IAC Paper (IAC-03-IAA.8.3.06: Do potential SETI signals need to be
decontaminated?)

DO POTENTIAL SETI SIGNALS NEED TO BE DECONTAMINATED?

Richard A. Carrigan, Jr.

Fermi National Accelerator Laboratory

Box 500 MS221

Batavia, IL 60510 USA

<carrigan@fnal.gov>

ABSTRACT

Biological contamination from space samples is a remote but accepted
possibility. Signals received by searches for extraterrestrial
intelligence (SETI) could also contain harmful information in the spirit
of a computer virus, the so-called "SETI Hacker" hypothesis. Over the
last four decades extraterrestrial intelligence searches have given
little consideration to this possibility. Some argue that information in
an extraterrestrial signal could not attack a terrestrial computer
because the computer logic and code is idiosyncratic and constitutes an
impenetrable firewall. Suggestions are given on how to probe these
arguments. Measures for decontaminating extraterrestrial intelligence
signals (ETI) are discussed. Modifications to the current SETI detection
protocol may be appropriate. Beyond that, the potential character of ETI
message content requires much broader discussion.

### [INTRODUCTION]{.underline}

Since the dawn of SETI four decades ago[^1] the potential for success
has risen substantially. With the advent of powerful new facilities and
sophisticated signal detection programs searches have taken on a new
crescendo both for radio and optical frequencies[^2]. Recent
extraterrestrial planet searches have shown planets are more common than
many anticipated. Year by year more is understood about the genesis of
life[^3]. Recent insights suggest the possibility that the pace and ease
of evolution could be faster through endosymbiosis and the increasing
temperature span suitable for life as exemplified by extremophiles.
There are glimmerings on the horizon of artificial computer "life" so
that we may be faced with this somber possibility in several decades.
Even with electronic technology barely a century old earth civilizations
have the capability to send signals to the stars. Earth no longer seems
so rare[^4]. This is no promise that there are intelligent signals in
the cosmos that will soon be discovered. On the other hand, the
possibility should be considered seriously and preparations made to cope
with a signal if one is received.

ETI signals could come in a variety of forms. One type of potential SETI
signal is a beacon containing little information. A second kind is a
message. In what follows it is argued that particular care needs to be
taken in the way messages are handled when they are received. A third
possibility is that the galaxy may contain artifacts like TV or radar
leakage signals[^5]. These artifact "signals" are the equivalent of
archaeological information from early civilizations on the earth. Few of
the archaeological artifacts discovered on earth were intended to inform
the future. At the outset of the SETI quest it was recognized that there
was a dark side to the emanation of TV and radar artifacts from earth. A
hundred year old sphere of high-tech signatures is radiating out from
earth at the speed of light signaling mankind's presence and
capabilities. A signal from the first radio activity on earth a century
ago could have reached a star fifty light years away and a civilization
there could have immediately responded and sent a signal back to earth.
Such a signal could be useful or possibly very harmful to us. There are
on the order of 400 stars within fifty light years of earth.

Interstellar signaling can also be broken into signals intended for
two-way conversations and one-way signals. Two-way conversations are
possible for stars that are close together. However even two bristlecone
pines separated by ten light years could only get in a hundred or so
exchanges in a lifetime, hardly the stuff of a good college education.
At first glance a one-way message sounds like watching TV or stuffing a
note in a bottle and tossing it in the ocean. On the other hand one only
needs to remember the impact of Greek culture on the West to realize how
significant one-way communication can be. (Interestingly, P. Morrison
estimated that the total written contributions from ancient Greece
amount to 10^9^ bits of information[^6].)

Finally, will a SETI signal be altruistic, benign, or malevolent? It
would help to understand the motivation of a message before reading too
much of it. Like Odysseus, we may have to stuff wax in the ears of our
programmers and strap the chief astronomer to the receiving tower before
she is allowed to listen to the song of the siren star.

The central premise of this article is that an ETI signal could be
malevolent. The concept is that the signal might be able to take over
the receiving computer or urge the construction of a translator with an
unknown agenda. I call this hypothesis "SETI Hacker." This concept is
not new. It is a theme in a large body of science fiction[^7]. What is
new here is an attempt to discuss the possibility in analytical terms
and look for means to denature SETI signals. Some aspects of this have
been discussed earlier in a paper prepared for Bioastronomy 2002[^8].

