Ham Radio / Off-grid communication

Obfuscous said:

Honest question, is a Nokia off the grid?

Click to expand...

Like, a cell phone? What do you mean exactly?

If the question is if the old ones are off the network, mostly yes, depending on where you are. 2G services are pretty much gone in most places people will be posting from.

I was reading the other day that the 'SMS blasters' that the chinks have been trying to deploy in Canada rely on modern phones having 2G enabled, doing a 4G handshake then downgrading to take advantage of old GSM encryption flaws. So it still has to be going somewhere.

If you mean it in a 'is this useful off grid communication' then for the most part lol no, cell networks are heavily monitored and probably pretty easy to take down. You could run your own GSM stations, people have software out there to do that, but then you'd be sharing a band with people who have radio towers to DF you all over the place and a financial incentive to get you busted.

1. Intro
This is a guide for using the one-time pad method as a means of secure, low tech communication.

The one-time pad (OTP) is a technique for unbreakable encryption when it is used correctly.

The plaintext (message we want to send) can't be reversed from the ciphertext (our encrypted message) without the key being known.

There is no form of cryptanalysis or method to reverse the ciphertext to the plaintext, even with infinite time and computational power. It's the only form of encryption that has perfect secrecy.

One-time pads can be created with something as simple as a pen, paper, and set of 10 sided die. Only basic mental math of addition and subtraction is required for encrypting and decrypting messages using the OTP.

This guide will also provide an example of the full lifecycle of a OTP encrypted message, covering both transmission and reception on HF using the JS8call software.


2. The OTP
The one-time pad technique is an evolution of the Vernam cipher. If you've ever listened to a recording of a shortwave number station where a string of numbers or letters is read out loud, chances are they're using a one time pad. Some stations like E11 still use this technique to this day.

A one-time pad is composed of multiple keys of randomly generated digits. You can think of the keys in the one-time pad as "noise" we combine with our message.

The basic process of both encryption and decryption is converting the characters of your message into digits using a checkerboard (map of characters to digits), and combining them with the randomly generated key digits either subtracting or adding, with modulo 10.

"Modulo 10" for our purposes just means looking at the last digit. So for addition if we added 9 + 7 together, 16 becomes 6. We look at the last digit and that's our result.

And for decryption, subtraction of the key digits from the encrypted digits with modulo 10. Then converting the resulting numbers back into characters with our checkerboard to get our message.


The process is very simple and only uses mental math. The next three sections will also go into more detail of this process.

Below is an example of a one-time pad. Each new line represents a new key:

The one time pad can be written on anything, like a piece of paper, index card, or notebook.

Both the sender and receiver must have an identical copy of the same one-time pad. It's best practice to label the sender's copy OUT, and the recipients copy IN, followed by the sequence if there are multiple one-time pads created.

The length of the keys in the one-time pad can be as long or as short as needed. There can be as many keys as required for however many messages are intending to be sent. It's best however to keep messages simple and set a maximum limit of 75 characters used per one-time pad (which includes all key material)

The main disadvantage to the OTP is the logistics of ensuring that both parties can securely maintain and exchange a copy of the key. OTP communications require coordination of when the message will be sent, and when one time pads will be exchanged.

The one-time pad is single use.
It must be destroyed after it's used.

Both the sender and receiver must do this. The integrity, and perfect secrecy of the one-time pad communications hinges entirely on this factor. Destroying it physically also prevents accidental re-use of the same one time pad.


3. Generating the OTP
To generate a one-time pad, we need a good source of randomness.

Computers are bad at generating random numbers, and awful at security. While it may appear "random", numbers generated by computers are deterministic, generated by an algorithm, and not truly random.

As a general rule of thumb, the plaintext (message you want to send) and key itself should NEVER touch a computer.
Do not use online number generators. Do not use online "one time pad generators".
Do not use software applications, mobile software, /dev/random, etc.

Nothing that isn't the encrypted message should ever be put on a computer.

A set of 5 D10 die, like the one pictured below, serves as a good enough source of unpredictability for generating our random digits.


To generate the keys for our one time pad, toss 5 at a time and write the digits down read in the order they landed from left to right. Do not pick the order. Write them as they landed.

