recording all qasars is not impossible, it is not really easy. And anyway, my first reaction when I started reading this story was exactly like the comment mad

The length of the "one time pad" is large, but the number of them available? I mean the number of quasars that are good enough receivable to use for this purpose. I have no idea, but I doubt if its more than 2^32. In that case, brute force would be quite easy: just try each of the available quasar signals.:
Record the signal of each of them at time T, also record the encrypted message at time T, and try them all out in a fast computer.[...]

Well, you have a big problem with your time T. How do you know it? If you do not know the source (which quasar is used), it is also unlikely that you know the exact time T used for the start of the random stream. It is unlikely that you know it with a better precision than a few seconds. If the two parties do not exchange messages frequently or do not re-negociate the start of the random stream frequently, then you may not even know T with a precision of a day.

The NewScientistTech article does not give details about the amount of data available from the quasars, but other articles mention that quasars are typically observed in relatively high frequencies (20-40 GHz). Even if the signal strength is sampled with a low resolution and only a few truly random bits are extracted from the stream, you would still have a stream of bits that is in the Gbps range. This is a reasonably large amount of random data.

So even if the number of usable quasars is rather low (say, a few thousands: 2^10 instead of 2^32 as you mentioned), you would need a lot of antennas and petabytes of storage to record all these random streams. You would have to store something in the order of 2^40 bits per second for several seconds or even days (the uncertainty on T). This is not impossible if you have a large budget, but this is difficult and expensive.

It could even be much worse than 2^40: a recent catalogue of quasars from March 2006 mentions 85221 of them, with new findings doubling this number every second year: 48921 in 2003, 23760 in 2001, etc. Let's say that 2^15 of them are usable (and that you have 2^15 antennas at your disposal). If the signal strength is sampled with a medium resolution of 8 bits at a frequency of 30GHz and your uncertainty interval on T is about one hour, you would need to store 2^15 * 2^3 * 2^35 * 2^12 = 2^65 bits of data before starting your brute force attack. Good luck!

Once you have all this data, you still have to do the brute force attack. You wrote "just try each of the available quasar signals." This is correct but you ignore the fact that the random stream is unlikely to be used as is. It will probably be used to seed a stream cypher. In the simplest case, the random stream would be hashed a couple of times before being xor'ed with the data. You will need a huge amount of computing power to perform all these operations and try each of the available signals at each possible time offset.

Note: it is unlikely that both parties can get the signal and be synchronized with a nanosecond or picosecond resolution. So they would probably negociate a time window (say, with a resolution of one second or so) and some kind of unique marker within that time window in order to know exactly when to start. If you are the attacker and you cannot know which source is used, you probably do not know the time window nor the marker. But even in the unlikely case that you would have a way to obtain one or both of these, you would still have the problem of storing the huge amount of data from all quasars until you know which part of it should be analyzed.

So although a brute force attack based on recording all qasars is not impossible, it is not really easy. And anyway, my first reaction when I started reading this story was exactly like the comment mad

The length of the "one time pad" is large, but the number of them available? I mean the number of quasars that are good enough receivable to use for this purpose. I have no idea, but I doubt if its more than 2^32. In that case, brute force would be quite easy: just try each of the available quasar signals.:
Record the signal of each of them at time T, also record the encrypted message at time T, and try them all out in a fast computer.[...]

Well, you have a big problem with your time T. How do you know it? If you do not know the source (which quasar is used), it is also unlikely that you know the exact time T used for the start of the random stream. It is unlikely that you know it with a better precision than a few seconds. If the two parties do not exchange messages frequently or do not re-negociate the start of the random stream frequently, then you may not even know T with a precision of a day.

The NewScientistTech article does not give details about the amount of data available from the quasars, but other articles mention that quasars are typically observed in relatively high frequencies (20-40 GHz). Even if the signal strength is sampled with a low resolution and only a few truly random bits are extracted from the stream, you would still have a stream of bits that is in the Gbps range. This is a reasonably large amount of random data.

So even if the number of usable quasars is rather low (say, a few thousands: 2^10 instead of 2^32 as you mentioned), you would need a lot of antennas and petabytes of storage to record all these random streams. You would have to store something in the order of 2^40 bits per second for several seconds or even days (the uncertainty on T). This is not impossible if you have a large budget, but this is difficult and expensive.

It could even be much worse than 2^40: a recent catalogue of quasars from March 2006 mentions 85221 of them, with new findings doubling this number every second year: 48921 in 2003, 23760 in 2001, etc. Let's say that 2^15 of them are usable (and that you have 2^15 antennas at your disposal). If the signal strength is sampled with a medium resolution of 8 bits at a frequency of 30GHz and your uncertainty interval on T is about one hour, you would need to store 2^15 * 2^3 * 2^35 * 2^12 = 2^65 bits of data before starting your brute force attack. Good luck!

Once you have all this data, you still have to do the brute force attack. You wrote "just try each of the available quasar signals." This is correct but you ignore the fact that the random stream is unlikely to be used as is. It will probably be used to seed a stream cypher. In the simplest case, the random stream would be hashed a couple of times before being xor'ed with the data. You will need a huge amount of computing power to perform all these operations and try each of the available signals at each possible time offset.

Note: it is unlikely that both parties can get the signal and be synchronized with a nanosecond or picosecond resolution. So they would probably negociate a time window (say, with a resolution of one second or so) and some kind of unique marker within that time window in order to know exactly when to start. If you are the attacker and you cannot know which source is used, you probably do not know the time window nor the marker. But even in the unlikely case that you would have a way to obtain one or both of these, you would still have the problem of storing the huge amount of data from all quasars until you know which part of it should be analyzed.

So although a brute force attack based on recording all qasars is not impossible, it is not really easy. And anyway, my first reaction when I started reading this story was exactly like the comment mad

The closest extrasolar planet is (according to this) Gliese 876. It's situatet 15 lightyears from us which works out to 15years*300000km/s*86400s*364.25days=472068000km. The planetary radius is 0.1 times the solar radius (our sun). Which gives a diameter of (according to wikipedia) 1392000km*0.1=139200km. Thus (according to Python) the angle is; atan(139200km/472068000km)~2.9*10^-4 radians, which is ~0.017 degrees. A supprisingly big number in my opinion.