DTP Foundations Week #9

Week 9 of the deductionist training program.

Answer for cipher #1.


Answer Cipher #1:

So you’ll need to know some vocabulary that are used in the codebreakers world.

  • Plaintext - A message before it is encrypted and after it is decrypted.
  • Ciphertext - Enciphered plaintext.
  • Pattern word - A word that uses the same letter at least twice. For example “That”.
  • Monoalphabetic - Cipher in which the plaintext is encrypted using one alphabet. For example “A” is always “O”.
  • Polyalphabetic - Cipher in which the plaintext is encrypted using multiple alphabets. For example “A” is sometimes “O” but can also be “K”, “Q”.
  • Transposition cipher - Encryption system in which letters change position, creating an anagram.
  • Substitution cipher - Encryption system in which letters are replaced but remain in the correct position.
  • Concealment cipher - Encryption system in which the ciphertext is unnoticed.
  • Breaking - To uncover a ciphers plaintext without knowing the method of encipherment.
  • Softbreaking - The use of a computer that systematically breaks an enciphered message almost automatically.
  • Hardbreaking - The use of frequency analysis or other methods of manually breaking an enciphered message.

[For more about information about ciphers]

So to solve this cipher you’ll need to figure out if it’s monoalphabetic or polyalphabetic. You do this by looking at patterns in the text.

  • If a lot of letter combinations are repeated in exact order then it’s probably monoalphabetic.
  • If it seems completely random it’s probably polyalphabetic.

There’s five words that are repeated “PIDM”, “GWC”, “BPM”, “LWV'B” and “Q”. So this is probably monoalphabetic. Monoalphabetic is relatively easy to break while polyalphabetic is much harder. Now that you know that it’s monoalphabetic you need to figure out if it’s a substitution or a transposition cipher.

You need to know this:

  • The most common letters of the English language, in order of frequency are: E, T, O, A, N
  • The least common letters of the English language are, from least to most common: Z, J, Q, X, K

With that information you can figure this out:

  • If the most frequently used letters are common in the ciphertext but the least frequently used are not, then it’s probably a transposition cipher.
  • If the least frequently used letters are common in the ciphertext but the most frequently used are not, then it’s probably a substitution cipher.

We see a lot of “Q"s and "Z"s so this is probably a substitution cipher.

Now to hardbreaking the cipher.

The other things you need to know about frequency:

  • The most common letters in the beginning of a word in order of frequency: T, O, A, W, B.
  • The most common third letter of a word in order of frequency: E, S, A, R, N.
  • The most common last letter of a word in order of frequency: E, S, T, D, N A.
  • Single letter is often "I” or “A”.
  • Most common two letter words in order of frequency: Of, To, In, It, Is.
  • Most common three letter words in order of frequency: The, And, For, Are, But.
  • Most common four letter words in order of frequency: That, With, Have, This, Will.
  • The most common double letters in order of frequency: LL, EE, SS, OO, TT.
  • The most common digraphs in order of frequency: Th, He, An, In, Er
  • The most common trigraphs in order of frequency: The, And, Tha, Ent, Ion
  • “-TION” is the most common word ending.
  • Common three letter pattern words are: All, See, Off, Too, Add
  • Common Four letter pattern words are: That, Will, Been, Good, Here, Well
  • Common Five letter pattern words are: Where, There, Still, Guess, Needs

Now you try using frequency analysis on the ciphertext.

PIDM GWC LQAXWAML WN BPM JWLG GMB? Q LWV'B EIVB BW JM QV XZQAWV ITT UG TQNM, AW LWV'B UISM UM LQAIXXWQVBML Q. QN GWC PIDM VWB, XTMIAM LW, GWC KIV NQVL QB VMIZ BPM TISM EPMZM Q OWB IZZMABML.

You know that one letter words are probably A or I, it’s not as common for sentences to start with an A as with an I, so put in I where every Q is.

PIDM GWC LiAXWAML WN BPM JWLG GMB? I LWV'B EIVB BW JM iV XZiAWV ITT UG TiNM, AW LWV'B UISM UM LiAIXXWiVBML I. IN GWC PIDM VWB, XTMIAM LW, GWC KIV NiVL iB VMIZ BPM TISM EPMZM I OWB IZZMABML.

Now the letter M is the most repeated letter in the ciphertext so this is probably an E, replace all M’s with E’s.

PIDe GWC LiAXWAeL WN BPe JWLG GeB? I LWV'B EIVB BW Je iV XZiAWV ITT UG TiNe, AW LWV'B UISe Ue LiAIXXWiVBeL I. IN GWC PIDe VWB, XTeIAe LW, GWC KIV NiVL iB VeIZ BPe TISe EPeZe I OWB IZZeABeL.

Now the word “JM” is “Je” which is probably the word “be” so replace all J’s with B’s.

PIDe GWC LiAXWAeL WN BPe bWLG GeB? I LWV'B EIVB BW be iV XZiAWV ITT UG TiNe, AW LWV'B UISe Ue LiAIXXWiVBeL I. IN GWC PIDe VWB, XTeIAe LW, GWC KIV NiVL iB VeIZ BPe TISe EPeZe I OWB IZZeABeL.

The word BPM comes up quite often and we think that M is an E and the most common three letter word is “the” which ends in E. So fill that in and replace all the B’s and P’s.

HIDe GWC LiAXWAeL WN the bWLG Get? I LWV’t EIVt tW be iV XZiAWV ITT UG TiNe, AW LWV’t UISe Ue LiAIXXWiVteL I. IN GWC hIDe VWt, XTeIAe LW, GWC KIV NiVL it VeIZ the TISe EheZe I OWt IZZeAteL.

