A Field Cipher

With the current availability of high-speed Internet connections, computer-based encryption, satellite uplinks, and secure communication channels one may wonder if there is really any need for manual (pencil and paper) field ciphers.  For intelligence agents, special operations personnel, and others operating in hostile areas, a field cipher is essential.  A secure Internet connection is always not available to the operator in the field.  Computer based encryption may not be installed on available computers, and secure communication channels tend to fail at the most inopportune times.  The field cipher serves as a back-up to other encryption methods and as a primary means of communication when communication must be sent over non-secure channels.

The field cipher is a tactical cipher used to communicate between personnel operating in the field and their operations base.  It is not intended for encryption of long and complex reports, rather it protects the substantive portions of tactical messages.   

The field cipher is similar to the American military DRYAD Numeral Cipher/Authentication System and the British military BATCO, Battle Code.  The field cipher consists of five parts, the header line, the syllabary table, the authentication table, the key and cipher table, and the brevity list.

 

(Click on Cipher Sheet to Enlarge)

 

The header line consists of a sheet number to identify a specific field cipher sheet, a copy number for accountability of the sheets, and a period/sequence number identifying when that cipher sheet is supposed to be used.  The syllabary table is a 10x10 grid containing the alphabet and common two and three letter pairs. Each cell of the grid is identified by numerical coordinates, thereby allowing the conversion of letters into numbers.  The authentication table is a grid containing number pairs, identified by numerical coordinates, allowing for authentication challenge and reply.  The key/cipher table is divided into two parts: the key columns which are used to indicate the cipher rows, and the cipher rows themselves, which are used to encrypt information.  Finally, the brevity list is a numbered list of common messages. 

The field cipher allows direct encryption of numerical portions of a message, such as grid coordinates, route and street numbers, telephone numbers, and dates and times.  The field cipher also allows the encryption of words after they are converted to numbers using a syllabary table, or through the use of a brevity list of commonly used words and phrases.

To encrypt a message, choose a number-letter combination from the key section on the key-cipher table. For example, we might choose the key 3a. In the number "3" key column move down that column until you come to the letter "a". This indicates that you will work with the cipher row beginning with the letters FXE RDV, etc. (Note that this cipher row may also be identified using the keys 2a, 5g, 7g, and 9c.) In the cipher row the letters are grouped under numbered columns.  FXE is in column 0, RDV is in column 1, AMB is in column 2, GC is in column 3, SJ is in column 4, and so forth over to WO which is in a column identified by a hash tag or pound sign.

Any letter from the cipher row may be used to identify the number of its column.  The number zero can be identified using the cipher letter F or X or E.  The number two can be identified using the cipher letter A or M or B.  Using the cipher row identified by Key 3a, we can encrypt the number 13579 as: RGZHQ.  We can also encrypt 13579 as XCLIU, or EGYIQ.

Encrypting Numbers with the Field Cipher

Numbers are encrypted by first identifying a key (such as 3A) and then using the letters in the cipher rows to represent the numbers above their columns. 

Some examples of numeric encryption are:

My telephone number is: 3A AF MZ YL RG HQ  (202-555-1379)

The camp is at grid: 7P VJ ZC HT EK  (9489 2671)

The meeting is at 5D RT PS Main Street on 2U QY NR IL, at MC BD.  (2437 Main Street on 06/26/12 at 7.30)  Note that in this message we switched keys after the street number, but continued with the same key for the date and time. 

 

The cipher is always represented by letter pairs, while the key is a number-letter combination.  Whenever you receive a new key as part of a message, simply change to the new cipher row indicated by that key. In practice you should transmit no more than 20 characters before sending a new key and changing cipher rows.

 

Spelling with the Syllabary Table

 

The syllabary table allows the conversion of letters and letter groups into numbers so that they may be encrypted using the field cipher.  There is some small degree of security afforded by the random coordinate numbers of the syllabary table, but the syllabary table should not be used alone to send messages without first encrypting the message using the key/cipher table.

 

When beginning and ending spelling with the syllabary table it is important to indicate this in the text of the message.  This is done by encrypting the indicator of two hash tags / pound signs “##” at the beginning and end of the syllabary spelling sequence.

 

To convert letters to numbers using the syllabary table, simply choose the number coordinates that identify the cell containing the letter or letters you want to encrypt.  For example, the message "Attack planned for dawn." could be converted to numbers using the syllabary table like this: 22 30 24 87 18 89 95 95 43 56 48 68 95.

 

The numbers are then encrypted using the key/cipher table:

3A WW AB GF MS NH RK KU QZ UY JC LT JK PN QY OW.

 

Authentication Using the Field Cipher

 

Authentication is a method of challenge and reply used to ensure that the person with whom you are speaking is an authorized operator in your communications network.  To issue an authentication challenge choose a number from the far left column of the authentication table, and a second number from the top most row of the authentication table.  Note that these are each single digits. For example you might choose the number five and seven.  At the intersection of row 5 and column 7 we find the number 22.  Thus the challenge is: "Authenticate 57" and the proper reply is "I authenticate 22". 

Likewise if the authentication challenge was 89, the reply should be 31.

 

Using the Brevity List

 

The purpose of the brevity list is to rapidly transmit a number of standardized short messages.  If one wanted to send the 18th message on the brevity list one sends “Message 3A DK”.  Note that this could also be sent a “Message 3A AN”. 

 

The messages included in the brevity list are developed and standardized locally to meet the needs of personnel operating in the field.  Once developed the brevity list remains constant on all field cipher sheets, but may be much longer that what is displayed here. Multiple additional columns for the brevity list could be included on the back of the field cipher sheet, or a separate brevity code book could be provided to signalers.

 

Security of the Field Cipher

 

The security of the field cipher is found in its short cipher period and, in its low volume of traffic.

Normally a new field cipher sheet is used every day.   

 

It should be noted that the only mathematically unbreakable cipher is the One-Time Pad, but the complexities of distributing and maintaining a one-time pad system in the field are far greater than that involved with the field cipher.