The following sections of this paper discuss the possible nature of an
interstellar signal, information transfer over interstellar distances by
electromagnetic waves and matter carriers such as DNA, possibilities for
denaturing or sterilizing a signal, and summarize the discussion.

### [THE NATURE OF AN ETI SIGNAL]{.underline}

The character and size of an ETI signal will shape the approach to
treating the signal content. A beacon with a message of less than a
hundred kilobytes repeating every 10 seconds or so could be handled
without too many precautions. Based on current earth-based programs even
a ten megabyte program or data set can be opaque. To appreciate the
potential impact of a signal it is useful to review the character of ETI
signals and the scale of information that might be transmitted.

Signaling or information transfer over interstellar distances might be
either by electromagnetic waves such as a laser, radio or TV signal, or
by a matter-based medium like an old-fashioned letter, panspermia[^9] of
DNA or silicon chips, or a spacecraft. This discussion focuses on
electromagnetic signals. For an electromagnetic signal there will need
to be a receiver and a decoder or demodulator. The signal will have to
provide some attractive "advertisement" or lure to enlist the help of a
host. Almost certainly actual messages will have gone through some sort
of compression. The signal will have to carry the compression decoding
algorithms embedded in the message. On a first level it is to the
advantage of the sender to have the compression algorithm totally clear.

The actual message content could be quite different than we imagine. For
example information about scent and odor might be much more important if
the originator was an advanced dog. What information would be
significant for an intelligent plant? Even for humans a large fraction
of the intelligence base is grounded in pictorial images. Images are
harder to characterize than text and numerically-based intelligence.
Finally, the sender will probably have much more sophisticated
technology available than we do today. For example we are just leaning
how to read genomes and we do not yet have quantum computer engineering.

One anticipates that the message would be grounded in science and logic
rather than magic incantations. Metaphors and analogies from
non-scientific areas may be of little help in understanding ETI signals.
My wife has recently completed a diptych that contrasts the view of an
ant and an astrophysicist. The astrophysicist asks "Can I unweave the
gauzy fabric of stars?" while the ant wonders "Can I untangle the matted
carpet of earth?" We can be sure the sender of an ETI message will be
looking out to the stars but its core perspectives may be more like the
ant than the astronomer. A particular feature of a non-technical view of
SETI is the temptation to try to apply moral arguments. Attempting to
attribute human legal or ethical values to our explorer ant is a
dangerous stretch. The one "ethical" framework we could have some
confidence in is a Darwinian "survival of the fittest." This behavior
standard is about as far from altruistic as one can get.

For message size one can consider examples based on human experience. A
typical desktop computer operating system is now 1 Gbyte while Microsoft
Word is about 0.01 Gbytes. (A Gbyte is 10^9^ bytes.) Although the human
genome consists of 3 billion DNA base pairs, the effective information
content is on the order of 0.05 Gbytes. A typical education through
graduate school could be subsumed in 1-10 Gbytes. A rough "memory" of a
casual acquaintance including images might take 0.001-0.010 Gbytes so
1000 acquaintances could be summarized in 10 Gbytes. A lifetime of
images stored once a minute might be 100 times larger. Crevier[^10] has
tried to determine the information needed to characterize a person's
memory from several directions and gets numbers in the 2.5 Gbyte range.
Taking 6\*10^9^ people on earth and assuming 2.5 Gbytes/person gives 15
exabytes to send memory profiles of everyone on earth (1 exabyte is
10^18^ bytes). This is a very large amount of data. About 1997 Lesk[^11]
speculated that all the information in the world including pictorial
information required 12 exabytes of storage. Adding profiles for all the
world's inhabitants including their DNA information brings this number
to about 25 exabytes. An even more ambitious transmission might include
DNA profiles for all the creatures and plants on earth. Even an advanced
civilization might have to draw the line somewhere on the scale of
message transmission.

