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.

HOW PENELOPE WAS USED

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.