The “PIDM” which is now “hIDe” is probably the word “have” because that is a common word. So change all the I’s to A’s and change all the D’s to V’s.

Have GWC LiAXWAeL WN the bWLG Get? I LWV’t EaVt tW be iV XZiAWV aTT UG TiNe, AW LWV’t UaSe Ue LiAIXXWiVteL I. IN GWC have VWt, XTeaAe LW, GWC KaV NiVL it VeaZ the TaSe EheZe I OWt aZZeAteL.

The word “ITT” is now “aTT” which is probably the word “all” change all T’s to L’s.

Have GWC LiAXWAeL WN the bWLG Get? I LWV’t EaVt tW be iV XZiAWV all UG liNe, AW LWV’t UaSe Ue LiAIXXWiVteL I. IN GWC have VWt, XleaAe LW, GWC KaV NiVL it VeaZ the laSe EheZe I OWt aZZeAteL.

“BW” is now “tW” which is most likely “to” change all the W’s to O’s.

Have GoC LiAXoAeL oN the boLG Get? I LoV’t EaVt to be iV XZiAoV all UG liNe, Ao LoV’t UaSe Ue LiAIXXoiVteL I. IN GoC have Vot, XleaAe Lo, GoC KaV NiVL it VeaZ the laSe EheZe I Oot aZZeAteL.

“WN” is now “oN” which is probably “of”, so replace all N’s with F’s.

Have GoC LiAXoAeL of the boLG Get? I LoV’t EaVt to be iV XZiAoV all UG life, Ao LoV’t UaSe Ue LiAIXXoiVteL I. If GoC have Vot, XleaAe Lo, GoC KaV fiVL it VeaZ the laSe EheZe I Oot aZZeAteL.

“LWV'B” is “LoV’t” that means probably “don’t” or “won’t” and that means that the V is an N, change all the V’s to N’s.

Have GoC LiAXoAeL of the boLG Get? I Lon’t Eant to be in XZiAon all UG life, Ao Lon’t UaSe Ue LiAIXXointeL I. If GoC have not, XleaAe Lo, GoC Kan finL it neaZ the laSe EheZe I Oot aZZeAteL.

Now thanks to the word “NQVL” that now reads “finL” and the word “LWV'B” that now reads “Lon’t” means that L must be D because that’s the only thing that fits in. So replace all L’s with D’s.

Have GoC diAXoAed of the bodG Get? I don’t Eant to be in XZiAon all UG life, Ao don’t UaSe Ue diAIXXointed I. If GoC have not, XleaAe do, GoC Kan find it neaZ the laSe EheZe I Oot aZZeAted.

“EIVB” now “Eant” is probably “want” so replace all E’s with W’s.

Have GoC diAXoAed of the bodG Get? I don’t want to be in XZiAon all UG life, Ao don’t UaSe Ue diAIXXointed I. If GoC have not, XleaAe Lo, GoC Kan find it neaZ the laSe wheZe I Oot aZZeAted.

Now we have “wheZe” which is “where” so change all Z’s to R’s.

Have GoC diAXoAed of the bodG Get? I don’t want to be in XriAon all UG life, Ao don’t UaSe Ue diAIXXointed I. If GoC have not, XleaAe do, GoC Kan find it near the laSe where I Oot arreAted.

“ArreAted” is “arrested” so change all A’s to S’s.

Have GoC disXosed of the bodG Get? I don’t want to be in Xrison all UG life, so don’t UaSe Ue disaXXointed I. If GoC have not, Xlease do, GoC Kan find it near the laSe where I Oot arrested.

Thanks to the words “disXosed” and “disaXXointed” we know that X is P. Change all X’s to P’s.

Have GoC disposed of the bodG Get? I don’t want to be in prison all UG life, so don’t UaSe Ue disappointed I. If GoC have not, please do, GoC Kan find it near the laSe where I Oot arrested.

“bodG” and “Get” proves G to be Y. Replace all G’s with Y’s.

Have yoC disposed of the body yet? I don’t want to be in prison all Uy life, so don’t UaSe Ue disappointed I. If yoC have not, please do, yoC Kan find it near the laSe where I Oot arrested.

“yoC” is “you” and “Ue” is probably “me”. Replace all the C’s with U’s and all the U’s with M’s.

Have you disposed of the body yet? I don’t want to be in prison all my life, so don’t maSe me disappointed I. If you have not, please do, you Kan find it near the laSe where I Oot arrested.

With all that you should be able to know the rest too.

Have you disposed of the body yet? I don’t want to be in prison all my life, so don’t make me disappointed I. If you have not, please do, you can find it near the lake where i got arrested.

You send your men to find the body as quickly as possible but it’s too late. The body is gone.


Now this is a “ROT-8″ cipher or a “Caesar cipher” with a shift of 8 as it’s sometimes called. This is where all letters is shifted a number of letters so it substitutes it for an other letter. On a ROT-1 cipher A becomes B and B becomes C and so on. ROT-2 cipher is when A becomes C and B becomes D and so on.

This could have been faster to break if one would notice that it is a ROT-n cipher via softbreaking. If this ciphertext was inserted in a ROT-n converter then you’ll get a couple of answers out, these answers are from each shift, and you would then notice that the ROT-8 shift is the correct answer. I did it this way so that you all can hardbreak a monoalphabetic substitution cipher.


And with that, I’ll see you my irregulars.

I got “Dipper’s and Mabel’s Guide to Mystery and Nonstop Fun” in the mail today, and it is one heck of a great book. There are secret messages all throughout it that are written in the newest symbol cipher:

And I just finished decoding them all. Holy crap you guys. I’m putting em under a cut, they’re really intense.

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