Exabyte data bases currently require transmission times of years over
optical fibers with capacities of 10 Gbytes/s. (Note that the fifty
light year SETI example below discusses a rate of 10^-5^ Gbytes/s.)
Based on exabyte-scale databases we can anticipate the possibility of
very long electromagnetic signals possibly interspersed with short,
interesting messages to act as a lure.

Actual translators for signal decompression could be relatively simple.
Even now these types of signals can be handled with PC-scale computers.
SETI Hacker virus type software would likely be much more complicated.

### [INFORMATION TRANSFER OVER INTERSTELLAR DISTANCES]{.underline}

The fundamental challenge for SETI searches is to determine the most
effective way to search. A useful tool to probe this is to ask what the
"energy costs" are to transmit and receive an ETI signal. One needs to
know how much energy is required to transmit a bit of information using
different approaches. The sender will be driven to get the bit rate as
high as possible and hold the cost per bit low. Folded into the
transmission cost is a supposition about the level of technology at the
receiving end as well as information about the degradation due to
attenuation or damage to the message as it moves across the interstellar
medium. Transmitting through the relatively dense galactic core is
harder than transmitting in the outer halo. Transmitting further takes
more energy. Transmitting to a small receiving antenna takes more energy
than sending to a large antenna. Knowing the associated costs is a good
tool to shape searches and direct the search to suitable SETI
candidates.

Transmission by radio waves is well understood and can be easily
quantified. The relative energy costs of radio and laser signals has
been discussed elsewhereError: Reference source not found. The ratio
depends principally on assumptions made about how the laser signaling is
handled. It should be noted that most so-called high power lasers are
pulsed while it is integrated power that is important for message
transmission. Lasers and laser detectors are still in a Moore's law
development phase so that technology assumptions depend on the assumed
framework for the calculation.

Information transfer across space breaks into three components: the
launch or broadcast of the signal, propagation, and detection. Message
costs can be factored into these three parts. The propagation part is
dominated by the so-called gain or focusing power of the transmitting
antenna. The gain of an antenna is:

$G_{e} = \frac{4\pi}{\lambda^{2}}A_{t}$ (1)

where λ is the wavelength and A~t~ is the effective area of the
transmitter. The gain increases as the square of the ratio of the
effective diameter of the antenna divided by the wavelength. This is
because the antenna cone narrows and the signal is more tightly focused.
The antenna broadcast to a smaller and smaller region of the sky as the
gain increases. This is generally satisfactory for a radio or TV signal
since the star is much smaller on the sky than the radio beam area.
Antennas employing interferometry can be an exception to this rule. The
gain for a laser system is higher since λ is much smaller. As a result
the cone of a laser beam can be smaller than a star's planetary system
for nearby stars. The gain of an Arecibo-sized transmitter operating at
3 cm is 10^9^.

For a noisy radio link, the Shannon limit for the channel capacity in
bits/s is:

![](media/image1.wmf){width="1.5in" height="0.5277777777777778in"} (2)

where B is the bandwidth, T~N~ is the receiver noise temperature, and k
is Boltzmann's constant[^12]. P~r~ is the power received by the earth
antenna. Leigh showed that for a low signal to noise ratio the maximum
channel capacity is:

$C_{m} \approx \frac{P_{r}}{\text{kT}_{N}\text{ln}2}$. (3)

In this case the channel capacity depends on the power received but not
the bandwidth. P~r~ is given by the Friis transmission formula:

$P_{r} = P_{t}\frac{A_{t}A_{r}}{\lambda^{2}R^{2}}$ (4)

where P~t~ is the transmitter power, A~r~ is the effective area of the
receiving antenna, and R is the distance between the transmitter and
receiver. With a 10 GHz carrier (λ = 3 cm) a 1000 kW signal at 50 ly
could transmit substantially more than 10 Kbytes/s assuming
Arecibo-sized antennas with a receiver noise temperature of 10 degrees K
and a 1% bit error rate. A 1 Gbyte program or computer encyclopedia
would take less than a day to transmit and cost several thousand dollars
assuming an energy cost of 10¢/kWH. This is only an order of magnitude
more expensive than buying software on a CD. For some technology
assumptions the energy cost for laser transmission could be
substantially smaller. The dispersion of the interstellar medium limits
the transmission bandwidth. Following Leigh the upper limit on the
bandwidth for a 1000 ly path is 3.5 MHz, quite sufficient for a
10 Kbyte/s transmission rate at 50 ly.

