Ode to A. M. Scott

Ode to A. M. Scott

Meet the AMSCO cipher

The AMSCO cipher was created by A.M. Scott in the 19th century. Unfortunately, little is known about him, apart from him being a member — known as “AMSCO” — of the American Cryptogram Association (ACA) [here]. It was first formally published in 1939 by Helen Fouché Gaines as “Cryptanalysis: A Study of Ciphers and Their Solution” within Elementary Cryptanalysis [here]:

With AMSCO, we initially create a 5x5 grid with a key. We then lay out our plaintext in a sequence of double and single characters. So let’s find the AMSCO cipher with a key of 41325, for the following:

apessemisticpestexists

We can now layout our key (4, 1, 3, 2 and 5), and then take each of the characters in sequence (with an alternating sequence of 2 then 1, and repeated):

4  1  3  2   5
ap e  ss e  mi
s  ti c  pe s
te x  is t  s

We then read the sequence of the key (1, 2, 3, 4, and 5). And thus is becomes ‘e ti x e pe t ss c is a ps t em i s s’, and so the cipher is ‘etixepetsscisapstemiss’

Now let’s do it in reverse. With a key of ‘32415’, what is the AMSCO plaintext for the ciphertext version of: nwrertht bapng kiereguo? In this we have 21 characters so it will be laid out as:

X  X  X  X  X
2  1  2  1  2
1  2  1  2  1
2  1  2  1

First we lay out the key, and then populate the first column:

3  2  4  1  5
         n
         wr
         e

And now the next column:

3  2  4  1  5
   r     n
   th    wr
   t     e

And next:

3  2  4  1  5
ba r     n
p  th    wr
ng t     e

And next:

3  2  4  1  5
ba r  ki n
p  th e  wr
ng t  re e

And finally:

3  2  4  1  5
ba r  ki n  gu
p  th e  wr o
ng t  re e

And the result is “barking up the wrong tree”

If you want to try your own, I have created a CTF here where we convert from plaintext to ciphertext:

https://asecuritysite.com/ctf/ctf_92

and for the ciphertext to plaintext:

https://asecuritysite.com/ctf/ctf_93

And, if you have time, here’s 93 ciphers for you to solve:

https://asecuritysite.com/encryption/ctf