The PENELOPE cryptosystem was designed for the encryption of call signs and address groups used on radio circuits, both CW and Voice. System components included keying materials and two associated devices designed for encrypting call signs and address groups in the form of four character groups, which were composed of any combination of letters and digits.

Encryption produced another four character group, consisting in this case of any combination of letters, digits and slant V signs. The call signs and address groups which could be encrypted by the system could be found in separate publications.

The PENELOPE keying material consisted of two parts:

(a) A substitution table for changing plain four character address groups or call signs into intermediate groups, which could be further encrypted.

(b) Key cards for use with one of the cryptodevices in encrypting and decrypting the intermediate groups obtained through the substitution table. INTERMEDIATE GROUPS WERE NEVER TO BE TRANSMITTED WITHOUT FIRST BEING ENCRYPTED. Substitution squares and key cards, being complementary, were together known as PENELOPE Key Lists.

The cryptodevices used in the PENELOPE system were:

(a) CSP 1750 (KA-2)  A manually operated mechanical device, used in conjunction with key cards to encrypt and decrypt intermediate groups.

(b) CSP 1756 (KA-3) - A device which used sliding strips, for the same purpose as the CSP 1750 (KA-2), and intended for use whenever the CSP 1750 (KA-2) was inoperable.


Any encrypted call sign, address group or other group could be transmitted phonetically on voice circuits. Furthermore, if desired, encrypted call signs and address groups could be changed to a pair of words for voice transmission by use of a "word square"
This was included with the appropriate PENELOPE keying material.

Procedure for Retarding Voice Call Sign Changes:

A procedure known as MISFIT was devised for retarding voice call sign changes when a force was engaged in action at a time scheduled for a call sign change. Implementation of MISFIT Procedure merely continued the use of voice call signs already in  effect past the time scheduled for their supersession; it had no effect on the supersession of RATT/CW call signs. Thus, when MISFIT was declared and the time scheduled for normal call sign change was reached, AMSP 298 key card supersession took place, new call signs (CW/RATT and voice) were derived, and all but the newly derived voice call signs were put in use.  As long as MISFIT was in effect, the voice call signs already in  use, derived by means of the superseded key card, remained in effect.

EXAMPLE: A force is engaged by the enemy at 233OZ, and has a call sign change scheduled for OOO1Z. The Force OTC feels that conducting the call sign change as scheduled on the Force's voice radio circuits will be deleterious to his tactical control. AMSP 298 key card "DH" is in use by the units of his Force. The OTC orders the following transmission made to all Force units concerned:

"MISFIT DELTA HOTEL" . As transmitted, this order meant "continue using voice call signs encrypted by means of AMSP 298 daily-changing key card "DH" until further notice."

CHARTER - When the situation requiring retardation of the voice call signs change had passed, replacement of the superseded voice call sign was effected as soon as possible. The OTC may have accomplished this by ordering transmission to units concerned by the use of the term "CHARTER", followed by the designation of the effective AMSP 298 key card by which the new voice call signs would be derived, and a time indicating when that key card was to be put in use.

EXAMPLE: The action which caused the OTC to order MISFIT has ended at 070OZ. The OTC wishes to allow time for regrouping before changing call signs, so decides to wait until 080OZ to put the deferred key card (e.g"DZ") into effect. The OTC orders the
following transmission made to all Force units concerned:


As transmitted, this order meant "Change to call signs encrypted by means of AMSP 298 daily-changing key card "DZ" at O8OOZ".

The OTC ensured simultaneous promulgation of MISFIT procedure on  all voice radio circuits under his control. Ordering MISFIT procedure into effect in the manner described above before engagement in action had occurred could betray to opposing forces that an operation is impending. Therefore, any orders issued prior to engagement in action which indicate that MISFIT procedure is to be implemented will be afforded physical or cryptographic security in their transmission.


csp_1750_a_687_2.jpg The CSP 1750 (KA-2) was a manually operated cipher device which was employed in enciphering and deciphering four character groups for which intermediate groups have been substituted. The cipher device is of metal construction. On its surface are four columns of characters with a movable chain beside each column. Below the four columns of characters are four windows in which appear the counter numbers for each column. A metal stylus is attached in brackets on the side of the device for use in moving the chains.

Before beginning encipherment or decipherment the cipher device must be zeroized, i.e., each of the four counters must read zero. Zeroizing is performed by holding down the lever at the lower left corner of the device and by turning the knob on the lower right side counterclockwise. When the knob locks and zero (00) appears on the counters, the cipher device is correctly zeroized. To register a character, the stylus was inserted in the chain link opposite the desired character and perpendicular to the face of the device. The stylus was pulled downward until it reached the metal bar below the columns of characters; the total will appear on  the counter below the column used. An example of the machine in the photo is on display at the MARCOM Museum, Halifax. (Photo courtesy John Alexander, G7GCK Leicester, England)


The CSP 1756 (KA-3) is a cipher device designed for the same purpose as the CSP 1750 and intended for use whenever CSP 1750 is not available. The device is a wooden board with a fixed plastic strip at the top, another fixed plastic strip at the bottom, and three grooved channels in the center, each containing a sliding  strip. The plastic strip at the top is marked with an alphabet-numeral sequence consisting of a normal alphabet, a slant sign, and the numbers one through zero. The plastic strip at the bottom is marked with a numeral sequence consisting of a series of numbers from 00 to 36 in reverse order. Each of the three sliding strips is marked with a normal alphabet, a slant sign, the numbers one through zero, a vertical, black line, and the same sequence of characters repeated.
CSP 1756/KA-3.  (Photos courtesy John Alexander, G7GCK Leicester, England)

This photo illustrates the shipping box for the CSP-1756/KA-3. (E-bay photo) 


Excerpts from document AMSP 295(B) dated 1965. This document was declassified per memo 100-80/4022 (DCOMSEC S-3) on Jan 15, 1980.

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Dec 19/10