Unknown 11-24-19

This one was a tough one to diagnose the type even though the correct type ranked fairly high in my analyzer. I needed the crib to solve it. It fit in multiple places.

Crib: APTLZAVTFWVVYZVUFVBYM

EGRXC TYNHQ TEBPQ XYKPB RYVYL FDYBY GFBVK AIMOS SLXUP SECOS WYCVJ CINTS ZGKPB RKEMC JXKGA UEIZX VAECV EUEQM VAKCB XRXRB FJTUL WTSAU ENFBM ITFGD F

3 thoughts on “Unknown 11-24-19”

    1. It continues: RFWNFSJ N YMNSP

      SPOILERS below.

      Before your hint I did have it narrowed down to some type of Quagmire. But that was by the process of elimination because the crib wouldn’t fit with the other cipher types except maybe periodic Gromark. And even with periodic Gromark there would be only one place for a crib that long.

      The strange thing about this being a Quagmire is the low periodic Index of Coincidence. My MIC stat was 44 for this cipher. I’ve never had a Quagmire with a MIC that low. Out of curiosity I computed the MIC stat for every 121 letter string in my file of about 30 books. There were over 13 million such strings. The lowest MIC was 50. Admittedly these were plaintext strings, but still nothing as low as 44.

      I finally solved it by trying a key search on every period where the crib would fit.

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