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…