The HC-520 Cryptomatic , another Crypto AG machine, can be thought of as a modern, electronic version of the CD-57. It features an LCD display with keyboard plus an external power supply for a rechargeable NiCd battery pack. Alternately, 6 to 7.5 VDC volts at 0.5 amps external power can be applied via the 'D' socket at the rear. To encrypt messages, the user would enter the plaintext via the keyboard, then read the encrypted text in the LCD display. The buffer in the machine will hold around 690 characters.