Portax

History

The Portax cipher first appeared in the American Cryptogram Association (ACA) Oct-Nov 1948 edition of the ACA's magazine The Cryptogram and was developed by ACA member RED E. RASER. It is based on the Porta cipher but enciphering is performed on pairs of letters using a slide containing four rows of letters.

Description

The top row of the slide, row 1, is fixed and contains the first half of the alphabet (A-M). The next three alphabet rows are on a moving slide. The first row on the slide, row 2, contains the second half of the alphabet (N-Z), and the next two rows contain alternate letters of the alphabet with row 3 containing A C E...U W Y and row 4 containing B D F...V X Z. The sliding alphabets are repeated to allow for the slide.

A B C D E F G H I J K L M Row 1 - Fixed

N O P Q R S T U V W X Y Z N O P Q R S T U V W X Y Z Row 2 - Sliding

A C E G I K M O Q S U W Y A C E G I K M O Q S U W Y Row 3 - Sliding

B D F H J L N P R T V X Z B D F H J L N P R T V X Z Row 4 - Sliding

To encipher text using the Portax cipher write down a keyword and below it write the plaintext in rows of the length of the keyword. There must be an even number of rows as each vertical pair of letters is enciphered as a pair. For example using the key word PORTAX and the plaintext "Everyone is entitled to be stupid, but some abuse the privilege.”

PORTAX

everyo

neisen

titled

tobest

upidbu

tsomea

buseth

eprivi

le

ge

The key letter at the top of the plaintext column is found in row 3 or 4 of the slide which is moved so that it falls below the A of row 1. The slide shown above is positioned for the first key letter, P. The first plaintext letter pair in the column below P, EN is then enciphered by locating the first letter in row 1 or 2 and the second letter in row 3 or 4. These are treated as being diagonally opposite each other in a rectangle and the ciphertext is the letter pair in the opposite corners of the rectangle, MX in this case. If the two plaintext letters are located in the same vertical column of the slide then the other two letters in the same slide column are used as the ciphertext. For example using the above slide the cipher pair NA would be enciphered as GB. This method is continued for each key letter column until the cipher is created.

Example

Keyword: PORTAX

Key: PORTAX PORTAX PORTAX PORTAX PORTAX PORTAX PORTAX PORTAX PORT

Plaintext: Everyo neisen titled tobest upidbu tsomea buseth eprivi lege

Ciphertext: MPJWPT XQYIWD WANGJL NENOID WWMKCN PEGYCO IAVIXG QOLANK JIKW

PORTAX PORTAX

everyo MPJWPT

neisen XQYIWD

titled WANGJL

tobest NENOID

upidbu WWMKCN

tsomea PEGYCO

buseth IAVIXG

eprivi QOLANK

le JI

ge KW

Solving

Solving methods: Brute force and Dictionary keyword search.

After selecting the Portax cipher, the cipher period can be selected from the Cipher Settings drop down menu. The default setting of 0 will result in all periods in the keyword range set in the Options window being tested.