Two types of authentic Civil War-era code were used in the Booth diary prop. Both are types of transposition cipher, in which the letters or words of the original message are rearranged according to a prearranged plan. If you know the plan, you can unscramble the message to read it.
The vertical bar in the middle of the message is the clue that the way to decode it is to begin by rewriting it with one line over the other, like so:
To make it easier to read, ignore the spaces and stagger the letters like so (kind of looks like a rail fence, hence the name):
S
W
A
D
E
L
H
F
E
C
M
A
Y
O
R
A
G
M
Q
I
I
E
O
I
H
O
D
X
A
C
L
W
L
C
A
F
Y
O
P
N
T
A
R
N
E
Y
U
N
N
T
R
C
M
N
X
X
To read the decoded message, read in a zig-zag pattern starting with the first letter of the top line, then the first letter of the bottom line, then the second letter of the top line, and so on. Following the zig-zag pattern on this particular message yields the following plaintext result (with reasonable punctuation marks added):
The three Xs at the end are what are called "nulls," meaningless characters inserted to keep the cipher format correct.
The second type of code is somewhat more complicated, a type of transposition cipher called a "route." One of the coded sections from the diary is written this way:
You'll set up a grid to decode the message. "Davis 21" is the key phrase that tells you how to set up and read the grid. "Davis 21" is a five-letter word followed by two numbers: this tells you the grid should have five columns and five+two (seven) rows. At the top of each column, you put one of the letters in the key word, "Davis," like so:
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
The columns in the grid are then filled with the words from the coded message, beginning with the column which comes first in alphabetical order. In this case, for example, you'd start with the A column, followed by the D column, followed by I, then S, then V. The numbers from the key phrase, 2 and 1, tell you the direction in which to fill the columns. "2" means start at the bottom and go up, "1" means start at the top and go down. Ordinarily, you alternate direction with each subsequent column (unless there's another number telling you to do otherwise). So, for example, in this case you'd take the first seven words of the coded message and put them in the A column starting at the bottom and going up, like so:
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
You'd put the next seven words into the D column, starting at the top and going down. You'd keep filling up the grid, in alphabetical order of columns and alternating direction from column to column, until all the words were used up. That gets you this result:
To find the decoded message, you simply read across the columns you've now created starting at the top left and reading across and down in the normal way. Thus, for this message:
That last word, "immortal," is a null, used just to fill the necessary extra space in the grid. But it shows you what was on Booth's mind...