"I then need to verify you are from an advanced civilization.  Do you have a suggested proof methodology of your own to offer?"

"We can potentially contact others of your world and speak into their own minds, given sufficient information about them; but that could take two half-minutes, and we expect you've come up with preferable methods.  We're happy to play it your way."

(One of the fundamental principles of civilizational design is that, once you build a dangerous mechanism, there is some chance somebody might trigger it, deliberately or by accident.  If you build thirty thousand nuclear weapons, and then somehow oops they all get set off, this is a kind of thing that happens when you build thirty thousand nuclear weapons in the first place.

If you ask what the mechanism for a First Contact situation ought to do, the answer is that it should do some things you'd want to avoid doing if there was not a First Contact situation going on.

Dath ilan can aggregate almost effectively enough to be something like a single entity in some ways, especially in emergencies.  Making a deliberate choice to strip the phrase 'dath ilan' of the agent-marker doesn't change that, whatever it may morally emphasize.

Dath ilan has become almost like a person in some ways, and the First Contact alarm produces something of a startle response.

So it should be as impossible as possible to set off the First Contact alarm unless First Contact is actually happening, even if some of the people in charge of the system decide they've gotten really bored.  Ninth-rank Keepers can go insane too.

So consider then this question:  How could you possibly verify that you are talking to an alien civilization, in a way that nobody in all of dath ilan should be able to fake?  After all, to ask some difficult question that is proof of great wisdom, and know the aliens' answer is correct, wouldn't you have to know the answer yourself?  And then, might not somebody else figure it out too?)

"Speaking into my head would be interesting, but not full verification from the standpoint of greater Civilization; I'd prefer you avoided that for now.  On our own plan, there's two roads we can take from here.  The first one is that I transmit a specification of a problem, in a simple programming language, which is too hard for our civilization to solve, but which we could easily verify a solution for.  That strategy was designed for higher-bandwidth First Contact channels than this one seems to be.  But it's the diamond standard if you do have a good way of receiving a higher-bandwidth transmission, and you have fast programming capabilities, and powerful computers."

Helorm reports back that the person on the other end says she's not a professional programmer, though she's connected to the Network and their civilization generally has more advanced computers than does dath ilan.

This general class of possibility has also been considered, and a fallback strategy devised.

The Keeper starts listing some possible math questions that require large amounts of computing power to answer, questions that seem of sufficient probable interest to alien civilizations that they might have computed them already.

Helorm repeats the math questions to the voice, and repeats back the voice's "Yes" after the Keeper mentions primes of the form 2^n-1 for some integer n.

Then "51", after the Keeper asks how far they've gotten.

51 is... a lot.

And Derrin was not expecting the other end of this call to cheerfully go along with any of his tests, at all.

Well, this is it.  The Keeper keys a preliminary acronym into a symbol pad, then says, "Give me the exponents for the 45th up to the 49th such primes.  Whichever five such primes come after 2^32,582,657 - 1."

"37,156,667," says Helorm, carefully and slowly.  (Baseline digits are all single syllables, and none of them sound like each other, but there's no point in taking chances.)  "42,643,801.  43,112,609.  57,885,161.  74,207,281."

The Keeper finishes entering those five numbers into his numpad, and then double-checks them by repeating them back to Helorm from the screen above his keyboards, before pressing the Completion key.

(How can you build an alarm for First Contact situations that nobody inside Civilization itself should be able to trigger, if you can't just send off a hard computational problem and verify the solution?

You could ask for a higher Mersenne prime than Civilization has ever searched, and begin to verify its primality.  But if it was possible to verify a large candidate Mersenne prime in a few seconds, it would be the sort of question where it was possible for a mildly rich investor to test lots of 8-digit numbers quickly and discover the answer themselves.

So instead, Civilization has found the 45th Mersenne prime via a large public-effort clustered computer, and then (according to Network repositories on the subject) declined to announce that prime as it announced the previous 44.  Instead, the discovered number was stored in a few secure places; as one of several potential resources for use in verifying First Contact with some more powerful civilization.

And this is literally true, so far as literal statements go.  It's not like the government makes a habit of literally lying; that would be a very bad habit for the government to acquire.  People would then need to expend mental effort, during every interaction with the government, on wondering whether the government's words were related to reality at all; and if things ever got anywhere near that bad, the dath ilani would toss out their current civilization and start over.