Repeat this for as many groups as we want to have. The one time pad should use keys that are all of the same group length, for as long or longer than the messages we want to send.

Once we're finished we'll have something like this:
We rolled our set of 5 d10 6 times to get 6 groups of random numbers. Each group needs to be the same length.

We can repeat this process for as many keys as we would like and add them as a new line.

The first group in our one time pad is always the key ID. The key ID is a unique identifier that allows the recipient to know which key from their one-time pad to use for decryption. This number isn't used in the decryption or encryption process itself, but tells the recipient which key to use for each message that we send.

It's important that the key ID is generated using the same random process as the rest of the key. Do not pick a "memorable" number, do not use the date or increment sequentially.

After we've generated our random digit groups, we label our copy of the pad OUT, and the recipients copy IN. This is to prevent accidental re-use of the same key.

This is what a completed example of our one-time pad might look like, with three keys:



It may be tedious, but do not copy the key electronically. Do not use a phone to take a picture of it and print it out.

Once we have both a IN and OUT pair, our one time pad is now complete and we are ready to use it.


5. The checkerboard
The checkerboard is what enables the conversion of characters into digits that we combine with the keys of our one time pad.

Checkerboards are ideal because they allow for fractonation and compression. That means in simple terms, the most common letters in the English language (like E, T, A, O, I, N) get a single digit, while less common letters and numbers get two digits. This keeps our encrypted messages shorter than they would be otherwise. They are not used in the encryption process itself, but for encoding and decoding letters and numbers.

Both the sender and receiver must use the same checkerboard for the message to be decodable.

The most well known checkerboard is the straddling checkerboard and can be remembered by the mnemonic "AT ONE SIR"


In this checkerboard:
A => 0
T => 1
O => 3
B => 20
V = > 53

and so on. You may notice that the checkerboard has no numbers. This is where the figure shift character // character (59) comes in. When we see 59 (//), we shift from letters to literal numbers. And shift back after we see another //.

Spaces should be omitted where the message is readable without them.

An example of the conversion of FARMERCHUCK using the checkerboard is:

23 0 9 29 6 9 21 25 52 21 27

and another example. Let's say we were planning a meeting at Farmer Chuck's at 18:45z. We need to use the figure shift to write the time.

When we use the figure shift to write numbers, we write each number twice. So 1845 => 11 88 44 55


23 0 9 29 6 9 21 25 52 21 27 7 0 1 59 11 88 44 55 59 57

For decoding, each number starting with 2 or 5 in the AT ONE SIR table means we use two digits for decoding the letter.


There are also additional checkerboards we can use, like the CT-37 table. The CT-37 table uses the mnemonic ESTONIA.

(Image from: https://www.ciphermachinesandcryptology.com/en/table.htm | archive )

The CT-37 table lets us make use of a codebook. It also provides mapping for punctuation. In this table, much like the // figure shift in the previous checkerboard, we use 89 to shift from letters to numbers, and 99 to use the codebook.

A codebook is a shorthand for a group of phrases that are used frequently. Instead of spelling out a long phrase character by character, we assign it a numeric code. This compresses the message further and reduces the amount of key material we burn through.

Both sender and receiver must have an identical copy of the codebook, just like the one-time pad and checkerboard. The codebook itself should also be kept secret, but can be kept after the one-time pad is destroyed.

A small codebook might look like this:


The codebook allows us to write "NEED RESUPPLY AT CHUCKS" as:


saving 15 full digits.

As with 2 and 5 in the AT ONE SIR table, the ESTONIA CT-37 table marks two digit sequences with 7 or 8. Numbers after the figure shift are also copied twice.
There are also a few other checkerboard variants. But for sake of simplicity, I recommend choosing either the AT ONE SIR or ESTONIA CT-37 checkerboard.


6. Encrypting a message

Let's encrypt the message MEETATCHUCKS1845z using key 001 from our pad earlier. For the encoding and decoding segments, we will use the simpler AT ONE SIR straddling checkerboard.

Our first step is to convert the plaintext to digits using the checkerboard.



When we are doing our conversion, write each digit in groups of 5 like our pad:

23092 96921 25522 12770 15911 88445 55957

With the message converted using the checkerboard, it's now time to combine it with our keys from our one-time pad to get our encrypted message.

