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From: mahamasoo on 16 Feb 2007 00:54
Hi,

I am new in cryptoanalysis. I have this problem and I am hoping
someone can help me solve it. I have been working on it for a while
but with no success. I tried looking at frequency analysis but it
din't help. I tried using Secret Code Breaker but again with no
success. So if someone out there enjoys this sort of thing and wants
to help, here's your chance.

Problem

P S E W O T D P S U I I M A W E W Q I I T U I I M
W E U I I M E Q I W J T W E U I I M E Q I E P S A
S J D

Hints:
The encryption algorithm used is a classic symmetric cipher that has
the following property:
Just as plaintext is entered into the cryptographic system to get the
cipher-text, the cipher-text can be
entered in the same place in the system, to retrieve the original
plaintext.
The key is only 1 letter.

Good luck

From: r.e.s. on 16 Feb 2007 01:57
<mahamasoo(a)gmail.com> wrote ...
> P S E W O T D P S U I I M A W E W Q I I T U I I M
> W E U I I M E Q I W J T W E U I I M E Q I E P S A
> S J D
>
> Hints:
> The encryption algorithm used is a classic symmetric cipher that has
> the following property:
> Just as plaintext is entered into the cryptographic system to get the
> cipher-text, the cipher-text can be
> entered in the same place in the system, to retrieve the original
> plaintext.
> The key is only 1 letter.

You might find a more receptive audience for puzzles like
this at rec.puzzles.

Anyway, this one turns out to be a varaint of what's called
a "caesar cipher", and the hints are suggesting that
Ciphertext = Key - Plaintext
because in that case
Plaintext = Key - Ciphertext
i.e., the "same thing" is done to encipher and decipher a
letter, namely subtract it from the Key. (Here letters
are treated as numbers, A=0, B=1,..., Z=26.)

You'll find that if K is one particular letter, subtracting
the given letters from it will give HESAIDTHECOOKWASA...



From: r.e.s. on 16 Feb 2007 02:04
I wrote ...
> Anyway, this one turns out to be a varaint of what's called
> a "caesar cipher", and the hints are suggesting that
> Ciphertext = Key - Plaintext
> because in that case
> Plaintext = Key - Ciphertext
> i.e., the "same thing" is done to encipher and decipher a
> letter, namely subtract it from the Key. (Here letters
> are treated as numbers, A=0, B=1,..., Z=26.)

... and the letters "wrap around", so for example A-C=Y.

> You'll find that if K is one particular letter, subtracting
^Key
> the given letters from it will give HESAIDTHECOOKWASA...

From: secretcodebreaker on 16 Feb 2007 10:38
mahamasoo(a)gmail.com wrote:
> Hi,
>
> I am new in cryptoanalysis. I have this problem and I am hoping
> someone can help me solve it. I have been working on it for a while
> but with no success. I tried looking at frequency analysis but it
> din't help. I tried using Secret Code Breaker but again with no
> success. So if someone out there enjoys this sort of thing and wants
> to help, here's your chance.
>
> Problem
>
> P S E W O T D P S U I I M A W E W Q I I T U I I M
> W E U I I M E Q I W J T W E U I I M E Q I E P S A
> S J D
>
> Hints:
> The encryption algorithm used is a classic symmetric cipher that has
> the following property:
> Just as plaintext is entered into the cryptographic system to get the
> cipher-text, the cipher-text can be
> entered in the same place in the system, to retrieve the original
> plaintext.
> The key is only 1 letter.
>
> Good luck
>
Secret Code Breaker says:

The plaintext = hesaidthelookwasagoodlookaslooksgoandaslooksgoshewent

The Key = WFXTSHQPOKMUNJIYCREDGLAZBV

The I.C = 0.0958 (which is a little high for English plaintext -0.067)
but the sample is small (53 characters)
From: r.e.s. on 16 Feb 2007 13:24
"secretcodebreaker" <invalid(a)email.address> wrote ...
> mahamasoo(a)gmail.com wrote:
> The plaintext =
> hesaidthelookwasagoodlookaslooksgoandaslooksgoshewent
>
> The Key = WFXTSHQPOKMUNJIYCREDGLAZBV

A simple substitution cipher with the above key doesn't
satisfy either of the two hints (the key must have only
*one letter*, and it must also have the property that
for any letter x, Encrypt(x) = Decrypt(x).

However there *are* keys that -- although violating the
one-letter-only hint -- do satisfy the other hint and
give the "meaningful" plaintext above; e.g., simple
substitution using a table of the following form:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
--------------------------
W.UTS.QPONM.KJIHG.EDC.A...

where the bottom row must give *both* the encryption
*and* the decryption of the top row. (Using your key
as the bottm row doesn't satisfy this requirement.)

So it seems to me that the only solution (so far) --
satisfying *both* hints -- is the one I mentioned ...

(Encrypt or Decrypt)(x) = Key - x (mod 26)

.... with Key = 'W' = 22, even though the resulting
plaintext about a _C_OOK seems a lot less "meaningful"
than one about a _L_OOK.

Possibly the hint about a one-letter-only key got
garbled somehow?
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