# Help catching this serial killer!

In the calm and peaceful Rurdem Town, a series of murders is driving the local police crazy. The police suspects this to be the deeds of a serial killer. They are right about that: the arrogant psycho got borred after the seventh murder, and decided to challenge them. He left behind a note that contain clues about his identity. But the jerk encrypted it. The police, frustrated, decided to set up a big bounty to whomever decipher the note and identify this serial killer.

The note left by the serial killer:

Decipher this and you'll know my name:

87E95 116M120 149W169 19D24 74X91 1R14 7Z30 10L27 135W153 102P111 101Q113 141Q159
27P40 25O37 35U51 75F82 43K49 2Y20 10A18 4W22 59S71 122B143 107N122 104V123 52V69
127L130 137L142 59R68 56M64 67J74 120C144 30N41 109W125 121J126 76D99 56U74 108R127
8Q30 5Y21 3R16 79L84 91H116 52W68 84Q96 125K133 11X28 22V37 75U93 4I8 126J133 8Z25
10D15 10C30 138P151 22W40 39Q47 56A78 95L110 9B34 69O79 135F138

I'll leave hints with each victim I kill next.


Note: The story is not necessary to solve the puzzle. You can skip it if you want.

Hint #1:

Subtraction is needed.

Hint #2:

If you give up, you'll never know who I am. (notice the added tag)

Hint #3:

 34O44 127W127   56H59 15P34 119K125 19C25 40P51 35B40   146V163 53M69 146N155 79B84
80N91 24D29   124D125 59F68 94O106 34G40 34F41 8J15 78M92 130J133
85C89 138L143 106P125 110H111 33A37 34W50 71F82 126R137 45K61 8U24
85J100 118D141 31V32   148J151 110D115 69W85   39J54 73H74
130F139 115D138 65L80 21H26 22C28 140G140

Hint #4:

 44U60 48M64 42O52 119C125   13N22 114D119 39V56 33R54 36G38 128P147 83T100
87B92 101P114   103L118 61Q73 74Q96   147H150 85L102 60M70 130K142 149P162 78U102
46U64 42F51 136H137   69P90 143C167 84J99 78F81
82L101 64B89 60Q82 96U114 83B90 116T131 28M42

The serial killer's name is

George Osten

Before I give the solution, I ask you to image the encryption mechanism as a wheel with an arrow pointing to a letter. Like a clock, but instead of going from 1 to 12, it goes from A to Z.

Solution:

Using the decryption formula (see Anne's answer and my previous one), one finds that the message is GIVEUPYOUMORONSGIVEUPYOUTHINKIAMTHATSTUPIDTOJUSTGIVEYOUMYNAME. Now, when applying the formula on the original letters, EMWDXRZLWPQQPOUFKYAWSBNVVLLRMJCNWJDURQYRLHWQKXVUIJZDCPWQALBOF, one finds that in most cases the "arrow" moves counterclockwise from the cyphertext to the cleartext. Except the following letters in bold: GIVEUPYOUMORONSGIVEUPYOUTHINKIAMTHATSTUPIDTOJUSTGIVEYOUMYNAME

This fits nicely with hint #3:

"My friend told me chngging directions may get me hanged"

And with hint #4:

"Some letters do not follow the same pattern." Where the pattern is the arrow moving counterclockwise.

• You got it man! Bounty will be sent in two days (don't relly on it though). Good job! – ibrahim mahrir Dec 10 '17 at 2:41
• Thanks! This was a very exciting puzzle! to figure out the original message, I analysed the distribution of values and compared it with letter frequencies in English; then I guessed that "give up you morons, give up" is such strange wording that the final solution might be a subset of, or based on, the actual message, and that realisation eventually led me to the guy's name. – J. Siebeneichler Dec 10 '17 at 2:48
• Yeah! I just had this idea: what if a serial killer is so arrogant that he really give his name to the cops but at the same time make sure they never get it, well, he must hide it in a message that says "I won't give you my name". Plus I love the concept of matryoshka dolls. – ibrahim mahrir Dec 10 '17 at 2:54
• Nice job (+1). I was (too) hung up on the word "chngging" in Hint #3, haha. – Anne Dec 10 '17 at 16:18
• Good job, @J.Siebeneichler – Joe Dec 11 '17 at 21:47

Each block of characters in the text is of the form $n_1sn_2$, where $n_1$ and $n_2$ are nonnegative integers and $s$ is a letter of the alphabet.