These formulas can be used to determine δE~t~, the transmitter energy
per received bit:

${\delta E}_{t} = \frac{P_{t}}{C_{m}} = 4\pi\frac{R^{2}}{G_{e}}\frac{\text{kT}_{N}\text{ln}(2)}{A_{r}}$
(5)

and δE~r~, the receiver energy per received bit:

${\delta E}_{r} = \frac{P_{r}}{C_{m}} = \text{kT}_{N}\text{ln}(2)$. (6)

The cost in the transmitted energy per bit increases as the square of
the separation and decreases as the gain is increased. At the receiver a
lower noise temperature and a larger receiving antenna decreases the
energy/bit. For the example δE~r~ = 10^-22^ Joules or .0006 eV and δE~t~
= 3.6 Joules. The energy per photon is δE~γ~ = hν where h is Planck's
constant and ν is the frequency. For the example the energy per photon
is 6.63\*10^-24^ Joules or .00004 eV. Detecting one bit in the example
requires 14 photons at the receiver.

Interestingly, it can be shown that the energy costs per bit for
electromagnetic processes and directed panspermia and spacecraft
transmission via DNA are similar for matter velocities on the order of
10^-4^ of the speed of light. Part of this arises because DNA
information storage is remarkably efficient. A 5 mm diameter sphere of
DNA can store on the order of 25 exabytes of information. Of course
matter transmission is much slower than electromagnetic transmission.
Naturally different assumptions about radio, laser, and matter
information transfer change the relative energy costs. The impact of
future technologies could also change the relative relationships. In
considering message delivery systems it would seem that none of them can
be definitely ruled out at this point.

The most important point is that large amounts of information can be
transferred inexpensively at the speed of light even with current
technologies. In addition, the message size can easily be so large that
the underlying intent of the message would not be apparent.

### [DENATURING A SIGNAL]{.underline}

As noted earlier, archaeological signatures and beacons appear to be
comparatively safe for SETI. Characteristically a beacon would transmit
little information. On the other hand it may be a short step from a
beacon to a message. For example a beacon could point to a signal in a
different wavelength band where a message was coming in. A message
should be approached with great care. It may be extremely dangerous! Put
simply the receiver needs virus protection and it had better have an
electronic condom.

At least two scenarios need to be considered in protecting against a
malevolent SETI Hacker signal. One is a computer virus in the message
that takes over the computer at the receiver. The other is an open
message that gives an impenetrable software code or instructions for a
hardware translator to handle an opaque message. Both cases are
dangerous. The damage may be done before the receiver appreciates that
it is under attack. This is the current experience even with earth-based
hacker attacks. There may not be an opportunity to pull the signal out
of the computer or turn off the power before the intruding signal has
taken over.

It is an open question whether an earth-based computer virus can
penetrate a computer if it is not familiar with the operating system.
The computer and computer security experts I have discussed this with
don't think it is possible. The argument goes that viruses typically
enter a computer by exploiting known features in the operating system.
Further, experts argue, typical computer operating systems are quite
idiosyncratic so that it can be difficult to analyze their structure
from a logical point of view.

However, it seems worthwhile to approach the question with an open mind.
For example, one could set up a thought or even a practical test with a
primitive "toy" computer, perhaps modeled along the lines of the first
Illiac and have programmers unfamiliar with the Illiac system try to
hack the program. I believe it would also be useful to convene a
workshop with diverse participants to discuss the subject in some
detail. This might be coupled with broader discussions on the topic of
denaturing ETI signals.

There are a number of approaches that might be taken to denaturing a
SETI signal. Breaking the information into packets as is currently done
for <SETI@home> would seem to make it much harder for a large program to
bootstrap itself back together. Isolating incoming message on a
dedicated computer would keep the potentially dangerous signal in an
isolated environment. There may be additional techniques such as the use
of one-time only coding and constant program integrity checking. Again,
a convocation of experts could identify approaches to handling this.