But that literally true statement is also a trap.  One can imagine some illdoer who wants to trigger a false First Contact alarm.  There would be all manner of ways an illdoer might imagine profiting from that situation.  Even dath ilani try to avoid leaving billion-dollar bills out where ill-intentioned people can too easily pick them up.

The 45th Mersenne prime, 2^37,156,667 - 1, is not heavily guarded.  It is not that impossible for a criminal to steal one of the stored copies.

The 46th Mersenne prime, 2^42,643,801 - 1, is stored in only one place, an advanced weapons silo already under strong guard.  It was searched-out and discovered under conditions of much higher secrecy.  Any criminal trying to pose as an alien civilization, one might hope, will not realize that they need to steal a 46th Mersenne prime at all.  They'll just show up with stolen knowledge of the 45th Mersenne prime, and no knowledge of the 46th one.

So if a 'contactee civilization' shows up with knowledge of the 45th Mersenne prime, but not the true 46th prime, some highly placed key figures will have a pretty good idea what happened.)

(...Of course, showing up with knowledge of just the 45th and 46th Mersenne primes is also a trap.)

(This about dath ilan:  Within it, there are a number of serious, altruistic, cooperative, highly and perhaps overly intelligent people, who have never gotten a chance to be a cunning mastermind in their whole lives, outside of maybe some fiction they write on the side.

One of those frustrated would-be masterminds, if you ask them to outwit hypothetical criminals, is not going to stop thinking, gaming, counterplanning, and building traps, until they have run out their full budget.

The 47th Mersenne prime was searched out under conditions of even greater secrecy, and never written down at all.  If you write something down, somebody might steal it.

Instead, after the 47th Mersenne prime was found, it was deleted.  Only a cryptographically secure hash of it was preserved; and that was used to build three black-box circuits that can verify, up to once per day, whether or not their input is the previously found exponent of the 47th Mersenne prime.

If somebody shows up knowing just the 45th and 46th Mersenne primes, but without the key input to the black boxes containing a secret hash of the 47th prime, the Government will have a good idea what happened.  And the Government will be very very impressed, though not at all amused.)

(Now consider: how secure is that arrangement, really?

The first question: could some lone genius devise a much faster way of finding Mersenne primes?  This does seem relatively unlikely.  The method for testing Mersenne candidates is already running nearly as fast as possible, according to some provable lower bounds.  Mersenne numbers are the largest known primes exactly because they are a special case whose primality can be verified in time very close to a lower bound.

Separately, there are a lot of would-be mathematicians in dath ilan, and a lot of them are looking at prime numbers, and the monetary and prestige bounties for fundamental discoveries there are already quite high.  It happens maybe once a year, in dath ilan, that some lone genius finds a single lump of low-hanging fruit that large; and so that is unlikely to occur with respect to this exact math result in particular.  A discovery like that is not something a would-be prankster can reliably force reality to give them, not with any amount of effort.  Furthermore, being a mathematical supergenius in dath ilan is a full-time job.  So far as Civilization knows, not a single such mathematician has yet found time to become a criminal mastermind on the side.

Then move on and consider the three black-boxes containing the hash of the 47th Mersenne prime.  All three were placed in guarded locations, but guarding is imperfect.  And perhaps, what artifice has built, artifice might be able to probe?  If you could read out the stored hash result inside the black-box device, you could test many possible inputs against that hash.  Which, if that hash can be checked immediately, during an emergency time-sensitive First Contact situation, will allow for parallel verification of candidate inputs much faster than a Mersenne candidate can be checked for primality.  From this, the original input, the Mersenne prime, could be recovered.

And above all - given that Civilization was able to find the 47th Mersenne prime in the first place, without coopting all of Civilization's computers for a week to look for it - might not some sufficiently enterprising criminal be able to collect enough computers, or break the security on enough Networked computers and coopt their computing power, in order to find the 47th Mersenne prime themselves?

In security, it doesn't matter how much higher you make a wall, if somebody can just walk around it from another direction.  Putting infinitely many security guards around the stored hash of the 47th Mersenne prime won't prevent somebody else from finding it again the same way you did.  The project that found the 47th Mersenne prime was not given an infinite budget or an unbounded draw on Civilization's computing resources; it is a fundamentally shaky assumption to imagine that nobody else in Civilization could do what that project did.  That's why the project leader didn't bother trying to make the whole system any more secure than it already is.

