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Chapter 7

Communications Security Operations

7-1. General

a. Success on today's battlefield depends on the commander's ability to concentrate superior combat power at critical times and places. A key to this success is superiority in command and control via communications. Effective communications is essential to winning.

b. The enemy realizes the importance of our communications systems and will continuously try to interfere with our ability to communicate. He will try to gather intelligence from our communications, and then he will try to disrupt them. He will attempt to interfere by breaking into our nets and trying to deceive us, or he will try to jam us. Failing at these measures, he will try to destroy communications by fire. Our battlefield success will depend heavily on how well we minimize his attempts to disrupt our communications systems.

c. Communications security is the protection resulting from the application of crypto security, transmission security, and emission security. These protective measures are taken to deny unauthorized persons telecommunications information. This chapter addresses primarily the cryptographic and transmission security portions of COMSEC.

d. Presently, AC and RC have a substantial shortage of COMSEC equipment. No quick or easy solutions are in sight. The need for COMSEC is essential; if adequate quantities of COMSEC equipment are not available, the commander must take other measures to secure his communications. Additionally, the active Army must also be skilled in the use of manual encryption techniques. Because most RC units possess little secure voice equipment, the active Army must anticipate transmitting and receiving traffic with the reserves using manual encryption techniques. Remember, manual encryption is also the backup for loss or failure of machine crypto systems. Therefore, all forces must maintain proficiency in the manual encryption area, regardless of interaction with other forces. Below are alternative methods and systems which can be used in lieu of on-line crypto systems. They present some difficulty when large volumes of traffic must be processed; however, these methods are essential to assure success and survivability on future battlefields.

7-2. Authentication Systems

a. An authentication system is designed to protect a communications system against the acceptance of fraudulent transmissions. Everyone who communicates in a tactical or strategic environment requires some method of authenticating. Good authentication practices contribute to combat survival and effectiveness, because they aid in establishing the validity of a transmission, message, or originator. All commanders must implement their use during training and actual operations.

b. Combat experience in Vietnam proved that IED by the enemy contributed to substantial numbers of casualties and caused many missions to fall short of desired results. Proper authentication procedures can prevent an enemy from posing as a friendly station. The enemy is adept at IED and needs only a moderate degree of skill to seriously affect our communications when we do not authenticate. A balance has to be struck so that effective communications is maintained without harassment of friendly communications. Guidance on the use of authentication systems is found in the unit SOI, ACP 122(D), AR 380-40, and TB 380-41.

c. IED is the easiest EW technique to counter. Authentication is one of the best means available to stop enemy IED efforts. Operators are required to authenticate when they--

  • Suspect a transmission is from an enemy station operating in the net (deception).
  • Direct a station to go to radio silence or to break that silence. (Self-authentication can be used if authorized by the SOI.)
  • Are challenged to authenticate.
  • Talk about enemy contact, give an early warning report, or issue any follow-up report.
  • Transmit directions which affect the tactical situation such as "Move to..." or "Turn off the radio." (Conversely, they challenge any directives like these with a request to authenticate.)
  • Cancel a message.
  • Open the net or resume transmitting after a long period of silence.
  • Transmit to someone who is under radio listening silence.
  • Transmit a classified message in the clear.
  • Transmit messages in the blind; that is, neither desiring nor expecting a reply.

d. Challenge if you are not sure that authentication is required. If a station takes more than 5 seconds to authenticate, rechallenge. Why 5 seconds? Because an enemy operator may try to contact another station and have it respond to that same challenge, thereby obtaining the appropriate reply to your challenge.

e. The two most commonly used authentication procedures are challenge-reply and transmission authentication. The main difference between the two is that challenge-reply requires two-way communications, whereas transmission authentication does not. Even though transmission authentication requires only one-way communications, it is neither as simple nor as flexible as challenge-reply. The challenge-reply procedure most often used has a more flexible application.

(1) Challenge-reply.


(2) Transmission authentication.

NOTE: Transmission authentication is used only when it is impossible or impractical to use challenge-reply authentication.


7-3. Transmission Security

Several categories of tactical information require transmission security protection. These are listed in FM 34-62, Appendix C. It is important that you learn these categories.

7-4. Codes

a. A code is a language substitution system that transforms plain language of irregular length, such as words or phrases, into groups of characters of fixed length. A code has an underlying plaintext of variable length, whereas a cipher has an underlying plaintext of fixed length (see paragraph 7-5). The codes that you will use are usually found in your unit SOI packet.

b. Two types of codes are normally used in tactical communications: security codes and brevity codes (only as authorized). A code used to hide meanings from another party is a security code. A code used to shorten transmissions is a brevity code. A brevity code only shortens transmission; it does NOT provide security. It is referred to as a brevity list. The international Q and Z signals found in ACP 131(D) and the police 10-code signals are examples of brevity lists. Brevity lists must be used in conjunction with an approved code to provide security.

c. Most codes can be placed into one of three categories: numerical, operations, and special purpose.