We can get

the position of a second letter (let's call it $t$), from the difference $n_2-n_1$.

Note that

there are two possible "distances" between $s$ and $t$ if we use the letters "circularly" (as hinted). The idea is basically to find the "middle" letter between $s$ and $t$ in the "shorter" direction.

Example:

We can say that A and Y are a distance of $24$ apart (going alphabetically) or $2$ apart (going reverse alphabetically). So the "middle" letter in the shorter direction is Z.

If we want to state this a bit more formally:

Let $d_1$ be the distance if we start from $s$ and traverse the alphabet to $t$ alphabetically, and $d_2$ be the distance if we do the same in the other direction.

The position of the new letter then ends up being:

$$\left(\text{position}(s) +\text{sign}(d_2-d_1) \times\left\lceil\frac{\min(d_1,d_2)}{2}\right\rceil\right)\mod26$$

This should fix all the "problematic" letters, I believe.

Example 1:

The 22nd block gives us $n_2-n_1=143-122=21$, which corresponds to V. $d_1=21-1=20$ and $d_2=(1-21)\mod26=6$. This gives us a position of $(1-3)\mod26=24$, which corresponds to Y.

Example 2:

For the third to last letter we get: $34-9=25$, which corresponds to Z. The shorter distance is clearly the one via "A", i.e., $2$. And the "middle" letter going in this direction is, well, A.

Hopefully all that was clear (apologies for any errors/poorly explained bits).

• Nice write-up. What do you make of the 3rd hint? – M Oehm Dec 9 '17 at 13:33

The hidden message is

giveupyoumoronsgiveuplouthinkinmthntstupiqtojustgiveloumlnnme

Which I interpreted as

Give up you morons, give up. You think I am that stupid to just give you my name?

The solution is:

Each element is composed of XYZ, where X and Z are numbers and Y is a letter. The value of a letter is its alphabetical position, starting at 0 (A = 0, B = 1, etc.) The decryption formula is ((Z - X) + val(Y)) / 2, which gives you the value of each cleartext letter. For some reason, it didn't work for Y, D and N, but the message is understandable anyway.

UPDATE: The messages from the hints are:

My friend told me chngging directions may get me hanged

and

Some letters do not follow the same pattern.

• You're on the right track. It didnt't work for some letters because you have to use the letters in a circular way: ....XYZABC...XYZABC... (and use the smallest distance). – ibrahim mahrir Dec 7 '17 at 16:45
• The puzzle is not yet solved though. – ibrahim mahrir Dec 7 '17 at 16:45
• Is the interpreted sentence correct? It sure seems like it, the formula is just not quite explaining how to get there for every entry – Joe Dec 8 '17 at 15:59
• @joe The interpreted sentence is right. For the entiries that didn't match I did explain how to use the formula correctly in my first comment. – ibrahim mahrir Dec 8 '17 at 17:16
• My confusion is that for all the entries that should be a Y, the subformula ((z-x) + y) is 23, so it doesn't roll over, nor does it equal Y. and when divided by 2, it ends up at L. – Joe Dec 8 '17 at 19:25

Partial - what I have so far

"Words" do not repeat themselves and are composed of two numbers separated by a letter, such that the second number is always higher than the first.

For B,D,F,H,J,L,N,P,R,V,X,Z, the difference between the two numbers is always odd

For A,C,E,I,K,M,O,Q,S,U,W,Y, the difference between the two numbers is always even

G and T are the only absent letters

• I would add that the difference between numbers is always between 3-25. Which conspicuously looks like 1-26. Still not sure what that means though :) – Joe Dec 7 '17 at 0:25