Hopefully, a clever criminal does not realize that finding the 47th Mersenne prime is necessary at all, to fool Civilization...

But that is security through obscurity.

Well, but seriously, though: this hypothetical criminal must now be anomalously rich, or anomalously good at breaking into Networked computers.  And rather than putting this prior resource to any more profitable and less attention-gathering purpose, they have become fixated on fooling Civilization about whether First Contact has occurred - briefly, for the weeks required to verify a higher claimed Mersenne.  Such a criminal now seems unlikely.

Absolute certainty is impossible to both bounded and unbounded agents.  So doesn't there come a point where you have to distinguish possibility from probability, and say you've done enough?)

(There is a family of sayings in dath ilan, all of which begin with the proverb, "A bounded agent cannot do all things", and continue onward to say that bounded agents can aspire not to overpromise what they can do.  A bounded agent can't achieve perfect discrimination by assigning 100% probability to all truths and 0% probability to all falsehoods; but a bounded agent can aspire to calibration, to only assign probabilities as extreme as 90% to events that actually happen 9 times out of 10.  If, being a bounded agent, it doesn't know with any greater surety and can't attain any greater surety, it can aspire not to claim that it is any surer.

For project managers, the saying goes, "The bounded budget you're given cannot attain all the customer's desires, but you can aspire to report back honestly which goals are feasible."

Even among a certain sort of frustrated dath ilani, they wistfully aspire to be sane masterminds, not mad ones.  They aspire not to be overconfident in their cunning plots, if they ever have a chance to plot properly.

The project manager in charge of cunning Mersenne plots reported back to his superiors that he'd made it reasonably difficult for a criminal mastermind to know the 47th Mersenne prime by means other than recomputing it themselves; which would require the following budget a factor of three greater than his own, absent public-good discounts; or breaking into the following number of Networked computers; and it would also require deducing that the whole series of traps existed, but that, unfortunately, any smart person might deduce.  The line of Civilizational code had been rendered very unlikely to ever execute in real life, realistically speaking; but the hypothetical illdoer had perhaps not been rendered less probable than an actual First Contact.)

(Of course, that entire project was a decoy.)

(It existed because a project like that would exist, if matters in dath ilan were generally as they appeared on the surface.)

(The 48th Mersenne prime is separated from the 46th and 47th primes by a noticeable gap.

It was found over the course of three days, using four percent of the power of a supercomputing cluster millions of times more powerful than is known publicly to exist.

Only one such supercomputer exists in dath ilan.  Every line of code which runs on it is accounted, monitored, and checked by multiple independent lines of reporting.  All the chips within it were manufactured by an entire technology base that is also not supposed to exist.  If a dark conspiracy is running unauthorized code on this supercomputer, or if they rebuilt that entire technology base with nobody noticing, dath ilan is already having a catastrophic civilizational failure and a massive emergency.  It is the sort of thing that genuinely should not happen, as security assumptions go.

In the next lines of the theorem-proven code that found the 48th Mersenne prime on that supercomputer, the program hashed the result and deleted the original.

That hash was a distributed hash.  It is not possible for some enterprising criminal to sneak into a single location containing the stored hash of an expensive 8-digit number, extract the stored hash, and try a hundred milion possibilities against a fast hashing algorithm.

Instead, there's twenty locations with shards of a hash; twenty nodes that need to send messages onward to each other, in order to arrive at a final result that each node will locally verify.  Trying even a single entry against them all is a rare exceptional operation that people will notice; even if you break into the system three times, you won't get nine tries.

These twenty locations are all the sort of places that were already under the heaviest level of operational security.  It is genuinely reasonable to say, as a security assumption, that clever criminals cannot break into 17 of them without notice.

Only if the distributed locations agree they have received 17-of-20 correct partially-hashed inputs from each other (so you cannot stop this by sabotaging any three of them), will the corresponding local alarms go off.  People at each of those locations will then start checking with each other by telescreen, to confirm that the alarm went off everywhere and wasn't a local anomaly caused by somebody hacking the local trigger that checks for the distributed confirmation.

Nobody on dath ilan knows the 48th Mersenne exponent; no conspiracy on dath ilan should be able to obtain all the pieces needed to reconstruct it; and no faction on dath ilan should be able to find that number the same way it was originally found, unless they have already gained control of Civilization's secrets nearly in toto.)

(It is actual security.)

(The designers really didn't want this alarm going off unless it's a genuine emergency.)

