# Reply To: Super-fun and often extra-hard ciphers from the crowd

The Body in My Library Forums Harry’s Meeting Place Super-fun and often extra-hard ciphers from the crowd Reply To: Super-fun and often extra-hard ciphers from the crowd

#92382
Rhydwen23
Participant

Harry, I hope you are okay to post the following partial solution to #91992.

Well done to anyone who managed to decrypt puzzle #91992. If you didn’t here are some pointers that should help. Feel free to post your own methods, observations and techniques.

Looking at the cipher.

Length (including spaces) = 1301 characters.
Length (excluding spaces) = 1086 characters.

Letter distribution:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
* * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * * * * * * * * * *
o o o o o o o o o o o
* * * * * *
*
*

Index of Coincidence = 0.0429

The letter distribution and IoC both indicate this is neither a mono-alphabetic substitution cipher, nor a transposition cipher.

A closer inspection reveals that the sequence HRMCKYKTDW occurs twice, once starting at position 213 and again at position 283. There are many other similar repetitions.

HRMCKYKTDW 213 283
MCKYKTDW 215 285
IWSBKX 59 479
NLVIWW 679 896

Looking at the spacing between the repetitions and the factors of the spacing gives…

HRMCKYKTDW 70; factors: 2, 5, 7.
IWSBKX 420; factors: 2, 2, 3, 5, 7.
KYKTDW 70; factors: 2, 5, 7.
NLVIWW 217; factors: 7, 31.

Seven is a common factor. Writing the sequence of characters into a block seven characters long gives:

KLDLWEF

BUHRMCK
YKTDWVW

XDHRMCK
YKTDWBQ

We then have the common characters lined up above each other. Examining the characters in each columns gives a much more promising letter distribution and allows a decryption.