Our pad from earlier is


Put our plaintext digits with below the key of the one time pad, ignoring the key IDs.

Our message here needs to use two key groups. I've made it purposely obtuse just to show how to handle messages that exceed one key's worth of material. Normally you should size your keys to comfortably contain a full message.

The process of combining the key is extremely simple. Take the first digit of the plaintext and add it to the key using modulo 10 addition. Modulo 10 addition just means that we only look at the last number.

So 1 + 2 is 3, but 9 + 9 ( 18 ) is 8. And we work our way from left to right writing down each number until we're complete.


06356 13440 97398 07987 56769 59181 74829 30516 82647 | key
[KEY] 23092 96921 25522 12770 15911 [KEY] 88445 55957 | plaintext
--------------------------------------------------------------------------------+
06356 36432 83219 22409 68439 64092 74829 18951 37594




Our final encrypted message is:




7. Decrypting a message

To decrypt, we combine the key with the ciphertext, but this time using subtraction mod 10 instead of addition. Write the received encrypted message above the key for decryption.

Modulo 10 subtraction just means if the ciphertext digit is smaller than the key digit, we borrow 10.

So 3 - 1 = 2, but 0 -7 becomes 10 - 7 = 3. We just pretend the ciphertext digit is 10 higher when we need to.

06356 36432 83219 22409 68439 64092 74829 18951 37594 | ciphertext
06356 13440 97398 07987 56769 59181 74829 30516 82647 | key
---------------------------------------------------------------
[KEY] 23092 96921 25522 12770 15911 [KEY] 88445 55957


This gives us back our plaintext: 23092 96921 25522 12770 15911 88445 55957

We can then use our AT ONE SIR checkerboard to convert it back


which gives us our message:

FARMERCHUCKS AT (figure shift) 1845 (figure shift)z




9. Tips & Planning Communications
To reiterate above before we continue:

The one-time pad is single use.
It must be physically destroyed after it's used, even if there is material left over.

Never use a computer to store or generate the one-time pad.
Keep messages short and concise. Omit spacing where it's not needed. A 75 character limit should be the upper bound for messages per each pad.

The disadvantage to the one-time pad method is the requirement the planning overhead, and ensuring that both parties can maintain a secure copy of the one-time pad.

All key material, codebooks, and schedules must be exchanged in person.

One-time pad commutations can be bidirectional, with both parties having a IN pad, and OUT pad.

While possible, it's not recommended to give multiple recipients the same copy of an IN pad.

There are two patterns of communication that can be used for one-time pad messaging: asynchronous and synchronous

Synchronous means that the sender and receiver coordinate an exact window of time to broadcast and receive. This is pretty much how every classic shortwave spy number station operates. It usually requires a fixed schedule that is known to both parties beforehand.

Asynchronous is could mean something as simple as leaving a sticky note with your encrypted message, or sending it over an email. It doesn't involve a fixed window of time.


The example in the next section will look at a synchronous one-way communication using HF radio.


10. Transmission
This section will cover an example of transmission of a one-time pad encrypted message using the JS8call software.

New to JS8call? See the post below:

First we need to understand our setup and threat model.

Any radio signal by nature of physics can be located. For sustained communications using this method it's ideal to operate from a portable setup that can be deployed and teared down . This can be anywhere from the top of a mountain on a trail, side of the road, or a public park.

Having a radio license for your jurisdiction with HF privileges gives you plausible deniability for both owning and using HF equipment. JS8call is frequently used in amateur radio communications as a part of the hobby.

We need to know the general location of where our recipient is before communications can take place.

This is imperative, as we need to plan around HF propagation conditions that allow us to reach them. The time of day we broadcast at and frequencies we use will be determined by this and are the two variables we are confined by.

If they are regional at a distance of 0 - 300 miles [500 km], it's ideal to use NVIS propagation and plan around the fof2 @ time of day for your latitude.

If they are beyond that distance, use VOACAP with general (province, state, city) estimates of where the recepient will be. Below is an example of what our propagation chart might look like for a transmission from Las Vegas to Denver

On the left you can see the chart of reliability over time of day. All times in VOACAP are in UTC.


From this chart, we can see that 40 meters and 30 meters are our two best options to reach them. Remember that HF band conditions are volatile. We need to account for this in our communication planning.