(1) Numerical codes are among the simplest and most useful types of codes and are used to encode numbers. They are almost always digraphs (two-letter configurations) and are designed to protect intelligence bearing QUANTITATIVE portions of tactical communications, especially voice communications. They provide a short term tactical advantage when it is impossible or impractical to secure information to any greater degree. They are intended for use through the lowest operational levels. They can be used to protect the when, where, and how many in communications that might otherwise be unencrypted. (For example, they may be mixed with plain language, operating signals, or a brevity list.) Numerical code examples are given below.

(a) Hours designation:

Plaintext--Meeting time is 1530.
Code value --Meeting time is I set XXBKWG.

(b) Frequency designation:

Plaintext --Change frequency to 14990 kHz at 1600.
Code value --Change frequency to I set XYFXMPE at HITV.

(2) OPCODE, in contrast with numerical codes, can be used to encrypt the what, who, why, how, and how many--the QUALITATIVE information in messages. They DO NOT, however, provide adequate protection for information if mixed with plain language. OPCODEs have a vocabulary of usually 1,000 to 3,000 entries and generally use trigraphic (three-letter) code groups. OPCODEs are usually multipurpose or general in that they may be used to encrypt different sorts of information. OPCODEs are intended for use down through the lowest operational levels. Operations codes examples are given below.

(a) A simulated message of tactical operation assignment:

Plaintext--Continue on assigned mission.
Code value--AAL.

(b) A simulated message of tactical operation report:

Plaintext--Battalion action at crossroad.
Code value --OXW RFM RFX WOX.

(3) Special purpose codes are OPCODE-type items but are generally designed for encrypting specialized types of messages such as radar reports and fire missions. Their vocabularies are usually limited in size and scope and may consist of single letter, digraphic, or trigraphic code groups. Frequently, they are intended for more sensitive application than general codes and are often used at higher echelons. Many such special purpose codes are of the one-time variety. Special purpose code examples are given below.

(a) Simulated message --Communications equipment, radar station reports:

Plaintext--TRC capability impaired, jamming suspected hostile.
Code value--OTV JNP.

(b) Simulated message --Artillery mission report:

Plaintext--Target altitude 2,500 meters.
Code value--XHY OHT.

d. Many codes are custom designed to meet requirements of specific users. They are fabricated in response to specific COMSEC needs. They can be produced for any commander who requires an individually tailored item. These codes are not to be produced without NSA approval. Most users, however, use only a few standardized systems. Standardized systems can be obtained and pre-positioned at appropriate levels in the distribution. This should be a consideration when requesting COMSEC support. If not regularly used, codes may not be on hand at the local COMSEC support agency. If not readily available, codes must be ordered through COMSEC logistic channels. Users and commanders should consider that custom designed codes require sufficient lead time to produce. Although a standardized system may not be the best solution to a tactical COMSEC problem, it may serve as an effective interim system until a more suitable custom designed product can be produced. You should never try to make up your own brevity codes since experience has shown they are too easily broken by the enemy. Only use authorized and approved codes.

e. The use of codes to gain advantage over an enemy cannot be overemphasized. Everyone using codes must be familiar with their capabilities, limitations, and intended usages for codes to be effective.

(1) Codes intended for tactical application are designed to provide ONLY that amount of security consistent with operational needs.

(2) Tactical OPCODEs usually require that messages be composed prior to being encrypted and transmitted. Users need a pencil, paper, and a place to write in order to work on OPCODEs.

(3) A tactical OPCODE is of specific but limited usefulness in the operational environment. It is difficult to use in the midst of hostilities or when riding in a vehicle. It cannot adequately protect high level communications.

(4) Numerical codes can usually be operated without pencil and paper. Numerical codes can provide protection to quantitative elements of information that pertain to an immediate tactical situation. However, they are not as secure as properly used OPCODEs or numerical ciphers.

(5) All codes have a cryptoperiod. They also have usage rules that outline restrictions on their employment. If a code is used in a way for which it is unintended, security can break down quickly. Total encryption using tactical codes is not always desirable or possible. Encryption of information the enemy already knows may help assist him in breaking our code system.

(6) One-time codes have special usage characteristics because of their one-time cryptoperiod. These codes provide a high degree of security and can be used for traffic with long-term intelligence value.

(7) The commander can request that the local INSCOM counter SIGINT personnel produce a code that will meet his needs when an emergency arises that does not allow a unit to use authorized codes (such as, compromise or cut off of distribution). Under no circumstances should unauthorized codes be used.

(8) Training must emphasize security and resupply procedures for codes to ensure that all personnel involved in their handling and use are properly trained.