Ultimately what may be needed is a protocol similar to the one for
biological contamination of a probe returning from space. The
International Committee on Space Research (COSPAR) developed such a
protocol through extensive international negotiations even involving the
United Nations[^13]. Concern about biological contamination grew out of
many experiences with situations where new biological species were
introduced to another ecology and then seriously impacted the
environment. Note that the SETI field does have a protocol[^14] to
follow if a signal is discovered but its purpose is to avoid public
relation problems if a signal is announced prematurely.

### [SUMMARY]{.underline}

The combination of energy cost and speed of transmission strongly favor
the use of electromagnetic signals for propagation of information over
interstellar distances. Present SETI efforts concentrate on
electromagnetic searches. An electromagnetic signal is feasible even
with current earth-based technologies and can carry enough information
to be dangerous. However, for an ETI signal some sort of translator is
required at the receiving end. As a result, the signal needs a lure to
induce the receiver to untangle the message. Such a lure would probably
be quite interesting and appear reasonable in intent. This implies care
should be taken in working with SETI signals.

This situation deserves serious attention from the SETI community. The
possibility of a malevolent SETI Hacker signal must be assessed and
protective measures should be put in place prior to the receipt of any
real signals.

[^1]: Cocconi, G., Morrison, P., "Searching for Interstellar
    Communications," *Nature*, **184**, 844-846, 1959.

[^2]: Tarter, J., "The Search for Extraterrestrial Intelligence (SETI),"
    *Annu. Rev. Astron. Astrophys*., **39**, 511-548, 2001.

[^3]: De Duve, C. "Life Evolving. Molecules, Mind, and Meaning,"
    *Oxford*, New York, 2002.

[^4]: For a contrary view see Ward, P., Brownlee, D. "Rare Earth,"
    *Copernicus*, Springer-Verlag, New York., 2000.

[^5]: Sullivan, III, W., Brown, S., Wetherill, C., "Eavesdropping: The
    Radio Signature of Earth," *Science*, **199**, 377-388, 1978.

[^6]: Cited in Sagan, C. et al., "Carl Sagan's Comic Connection," p.
    236, *Cambridge University Press*, Cambridge, 2000.

[^7]: Carrigan, R., Carrigan, N., "The Siren Stars," serialized in
    *Analog* Mar.-May, 1970, published by *Pyramid*, New York, 1971,
    Hoyle, F., Elliot, J., "A is for Andromeda," *Souvenir Press Ltd.*,
    London, 1962.

[^8]: Carrigan, Jr., R., "The Ultimate Hacker: SETI signals may need to
    be decontaminated," to be published in \"*Bioastronomy 2002: Life
    Amongst the Stars*\" Astronomical Society of the Pacific IAU
    Symposium Series, **213**, editors Norris, R., Stootman, F, 2002.

[^9]: See, for instance, Secker, J., Wesson, P, and Lepock, J.,
    "Astrophysical and biological constraints on radio panspermia,"
    *Journal of the Royal Astronomical Society of Canada*, **90**,
    17:184-192, 1996.

[^10]: Crevier. D., "The tumultuous history of the search for AI,"
    *Basic Books*-Harper, New York, 1993.

[^11]: Lesk, M., in comments that appear at
    <http://www.lesk.com/mlesk/ksg97/ksg.html>, 1997.

[^12]: Leigh, D., "An Interference-Resistant Search for Extraterrestrial
    Microwave Beacons," submitted to The Division of Engineering and
    Applied Sciences, Harvard Ph.D. thesis, 1998.
    (http://seti.harvard.edu/grad/d_thes.html).

[^13]: Nealson, K., et al., "Mars Sample Return: Issues and
    Recommendations," *Space Sciences Board of the National Academy of
    Sciences, Task Group on Issues in Sample Return*, 1997.
    \<http://www.nationalacademies.org/ssb/mrsrnot.html\>.

[^14]: Billingham, J., Chairman, "SETI Committee. Declaration of
    Principles Concerning Activities Following the Detection of
    Extraterrestrial Intelligence," *International Academy of
    Astronautics Position Paper Annex,* 1996.