The Keeper presses Completion on his numpad.

The number 57,885,161 goes in and is split up into a distributed hash, which is then transmitted on to twenty different special-purpose computers all over the planet, sending partially hashed inputs to each other that can be locally verified given other remote inputs.

The verifications check out.

No one in Civilization should have known this mathematical fact, or been able to learn it.

Something knows it anyways.

Six thousand key people are paged, woken up out of bed, or have VTOL aircraft dispatched to grab them.

Emergency-alert melodies play in a tiny remote causally-sealed passive-observation bunker containing some people who actually learned history, in case this is a situation where remembering the past would help enough that the seal on it ought to be broken.

All the anomaly filters are disabled on all of the software; the last 24 hours of stored data start to undergo re-review without those filters; and everything anomalous is queued for review by millions of idle or interruptible dath ilani mechanical-turkers who have just this second been paged with override project-requests.  Civilization turns a massive and unsustainable level of attention on inspecting itself for anomalies, starting with military installations, key infrastructure, and anything to do with children.

A massive three-stage fission-fusion nuclear weapon on top of a space-capable missile is armed and released to Governance control, along with a sequence of smaller fusion weapons, in case anything on or near the planet needs to be vaporized.

An additional cryptographically hashed message is recovered.  This one combines with a code in possession of the current head Keeper.  It permits her to broadcast a radio signal that, if its one-way hash matches a stored result built into secret hardwired circuits, will destroy every Networked computer in dath ilan on ten seconds' notice.

Green lights flash in the streets of a massive secret city housing all the Artificial Intelligence researchers.  Dath ilan has not built an emergency best-guess-alignment Artificial General Intelligence to keep in reserve; they have Earth-orbit asteroid monitoring, they are carefully monitoring the Yellowstone caldera.  The chance that an Artificial General Intelligence will be wrongly set off, if built, has been evaluated by secret prediction markets to be greater than the chance that such a tool will be suddenly needed on short notice.  This prediction has now been revealed as horrifically wrong.  The green lights are telling everyone to drop everything and take their best current plans and move forward now, trying to get dath ilan as quickly as possible to the point where in an emergency it can turn up a dial on the intelligence level of hopefully-aligned AGI; though of course they are not supposed to actually turn up that dial, unless everybody is about to immediately die otherwise.

Emergency testing of the purported 49th Mersenne prime is now in progress.

After Derrin presses the number-finished key, there's a five-second pause, and then his keypad flashes blue.

Keeper Derrin is as shocked as he's ever been in his life.  He lets his eyebrows go up visibly, since trying to present a perfect composure he doesn't actually possess would contradict his pride; if he wants to look that imperturbable, he needs to practice until he really is that imperturbable, not fake it.  "Checks out," he says.  "Observers, one of you go check the date of the next total lunar eclipse.  There's no preset code for that scenario and you need to start emergency rupture of our causal containment if that event is less than one week away.  Also, observers, if my keypad did not just flash blue, tsi-imbi."

Helorm actually chuckles.  Part of her is starting to go along with the pretend reality, especially given that she's sitting right next door to a psychiatric hospital and should really have been sedated by now.  [He said 'tsi-imbi',] she tells the voice in her head, and hears a startled laugh from the other end.

"The voice in my head says to tell you that you're not insane either," Helorm says a moment later.  "Wait.  How does the voice know what tsi-imbi even means, if it's from an alternate civilization that diverged long ago?"  If she's going to take this seriously as reality, there are Many Additional Questions here.

The Keeper is currently entering acronyms on his letterpad, preset codes which roughly translate to 'Putative parallel timeline of humanity, pre-curtain divergence point' and 'Contact is with small subfaction of contactee civilization' and 'There might be superintelligences hanging around near the other end of this call, though they are not putatively directly on the line' and 'Contactees probably don't understand their contact method and got it from elsewhere' and 'Contactees have not figured out Artificial Intelligence is dangerous' and 'Contactees have already interacted with at least one invited or abducted dath ilani' and 'Contactees claim to be interested in positive-sum reciprocal cooperation.'  Obviously, direct transcripts of this conversation can't go out of causal containment until somebody has thought about unknown infohazards of unknown magnitude.  These preset codes, transmitted in deterministic order, will get things started.

"Time-dependent processes have now been set in motion on my end," replies the Keeper with the auditory track of his mind.  "If there's nothing time-dependent on your side, you can go ahead and tell her."