The TSEC/KL-42 is a portable, electronic, off-line or on-line crypto-equipment or encryption and decryption of alphanumeric information of all categories and classifications. It has a built-in keyboard for information input, a liquid crystal display (LCD), a built-in modem and a telephone handset couplerfor transmission/receipt of encrypted/decrypted data over standard telephone lines, and printer connection capability. The KL-42 is not a ruggedized device.


The KL-42 without printer, or with an approved printer, is authorized for unrestricted use for classifications up to and including TOP SECRET. Approved printers compatible with the KL-42 are included in the Preferred Products List (PPL). Other compatible printers and restrictions on their use are included in a separate list available from the Commanding Officer, Naval Electronic Systems Security Engineering Center or Commander, Naval Investigative Service Command.

Since KL-42 printers do not automatically mark printed messages with the classification level, a statement of classification shall be included in printed messages processed by the KL-42. If knowledge of the time a message is sent is critical to proper interpretation of the message, that message shall include a date/time/message count feature in the plain texts.

KL-42 encrypted messages may be transmitted via any medium (Morse Code, verbal, modem, etc.).


KL-42 Traffic Encryption Keys (TEKs) are operational keys (KAK) or Exercise Keys (KXK) having a 24-hour cryptoperiod. Each 31 segment TEK canister is superseded monthly. A used TEK tape segment may be retained up to 72 hours after its supersession to accommodate delayed messages; however, the key must be protected to preclude traffic compromise through loss or espionage until itsdestruction. KL-42 training keys (KTK) and maintenance keys (KMK) have no cryptoperiod and each key segment may be reused until unusable. Training keys and maintenance keys will not be used forover-the-air transmissions.

Cryptonets should be kept as small as the operation permits to reduce the possibility of compromise but some KL-42 nets are by necessity large. In such cases, KL-42 cryptonets are limited to 200 members when traffic is TOP SECRET and up to 1000 members when traffic is SECRET and lower. Controlling authorities requiring larger nets must request approval from the CMC (CCT).

Reference:  Excerpts from Marine Corps Order MCO 2250 .1; C4-CCT-635; 17Jan 1990.

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Mar 20/01