Primerless Sequence Transposition

Here’s a Sequence Transposition cipher:

YUOTW TOEAH NHCSS TFEND WDEOT MNEAA TNOSO INBOO OULOW TNPNI RTSPT NKOTT ONOUT WGTHI IDAEA LDINE HAUAE OFRTA OTBDH OTOC 6

The crib is “TOPOFAMOUNTAIN” located above OOOULOWTNPNIRT, or more accurately, beginning at the 39th letter of the plaintext since the ciphertext letters at that point do not relate to the crib. I wrote a program that identifies the most likely primers given the ct, a crib, and the crib placement, assuming sufficient crib length. For this example my program placed the correct primer tied for 5th most likely along with 39 other primers. As an added hint, the check number at the end is 6.

The text is a random sentence from Gutenberg.org.

2 thoughts on “Primerless Sequence Transposition”

  1. I solved it without the crib using a hill-climber that selected a primer at random, did 3000 hill-climbing trials, and then modified a random digit of the primer, did 3000 more hill-climbing trials, etc.. The hill-climber got the solution after going through 2000 primers.

    The solution primer chain did not end in 6, but every solution primer has 4 equivalent solution primers. I got all 4 equivalent primers and their corresponding numeric keys. I chose the numeric key whose primer chain ended in 6 and did a search for a key word with that numeric pattern. There was only one such word. A caesar shifted antonym for it is: HTSHWJYJQD

    The plaintext continues: GZYNYINI

  2. Yes, that’s it. I was in the middle of typing a similar explanation of the four primers that work when your solution post came in, but I’ll forgo that. It’s the same idea as with Gromark keys. I’ll do a separate post on how I identified likely keys. As you point out, the different primers produce different keyword sequences. It’s possible for the correct primer, or one of its equivalents, to be consistent with other keywords. This con is probably rather unusual in that there is only one valid English keyword that works with any of the four valid primers.

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