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The current goes through a wire in the rotor and comes out at another position, say L. However, after that keystroke, the rotor rotates one or more positions. So the next time the user hits the W key, the letter will be encrypted not as L but rather as some other letter. Though more challenging than simple substitution, such a basic, one-rotor machine would be child's play for a trained cryptanalyst to solve.
So rotor machines used multiple rotors. Versions of the Enigma, for example, had either three rotors or four. In operation, each rotor moved at varying intervals with respect to the others: A keystroke could move one rotor or two, or all of them. Operators further complicated the encryption scheme by choosing from an assortment of rotors, each wired differently, to insert in their machine. Military Enigma machines also had a plugboard, which swapped specific pairs of letters both at the keyboard input and at the output lamps.
The rotor-machine era finally ended around , with the advent of electronic and software encryption, although a Soviet rotor machine called Fialka was deployed well into the s.
The HX pushed the envelope of cryptography. For starters it has a bank of nine removable rotors. The unit I acquired has a cast-aluminum base, a power supply, a motor drive, a mechanical keyboard, and a paper-tape printer designed to display both the input text and either the enciphered or deciphered text. In encryption mode, the operator types in the plaintext, and the encrypted message is printed out on the paper tape.
Each plaintext letter typed into the keyboard is scrambled according to the many permutations of the rotor bank and modificator to yield the ciphertext letter. In decryption mode, the process is reversed. The user types in the encrypted message, and both the original and decrypted message are printed, character by character and side by side, on the paper tape.
While encrypting or decrypting a message, the HX prints both the original and the encrypted message on paper tape. The blue wheels are made of an absorbent foam that soaks up ink and applies it to the embossed print wheels. Beneath the nine rotors on the HX are nine keys that unlock each rotor to set the initial rotor position before starting a message.
That initial position is an important component of the cryptographic key. To begin encrypting a message, you select nine rotors out of 12 and set up the rotor pins that determine the stepping motion of the rotors relative to one another.
Then you place the rotors in the machine in a specific order from right to left, and set each rotor in a specific starting position. Finally, you set each of the 41 modificator switches to a previously determined position. To decrypt the message, those same rotors and settings, along with those of the modificator, must be re-created in the receiver's identical machine. All of these positions, wirings, and settings of the rotors and of the modificator are collectively known as the key. The HX includes, in addition to the hand crank, a nickel-cadmium battery to run the rotor circuit and printer if no mains power is available.
A volt DC linear power supply runs the motor and printer and charges the battery. The precision volt motor runs continuously, driving the rotors and the printer shaft through a reduction gear and a clutch. Pressing a key on the keyboard releases a mechanical stop, so the gear drive propels the machine through a single cycle, turning the shaft, which advances the rotors and prints a character.
The printer has two embossed alphabet wheels, which rotate on each keystroke and are stopped at the desired letter by four solenoids and ratchet mechanisms. Fed by output from the rotor bank and keyboard, mechanical shaft encoders sense the position of the alphabet printing wheels and stop the rotation at the required letter.
Each alphabet wheel has its own encoder. One set prints the input on the left half of the paper tape; the other prints the output on the right side of the tape. After an alphabet wheel is stopped, a cam releases a print hammer, which strikes the paper tape against the embossed letter.
At the last step the motor advances the paper tape, completing the cycle, and the machine is ready for the next letter. As I began restoring the HX, I quickly realized the scope of the challenge. The plastic gears and rubber parts had deteriorated, to the point where the mechanical stress of motor-driven operation could easily destroy them.
Replacement parts don't exist, so I had to build such parts myself. After cleaning and lubricating the machine, I struck a few keys on the keyboard.
I was delighted to see that all nine cipher rotors turned and the machine printed a few characters on the paper tape. But the printout was intermittently blank and distorted. I replaced the corroded nickel-cadmium battery and rewired the power transformer, then gradually applied AC power. To my amazement, the motor, rotors, and the printer worked for a few keystrokes. But suddenly there was a crash of gnashing gears, and broken plastic bits flew out of the machine. Printing stopped altogether, and my heartbeat nearly did too.
I decided to disassemble the HX into modules: The rotor bank lifted off, then the printer. The base contains the keyboard, power supply, and controls. These snubbers had disintegrated. Also, the foam disks that ink the alphabet wheels were decomposing, and gooey bits were clogging the alphabet wheels.
