While the Enigma was designed for battlefield use, the machines in this area of the museum were generally
suited for fixed station secure communications. The Sigaba (si-GAH-bah), used by U.S. for high-level communications, was the
only machine system used by any participant to remain completely unbroken by an enemy during World War II.
(The Germans referred to it as the "Big" machine)

"There is a general misunderstanding regarding codes and ciphers. Code is a system whereby one word or several words can be translated by use of a code book; which is much like a dictionary, giving first the word and then the meaning of that word. For example: The word "squirrel" might mean in code, "advance to a certain position", or "attack at a certain time." The word "gopher" might mean retreat from a position. [It] was absolutely necessary, you see, for both the sender and the receiver to have the same book! And in order to cover all the requirements of military necessity, that book would be quite voluminous in nature. And so, it was necessary for the army to develop some other technique for the communicating of messages as well as their security in transmission, instead of the utilization of the book. This was a code.
        The next step was the use of ciphering. So actually we were not encoding messages, we were enciphering and deciphering messages. The difference being that either by mechanical or electromechanical means in a cipher, letter for letter transposition is accomplished and then the text is broken down from the clear message, which the sender writes, to a group of non-intelligible five letter groups. Which are then transmitted one letter at a time and since the receiver has the same facility, and the same direction and the same key words, he [the sender] can then encipher it by any one of a commonly held system."—Hall Derkin

How a cipher works:
Let's say you have this message which you want to send to a confederate secretely:
        "There are rabid chia pets in the forrest of Ardennes."
      First, choose a keyword, like RABID. Since the keyword has five letters, put the plaintext into five columns with the keyword above it, like this:

R A B I D
- - - - -
T H E R E
A R E   R
A B I D  
C H I A 
P E T S
I N   T H
E   F O R
R E S T
O F   A R
D E N N E
S X X X X

If the last line does not complete a column, then pad it with X's. This is why the cipher is called Complete Columnar. A slightly different cipher called Incomplete Columnar does not need the padding.
        Now put the letters of the keyword into alphabetical order and you get RABID=ABDIR. Read down from the A column, then the B column, and so on. This provides the ciphertext, like this:
        "HRBHEN EFEX EEIIT FS NX ER    HR REX R DASTOTANX TAACPIERODS."
      To decrypt, put the ciphertext into five columns (because you know the key word) and arrange the columns in the correct order. Reading across will give you the plaintext.
       
Advantages:

• Fast.
• Relatively easy to understand.
• Cryptanalysis is somewhat more complicated than the Railfence cipher, but not much.

Disadvantages:

• More than one keyword can be used to encrypt and decrypt the same text. For instance, the keywords ABCDE and FGHIJ produce the same ciphertext because the columns are in the same alphabetical order. Good encryption would means each possible keyword creates unique ciphertext.
• Letter frequency is not disguised at all.
• Poor choice of keyword can leave large portions of plaintext visible in the ciphertext. For instance, using keyword ABCDE creates ciphertext which is identical to the plaintext. Keyword ABCED will leave much of the ciphertext readable.