7-5. Ciphers

a. The one-time pad is a language substitution cipher system which transforms plain language formations of fixed length (numbers and/or letters) into characters or groups of characters of fixed length. In a cipher system, the underlying plaintext is fixed in length; in a code system the underlying plaintext is variable in length. A one-time pad has no vocabulary as such, and almost anything can be said using pads. One-time pads are highly secure and are used mainly for special operations.

b. All one-time pads have variables which are used to transform plaintext into cipher text. These variables are presented in the form of recognizable characters such as letters and/or numbers. Each individual key is used only once, from which is derived the name one-time pad.

c. The substitution of cipher text for plaintext and vice versa is performed according to a specified rule which uses the key variables discussed above. The rule, how to work the pad, is what distinguishes one type of system from another.

d. The three basic varieties of one-time pads are literal, digital, and literal/digital.

(1) A literal pad can encrypt letters only, so numbers must be spelled out before encryption. This gives great flexibility in the variety of plaintext that can be encrypted, but also results in a longer encryption time than would be experienced with a code.

(2) A digital pad encrypts digits only. If information to be protected is strictly numerical, digital pads can directly encrypt the plaintext. It is not uncommon for it to be used to directly encrypt narrative text if the text can be taken from a brevity list whose equivalent groups are numerical. This technique is especially valuable between speakers of different languages, as operators need no linguistic skills since transmission involves only digits.

(3) Literal/digital pads are used to encrypt both letters and numbers directly. Their applications are similar to literal-only pads except they can directly encrypt numbers without spelling them out. They are most useful over good quality circuits which are least likely to require spelling numbers.

(a) Standardization systems constitute the majority of one-time pads. Standard systems accommodate most operational systems, but custom designed pads can be ordered through the local COMSEC support agency by any commander who has a legitimate need for special material.

(b) Pads can be used to protect highly sensitive traffic since they provide security for an indefinite time. Pads require a pencil for their operation, and some require that messages be composed before encryption. Writing space is normally provided for writing directly on the pad. A pad key is intended for one-time use only. If more than one message is enciphered in the same stretch of key, it is possible to break both messages.

(c) One-time pads, like one-time codes, are most effective on point-to-point or broadcast nets.

(d) One-time pad example:

Plaintext--l4.5. Code value--XKBQ.
Message transmitted --Page 030, set 3, XKBQ.

e. Numerical ciphers, as with a pad, are characterized by the fixed length of the underlying plaintext. In all cases, this is a one-for-one substitution.

(1) The two types of numerical ciphers are the one-time ciphers and the standard cipher system. The one-time ciphers are an easy-to-employ, highly secure, numerical enciphering system. They can be used on basic numerical data or on a fixed format, such as specific data reports for personnel summaries. The standard numerical cipher system for enciphering numbers is DRYAD.

(2) A limited transmission authentication capability and a challenge/reply authentication capability are also provided with this system.

7-6. Brevity Lists

a. There may be instances where the types of information to be exchanged are not sufficiently varied to warrant the use of an extensive operations code. In such cases, it may be preferable to use a brevity list (Figure 7-1) in conjunction with a numerical cipher. Security is provided by encrypting numerical equivalents with an approved cipher system such as the DRYAD system.

b. Use of a brevity list has certain advantages over use of an operations code. Use of a brevity list eliminates the need to distribute, account for, and destroy an operations code in which the greatest part of the vocabulary is never used.

c. Brevity lists may be found in the supplemental instructions in the unit SOI. If there is no list present, it may be added at the unit level permanently or temporarily for a particular exercise.

d. A brevity list approach is an extremely practical alternative to an operations code--

  • When the information exchange requirement is relatively limited.
  • When the entries can be held to a minimum number of sentences, phrases, and/or words.
  • When the messages are generally short.
  • Where the vocabulary entries consist primarily of complete, independent thoughts.

7-7. COMSEC Operations Support

a. A unit must have a COMSEC account or have access to a COMSEC account, before it can conduct meaningful COMSEC training. Most units either have approved containers or a facility for storing COMSEC material or can obtain an approved container through supply channels. Command is responsible for establishing a COMSEC account. The unit must then train and operate using COMSEC equipment and/or systems. Commanders must use AR 380-40 and the TB 380-41 series in establishing COMSEC support for their unit. Although it places an additional burden on the commander to use COMSEC systems, their use is essential to success and survival on the battlefield. COMSEC IS NOT AN OPTION; IT IS MISSION-ESSENTIAL.

b. Unit SOPs must be clear on the use of COMSEC equipment and systems, both for administrative operations and tactical operations. All personnel should be familiar with SOP instructions and SOI instructions on the unit's particular COMSEC systems. Personnel must be familiar with procedures for resupply of COMSEC material during operations, and the COMSEC custodian must provide for an adequate supply of COMSEC material to be on hand for both training and/or operations.

c. Training programs must ensure that all necessary personnel receive adequate instructions and training on COMSEC procedures by both formal and on-the-job training. The INSCOM support activity can provide invaluable assistance in establishing, maintaining, and evaluating your unit's COMSEC account, training program, SOP, and storage facilities. Their support must be scheduled well ahead of time due to the number of units each activity supports. Information on storage and accounting for COMSEC equipment can be found in AR 380-40, AR 640-15, and the TB 380-41 series.

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