I made some happy, serendipitous finds. To rebuild the broken printer parts, I needed a dense rubber tube. I discovered that a widely available neoprene vacuum hose worked perfectly. Using a drill press and a steel rod as a mandrel, I cut the hose into precise, millimeter sections.
But the space deep within the printer, where the plastic snubbers are supposed to be, was blocked by many shafts and levers, which seemed too risky to remove and replace. So I used right-angle long-nosed pliers and dental tools to maneuver the new snubbers under the mechanism. After hours of deft surgery, I managed to install the snubbers.
The HX has nine rotors and also uses a technique called reinjection. Each rotor has a set of conductors that connect each and every electrical contact on one side of the rotor with a different contact on the other side.
For every rotor the pattern of these connections is unique. When the operator strikes a key on the keyboard, representing one of 26 letters, current travels through the set of nine rotors twice, once in each direction, and then through a separate set of 15 rotor contacts at least two times. This reinjection technique greatly increases the complexity of the cipher. The ink wheels were made of an unusual porous foam.
I tested many replacement materials, settling finally on a dense blue foam cylinder. Alas, it had a smooth, closed-cell surface that would not absorb ink, so I abraded the surface with rough sandpaper. After a few more such fixes, I faced just one more snafu: a bad paper-tape jam. I had loaded a new roll of paper tape, but I did not realize that this roll had a slightly smaller core.
The tape seized, tore, and jammed under the alphabet wheels, deeply buried and inaccessible. I was stymied—but then made a wonderful discovery. The HX came with thin stainless-steel strips with serrated edges designed specifically to extract jammed paper tape. I finally cleared the jam, and the restoration was complete.
One of the reasons why the HX was so fiendishly secure was a technique called reinjection, which increased its security exponentially. Rotors typically have a position for each letter of the alphabet they're designed to encrypt.
So a typical rotor for English would have 26 positions. But the HX's rotors have 41 positions. That's because reinjection also called reentry uses extra circuit paths beyond those for the letters of the alphabet.
In the HX, there are 15 additional paths. Here's how reinjection worked in the HX In encryption mode, current travels in one direction through all the rotors, each introducing a unique permutation. After exiting the last rotor, the current loops back through that same rotor to travel back through all the rotors in the opposite direction.
However, as the current travels back through the rotors, it follows a different route, through the 15 additional circuit paths set aside for this purpose. The exact path depends not only on the wiring of the rotors but also on the positions of the 41 modificators.
So the total number of possible circuit configurations is 26! And each of the nine rotors' internal connections can be rewired in 26! In addition, the incrementing of the rotors is controlled by a series of 41 mechanical pins. Put it all together and the total number of different key combinations is around 10 Such a complex cipher was not only unbreakable in the s, it would be extremely difficult to crack even today.
Small, at the U. Army's Signal Intelligence Service. It was the subject of a secret patent that Small filed in and that was finally granted in No. Meanwhile, in , Hagelin applied for a U. Perhaps surprisingly, given that the technique was already the subject of a patent application by Small, Hagelin was granted his patent in No. Friedman, for his part, had been alarmed all along by Hagelin's use of reinjection, because the technique had been used in a whole series of vitally important U.
Friedman Collection. After a career as an electrical engineer and inventor, author Jon D. Paul now researches, writes, and lectures on the history of digital technology, especially encryption. In the s he began collecting vintage electronic instruments, such as the Tektronix oscilloscopes and Hewlett-Packard spectrum analyzers seen here.
The revelation of Crypto AG's secret deals with U. Nor are Apple Macs safe. Malware that allowed attackers to take control of a Mac has circulated from time to time; a notable example was Backdoor.
Eleanor, around And in late , the cybersecurity company FireEye disclosed that malware had opened up a backdoor in the SolarWinds Orion platform, used in supply-chain and government servers. The full extent of the damage is still unknown. The HX machine I restored now works about as well as it did in I have yet to tire of the teletype-like motor sound and the clack-clack of the keyboard. Although I never realized my adolescent dream of being a secret agent, I am delighted by this little glimmer of that long-ago, glamorous world.
And there's even a postscript. My wry old Swiss friend had known everything all along! The Crypto AG affair was described in a pair of Swedish books. Chip Hall of Fame: Transmeta Corp. Crusoe Processor.
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