Its been bugging me a bit, that community advertisement else site. I'm curious about what it says, but I don't know if asking about it is on topic or not.

I note that one of the off topic reasons is:

Requests for analyzing or deciphering a block of data are off-topic here, as the results are rarely useful to anyone else.

Which would seem to indicate that "no, asking about how to decrypt this is off topic"

Are there on topic questions that can be asked for that given block of text?

(An asside, if there aren't any, next time the community advertisements come around, could you please not tempt me to ask an off topic question about what it says... or create a historically locked question that does have the answer for those who search for it?)

I would tag this (link), it says for discussion about community ads, please use the advertisement tag, but that one doesn't exist and I can't create it... so only one applicable tag at this time.)

# On-Topic or Off-Topic

That “temptation” you're worried about is actually the teaser of the ad… and it obviously works. ;)

If you look at the advertisement again, you'll notice that the advertisement goes hand-in-hand with the rule you're referring to. For a good reason, the byline clearly states “Don't be puzzled anymore… learn about cryptography instead of “Come over and we'll decrypt whatever you throw at us” .

Like mikeazo already commented, it depends on what exactly you're asking. Generally, questions are on-topic at this site and just like the two example questions mikeazo mentioned (1,2), such questions can include ciphertext and still be on-topic. Just don't post ciphertext-only questions asking to decrypt a block of scrambled characters.

Taking your example: “…asking about how to decrypt…” would indicate you're probably on-topic, while you're most probably losing yourself in the off-topic area when you're just “…asking to decrypt…”.

See, what that rule you're referencing to actually tries to make clear is that questions that look like

decrypt this: $tTr3t§4%$/&%23§er3Dt8/%"$&"9%$&e!\$§H9Hg9?5f§


are off-topic because “the results will be rarely useful to anyone else”.

Yet, you are obviously interested to learn about how to analyze and decrypt the ciphertext shown in a Crypto.SE advertisement. In this particular case, the answer could be useful to multiple Crypto.SE members, which pulls your question somewhat into an on-topic area.

But you're correct in saying that it's a bit confusing. As Reid pointed out, there was some back-and-forth discussion about the ad, but somehow we stuck with it.

Looking at it, I think you've chosen the optimal path: in case of doubt, ask on Crypto.META.

# Solving the Mystery

Since I was the one who created that advertisement (yep, you can put all the blame on me), and since some people (including myself) might argue that Crypto.META is the best place to handle questions related to a Crypto.SE advertisements, I will simply provide the solution here for your convenience (and for reference purposes).

Let's walk through this together…

## Analyze

What we know is that there are 3 lines of ciphertext:

1. DSZQUP.TUBDLFYDIBOHF.DPN
2. ETARVQ.UVCEMGZEJCPIG.EQO
3. FUBSWR.VWDFNHAFKDQJH.FRP


If you look at it closely, you’ll notice . characters, which appear at the same position on each line. This indicates you might be looking at a repetition of something, because those dots appear at the same position on each line. So the first hint you have is that you're most probably looking at information containing: 6 characters + . + 13 characters + . + 3 characters.

While staring at the ciphertext, another thing you might notice is that the three lines seem to be somehow related. Line 1 starts with a D, line 2 with an E, and line 3 with an F. This hints that the first letters increase with +1 each line, as if the line number is somehow related. That might influence all letters on the individual line… something to keep in mind.

## “Attack” (better: Guess)

Let's take the hints we've got and play a bit with those letters by guessing that the first line contains letters what are +1 of the original plaintext letters. After all, each following line seems to be +1 too… at least, when looking at the first letters of each line. So why shouldn't there be a line 0 which contains the plaintext which was then calculated +1? Let's stop wondering and simply see if we're correct:

1. DSZQUP.TUBDLFYDIBOHF.DPN

D - 1 = C
S - 1 = R
Z - 1 = Y
Q - 1 = P
U - 1 = T
P - 1 = O
. (dot character)
…


If you go through the rest of the letters yourself, you'll quickly be able to discover the plaintext.

Great, but what about the next two lines? Well, to find out what line 2 says, remind yourself of the fact that the first letter of the 2nd line was +1 of the first letter of the first line. It's logic to try to use that information. As we decrypted the ciphertext of the first line by calculating “letter - 1”, let's now take the letters of the 2nd line and see if “letter - 2” works:

2. ETARVQ.UVCEMGZEJCPIG.EQO

E - 2 = C
T - 2 = R
A - 2 = Y (If we assume A-1=Z, then A-2=Y. The characters obviously wrap in range [A-Z].)
R - 2 = P
V - 2 = T
Q - 2 = O
. (dot character)
…


Work your way through the rest of the letters. Knowing what plaintext line 1 returned, you'll quickly discover it produces the same plaintext we already retrieved in line 1.

By now, your gut feeling should already decipher line 3 for you… dare to trust that gut feeling and simply deduct that if “line 1 was each plaintext letter + 1”, and “line 2 was each plaintext letter + 2”, then line 3 might simply be each plaintext letter + 3. To get the plaintext, all we would have to do is to calculate ciphertext letter - 3:

3. FUBSWR.VWDFNHAFKDQJH.FRP

F - 3 = C
U - 3 = R
B - 3 = Y (again, wrap A-Z)
S - 3 = P
W - 3 = T
R - 3 = O
. (dot character)
…


Again, work your way through the rest of the letters. Already having successfully decrypted line 1 and line 2, decrypting line 3 should be easy (and almost boring).

## Understand

In case you want to verify if you've correctly decrypted it: the plaintext represents a (repeated) answer to the question “Where can I learn about Cryptography?”

Spoiler 1 – the plaintext:

crypto.stackexchange.com
crypto.stackexchange.com
crypto.stackexchange.com

Spoiler 2 – what it’s based on:

I based the whole thing on a real crypto-classic: the Caesar cipher (ROT13). But instead of using the regular ROT13, the ciphertext in the advertisement was gained from rotations which relate to the individual line numbers… meaning: line 1 is ROT1, line 2 is ROT2, and line 3 is ROT3.

That's it. Mystery solved and (let's just call it) “cipher” explained.

By the way… welcome to Crypto.SE! ;)

• I'd like to see that poster on a dark, seedy, half-forgotten alley of a metropolis in some post-modern sci-fi thriller. That's probably where the protagonist would find clarity right before their ultimate (and foreboded) demise. Joking aside, good work.
– rath
Jul 25 '14 at 22:27