Bifid uses a Polybius square to encrypt a message with a chosen alphabet.

To encrypt the message transpose me:

  1 2 3 4 5
1 B G W K Z
2 Q P N D S
3 I O A X E
4 F C L U M
5 T H Y V R

The message is read by rows and columns:

TRANSPOSE ME
------------
453222323 43
153352255 55

and read out row-by-row by making pairs:

45 32 22 32 34 31 53 35 22 55 55

And then divided up into pairs and turned back into letters:

45 32 22 32 34 31 53 35 22 55 55
--------------------------------
M  O  P  O  X  I  Y  E  P  R  R

and read out: MOPOXIYEPRR.

Knowing the secret alphabet, the procedure is simply reversed.

The Nihilist cypher uses a 5x5 Polybius square using a keyword.

For example, using the keyword ship, we obtain the square:

  1 2 3 4 5
1 S H I P A
2 B C D E F
3 G J K L M
4 N O Q R T
5 U V W X Y

Given a plain-text abandon hands and a key of move, the cypher-text is obtained by combining the plain-text with the key:

plaintext : 15  21 15 41 23 42 41    12 15  41 23 11
key       : 35  42 52 24 35 42 52    24 35  42 52 24
---------------------------------------------------- +
cyphertext: 410 63 67 65 58 84 93    36 410 83 75 35

Which yields the cypher-text: 410 63 67 65 58 84 93 36 410 83 75 35 and can be sent as-is. In order to reverse the operation, the keyword used to construct the Polybius square has to be known, as well as the key used for encryption.