The sequence transposition cipher was introduced by MSCREP (Mike Schroeder) in the ND 2015 issue of the Cryptogram magazine.
To encrypt a plaintext using this cipher:
(1) Chose a 5-digit primer, say 75639, and extend it to the entire length of the plaintext by adding the the fourth and fifth digits to the left of each plaintext letter position. If the sum is a two-digit number, use the rightmost digit.
For example if the plaintext is:
Scientists say their investigations of the closest star, Proxima Centauri, show it to have an Earth-sized planet orbiting about it.
then extending the primer 75639 gives:
scientistssaytheirinvestigationsofthecloseststarproximacentaurishowit
756392192130134314774514196550510556150176518316914750512556370190710
tohaveanearthsizedplanetorbitingaboutit
978196590514956534118752952714798516736
(2) Choose a key by writing the digits 0-9 in some order. The order may be based on a 10 letter key word, or in some other way. As an example we will use a key based on the word: BROADCLOTH. Writing the letters in 1-0 order gives the numeric key:
B R O A D C L O T H
2 9 7 1 4 3 6 8 0 5
(3) Following the numerical order of the key, write the plaintext letters in columns based on their digits in the extended primer in step (1).
This gives:
2 9 7 1 4 3 6 8 0 5
t n s i h e i s a c
t s i s r s a h o e
e g n y v t h a o t
t p s i t e e a l i
b h x s o t r - m n
- t r i t u a - i f
- v w s d e e - o t
- n o e i i i - t c
- h n o - - u - e s
- o i t - - t - - i
- g n a - - - - - a
- - t r - - - - - n
- - - c - - - - - t
- - - s - - - - - a
- - - i - - - - - a
- - - a - - - - - s
- - - r - - - - - z
- - - p - - - - - e
- - - l - - - - - r
- - - t - - - - - b
- - - o - - - - - -
(4) Form the ciphertext by taking off the columns above from left to right. Write the letters in blocks of 5, and append the digit of the final plaintext letter as a check. Display the primer in front of the ciphertext. This gives:
75639 TTETB NSGPH TVNHO GSINS XRWON INTIS YISIS EOTAR CSIAR PLTOH RVTOT
DIEST ETUEI IAHER AEIUT SHAAA OOLMI OTECE TINFT CSIAN TAASZ ERB 6
Below is a program for encoding and decoding this cipher type.