To decrypt, use the same Polybius square.
Convert the ciphertext to its coordinates in the usual manner, but keep it written in a single row.
U
A
E
O
L
W
R
I
N
S
44
33
35
32
43
13
55
31
23
25
4
4
3
3
3
5
3
2
4
3
1
3
5
5
3
1
2
3
2
5
Find the midpoint of the row.
4
4
3
3
3
5
3
2
4
3
X
1
3
5
5
3
1
2
3
2
5
Take all of the digits to the right of the midpoint and move them to a second row directly below the first.
Convert these vertical pair coordinates to find the plaintext.
4
4
3
3
3
5
3
2
4
3
1
3
5
5
3
1
2
3
2
5
F
L
E
E
A
T
O
N
C
E
Longer messages are first broken up into blocks of fixed length, called the period, and the above
encryption procedure is applied to each block. One way to detect the period uses bigram statistics on ciphertext letters separated by half the period. For even periods, p, as in the example above (p=10), ciphertext letters at a distance of p/2 are influenced by two plaintext letters (e. g., U and W are influenced by F and L), but for odd periods, p, ciphertext letters at distances of p/2 (rounded either up or down) are influenced by three plaintext letters. Thus, odd periods are more secure than even against this form of cryptanalysis, because it would require more text to find a statistical anomaly in trigram plaintext statistics than bigram plaintext statistics.[1]
