How to use One Time Pad cryptography with a Raspberry Pi

It has never been harder to be entirely certain that any message you send can be transmitted or received in absolute secrecy.

This holy grail of cryptography has long frustrated security experts, and most people are willing to settle for using encryption programs like gpg, which while theoretically breakable will resist all cracking attempts long after you’re pushing up the daisies.

What if, however, there were a way to be certain that your personal emails, pictures of your pet kitten, backups of your tax returns for the past decade and so on were safe even if intercepted? Enter the One Time Pad.

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The Notorious OTP

In simplest terms, a One Time Pad is a series of random numbers which you agree upon with someone with whom you wish to communicate, usually by meeting in person and exchanging pads.

When sending a message, you first need to convert it to numbers, then add each of these numbers to the numbers in the pad. Once the recipient receives the message, they can work backwards using their copy of the pad, deducting the numbers to retrieve your original message.

Provided that the numbers are truly random, no one else sees the pad, and the same text isn’t encoded twice with the same pad, then even the world’s fastest supercomputer couldn’t decode the message.

The strength of the encryption lies in the randomness of the pad numbers. Without knowing these, anyone who intercepted a message might see the word ‘LINUX’ encrypted as ‘OSYAJ’ but would have no way of knowing it isn’t another five letter word, like ‘CHILE’.

The One Time Pad has been kicking around in some shape or form since the 1880s but it wasn’t until 1917 that Gilbert Vernam et al formally patented a machine for automating the process. 

In this case two reels of punched tape were used, one with the original message and one with the pad of random numbers.

KGB agents in the US were quick to exploit this during the Cold War, placing small pads inside hollowed-out nickels, fake walnuts and any number of dastardly hiding places.

In the 21st century, computers like Raspberry Pi lend themselves well to being carried around easily and are perfect for generating and processing One Time Pads. But in order to understand why, it’s necessary to understand the pitfalls of the One Time Pad.

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Entropy isn’t what it used to be...

Bruce Schneier once described the One Time Pad as “theoretically secure, but... not secure in a practical sense.”

This reflects the fact there have been times that OTPs have been broken in practice despite their theoretical security. In the 1940s, for instance, US SIGINT’s counterintelligence program Venona was able to decrypt a number of Soviet OTP messages simply because some pads had been reused.

This crypto-cardinal sin was committed because the Soviets simply couldn’t generate pads fast enough for the thousands of daily messages sent during wartime.

A similar chink is found by German codebreakers in Neal Stephenson’s Cryptonomicon. The British employ a raft of old ladies with small bingo machines to draw numbered balls to generate pads. 

Unfortunately the old dears don’t always obey best practices – they fail to look away each time they draw a ball as instructed, meaning that they subconsciously select predictable numbers.

Fast forward to the 21st century and the issue hasn’t improved much. Entire books have been devoted to this subject but suffice it to say that computers generally aren’t very good at generating true randomness. 

Usually when it’s required a website or program will ask you to wiggle your mouse to provide a so-called “noise source” to work from.

Mostly however when a computer requires larger amounts of randomness it will form a string of pseudo-random data from your entropy pool, which, while ideal for determining where the next block will fall when you play Tetris, is less than perfect when it comes to security.

Thankfully one of the lesser known features of the Raspberry Pi is that it has its own built-in hardware random number generator, which in combination with the rng-tools suite can generate exactly the kind of high-quality randomness needed for an OTP.

The second obstacle faced by those using OTP has to do with key generation and distribution. The Soviets were unable to keep up with the demands of war, and in real life the bingo machines of Britain’s little old ladies would probably end up smoking with the number of times they’d need to be churned.

Fortunately the hardware RNG built into the Raspberry Pi can generate large amounts of data in a short amount of time. For instance a pad with 10,000 sets of five-digit random numbers can be generated in seconds by activating the hardware RNG (see step 1 of our guide on the next page) and then entering:

$ sudo base64 /dev/hwrng | tr -dc '0-9' | fold -sw 5 | head -c 10000 > bobtoaliceotp.txt

The resulting text file can then be printed out, for instance by Adafruit’s thermal printer. This printer has the advantage that unlike most laser printers it doesn’t record the serial number, make or model on each sheet it prints out (on this, see this website). 

It also holds up to 15m of paper, which will be plenty for generating long messages. Using this in combination with the excellent program otp-gen, you can print off your own pads automatically at the touch of a button too (see the ‘OTP goes Thermal’ box on this page).

Once the reams of paper are safely printed and tucked away, next comes the issue of physically distributing pads.

This is no doubt the main reason why the OTP hasn’t seen much widespread use. It’s rumoured that the red phone running between Washington and Moscow for instance is secured by an OTP, mainly because both Superpowers are wealthy and melodramatic enough to have men with dark glasses to handcuff themselves to briefcases and swap pads at regular intervals.

Of course if you have printed paper pads, it is possible to mail them to your recipient, but then your OTP would be no safer than a regular letter because it may have been intercepted and copied along the way. The same applies for using regular encryption programs like GnuPG.

The only way to be sure that your pad cannot be intercepted is to deliver it in person. Again the humble Raspberry Pi comes to the fore here because it’s extremely small and easy to carry. Upon meeting the person with whom you want to exchange messages, you can give them a copy of your Raspberry Pi’s MicroSD card or the Raspberry Pi itself.

To ensure perfect security of your messages, it’s necessary to delete pads that have previously been used.

If you have printed yours out, a little tearing off and a Zippo lighter is likely to be helpful here. (Other lighters are available.)

Otherwise running the shred command on the pad you just used should be enough to prevent recovery. Both SD cards and Raspberry Pis are inexpensive also, so if you really feel you have to destroy them once the pads have changed hands, you can do this too.

The low cost of a Raspberry PI is also a great answer to another common criticism, which is that an OTP is usually very difficult to scale beyond two people exchanging messages.

If you do decide to form a secret society, it may be best to designate one person to meet each member and exchange pads regularly. That person can then sit at the centre of the web and act as a clearing house for messages, forwarding them between members as need be.

Even using a Raspberry Pi, however, it is still possible for pads to be intercepted, and communicating can be cumbersome. This is why it’s good to employ some best practices for your OTP.