Transposition Ciphers

Transposition ciphers encode messages by simply rearranging the order of the characters in the message. As opposed to replacing letters with a substitute character. Note that what I've written about to date are types of Substitution Ciphers.

Transposition ciphers have keys with two parts. One is the size of the grouping of characters that are to be rearranged. The second is the ordering of the characters.

Let's try an example. We decide to rearrange groups of 7 characters. The rearrangement pattern is 6215734. The message is:

THE COURIER IS ENROUTE TO YOUR LOCATION

We start by group the characters in the size by which we are rearranging letters:

THECOUR IERISEN ROUTETO YOURLOC ATIONEE

Note we added some dummy letters to produce a multiple of 7 characters. Now, each group gets reordered by pulling characters in the order specified by our key (6215734).

We take character 6 (U), then character 2 (H), then character 1 (T), and so forth. The first group now becomes:

UHTOREC

We repeat this with the remaining groups and get:

UHTOREC EEISNRI TOREOUT OOYLCUR ETANEIO

Now regroup as sets of 5 characters.

UHTOR ECEEI SNRIT OREOU TOOYL CURET ANEIO

Think about what it might take for a cryptanalyst to decipher such a message. Next time, we will look at that problem a bit closer.