8
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I devised this cipher independently, but I have no idea whether it's easy or hard because I have little knowledge of cryptography. The algorithm is simple but not trivial.

AYVEKCI0 3D3 1UDTY1NWBYFJFUJDLQOFQIDWZ0UQMVWKLUCO3GWXWO

Note that both 0s and Os are present.

Hint:

Here are symbols you might think are possible in this cipher, but are not: 456789
Here is a symbol you wouldn't know is possible from the given code: ?

More code:

FYW?HIJUHZ0LPENNS0SZBF?N3DUZTECJMJVTMY03HDPJWH2Q0NZIE31SNUV YOBK
MRKTBFFASDXBKLKR?FAL3PWO30IIBM HKGO PCSHD1PDYZPUIQVHHVIZYB BTHHP
M IPEX2R L3Z?FAL3YH?120QD 3D3KOVTU?V2B2IBVEV 23XDSHMYUJQSV?SDMJH
UNYJJUJVATFE  TCX0D3KOVPY? WFSIF2LFADUYJG22S1?SWAUFEIF?ORKDNYLUX
NC2MLFPN1AMBY3VG?R2LMRD1BKMJVOBXGVTN OVHIR0JBZLJSXTAXZ2XEJYJITJY
AEFYCCR12REYJMOAIH2ITSC3SLVC01YMVIOFMZNNMGHEWQHLAGIOTM1QSHA1AYVE
KCI0 3D3 1UDTY1NWBYFJFUJDLQOFQIDWZ0UROTVKXKG32HLPM CZDS3NRHGVCF?
O2?D2IS0GPLM0KEYYSDZIYSUL3UHOGJ3GJ0KSJ?BYUFSHFNLVAL1DEKEULGPL1YH
B1 XMQLKLDSL1FLKNWF3YEGXOSO1XJTX FT VOTYNZ2WDQFMH0W3L VMVVDUHFO0
MRDMCJLKWNJ?G2GG?JWLUXT3LWZTGYGB 1D 0XII3T1MQSHA1AYVEKCI0 3D3 1U
DTY1NWBYFJFUJDLQOFQIDWZ0UROTVKXKG32HKWO2DVSTM MRIMQKH3O?N22YSYNQ
UNUIGUD2 DE0JUHIPUAZTHMUI2PVG03QUFUYUAFU N1QG3TOYEUDW GFVMXNOXQO
AFVSAV2BZ02W1NCZ3QSB 1?JCSULEQO3ETXE2IU2UYQCJQMQON31AURE32ESO3BV
0F DV TGO3P3O

Everything has been generated by computer instead of by hand, so there should be no more errors. So sorry for it being messed up at first!

2014/12/8 - More clues:

Characters are processed in groups of five.
The plaintext contains every letter of the alphabet except Q.

2015/6/30

? immediately precedes A.
The fact of 32 possible characters requires that characters be processed in groups of 5: 2^5 characters in the alphabet, 5 characters per group.

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  • 1
    $\begingroup$ @For I In Range: That's a convenient 32 character total. Binary mess maybe? $\endgroup$ – Lopsy Nov 25 '14 at 22:02
  • 16
    $\begingroup$ @MackTuesday: Let me grant you some knowledge of cryptography. Without a very big hint, or at least a full paragraph of ciphertext, this kind of puzzle is very boring to solve. All we can do is throw random attempts at the wall. With a paragraph of ciphertext, at least we can do frequency analysis. In my opinion, cryptographic puzzles are much more interesting when we know the algorithm but not the key. $\endgroup$ – Lopsy Nov 25 '14 at 22:05
  • 3
    $\begingroup$ The downvotes are probably because you posted it with mistakes. $\endgroup$ – pacoverflow Nov 28 '14 at 2:15
  • 2
    $\begingroup$ @MackTuesday please don't post clues in comments, especially after a ton of comments $\endgroup$ – dmg Dec 8 '14 at 15:34
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    $\begingroup$ It appears that each letter has been given a 5 bit representation. This leads to 32 possible characters - a-z,0-3 and space (which is why 4-9 are not allowed). $\endgroup$ – ghosts_in_the_code Jan 19 '15 at 13:37
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The hints:

? does not come after 3

and

? immediately precedes A

suggest that the characters map to some sort of numeric sequence. I started with the assumption that the letters and numbers would be contiguous, which leaves us three parts to move around:

[?ABC...XYZ]   [_]   [0123]

(I'll use _ to unambiguously designate a space.)

I then assumed that the digits must come at the end: otherwise I think we would end up with 0-9 and A-T (i.e. the digits are used to 'pad out' the list of characters).

Thus our only choice is where to place _ in the sequence, with three possibilities. It turns out that _ in the middle works for the next part.

We can use this ordering to map characters to numbers, from ? as $0$ to 3 as $31$. Then based on the hint:

Characters are processed in groups of five.

I broke the numbers into groups of five. The number $5$ matches nicely with the number of characters: $32=2^5$. This hints that we need to convert the numbers into binary. This gives us $5\times 5$ bits in each group.

While playing around with the blocks as bitmaps, I had the idea to transpose them, then convert back to numbers. For example, the first group of five characters:

$$ \begin{array}{rr|cccc} && \text{L} & \text{I} & \text{T} & \text{T} & \text{L} \\ && 12 & 9 & 20 & 20 & 12 \\\hline \text{F} & 6 & 0 & 0 & 1 & 1 & 0 \\ \text{Y} & 25 & 1 & 1 & 0 & 0 & 1 \\ \text{W} & 23 & 1 & 0 & 1 & 1 & 1 \\ \text{?} & 0 & 0 & 0 & 0 & 0 & 0 \\ \text{H} & 8 & 0 & 1 & 0 & 0 & 0 \\ \end{array} $$

This gives us, for both ciphertexts, the plaintexts:

LIFE GOES ON LONG AFTER THE THRILL OF LIVING IS GONE
and
LITTLE DITTY ABOUT JACK AND DIANE TWO AMERICAN KIDS GROWIN UP IN THE HEARTLAND JACKIE GONNA BE A FOOTBALL STAR DIANE DEBUTANTE BACKSEAT OF JACKIES CAR SUCKIN ON A CHILI DOG OUTSIDE THE TASTEE FREEZE DIANES SITTIN ON JACKIES LAP HES GOT HIS HAND BETWEEN HER KNEES JACKIE SAY HEY DIANE LETS RUN OFF BEHIND A SHADY TREE DRIBBLE OFF THOSE BOBBY BROOKS LET ME DO WHAT I PLEASE OH YEAH LIFE GOES ON LONG AFTER THE THRILL OF LIVIN IS GONE JACKIE SITS BACK REFLECTS HIS THOUGHTS FOR A MOMENT SCRATCHES HIS HEAD DOES HIS BEST JAMES DEAN WELL YOU KNOW DIANE WE OUGHTA RUN OFF TO THE CITY DIANE SAYS BABY YOU AINT MISSIN NOTHIN OH YEAH LIFE GOES ON LONG AFTER THE THRILL OF LIVIN IS GONE GONNA LET IT ROCK GONNA LET IT ROLL LET THE BIBLE MAN COME AND SAVE MY SOUL HOLD ONTO SIXTEEN AS LONG AS YOU CAN CHANGES COME AROUND REAL SOON MAKE US WOMEN AND MEN

Which are

lyrics from Jack & Diane by John Mellencamp.

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  • $\begingroup$ Bravo! And also bravo. $\endgroup$ – MackTuesday Jun 30 '15 at 21:37

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