If radio waves are just non-visible light waves that are picked up by a vibrating rod, how does a radio wave get “encrypted” so that it can’t be picked up unless it’s unencrypted?
Edit: Everyone keeps commenting that the content of the message is what’s encrypted, not the radio waves itself, but that’s not what I mean. Someone answered that digital signals themselves can be modulated or disguised, which is what I meant when I asked
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Human beings cannot hear, see or sense radio waves. For the wave to carry information, it has to be coded – modulated. Let’s take a very simple example – say you wanted to create a way of communicating across a swimming pool with waves. You could agree with your friend at the other end of the pool that big waves mean yes and small waves me no, and then, you can give yes or no answers with the waves. That is modulating – changing the waves to convey a message.
Now, imagine you create a whole alphabet with waves. Three big waves in a row are the letter S for example, and three tiny waves are the letter O. Now, you can send a novel across the pool with waves.
Now, lastly, imagine you send a secretly encoded message across the pool. Every letter is shifted one to the left, for instance, so to send SOS, you now send RNR. Now you are sending encrypted messages with waves.
Lastly – replace the water waves with radio waves.
>how does a radio wave get “encrypted” so that it can’t be picked up unless it’s unencrypted
There are two separate, but related, schools of techniques here.
A lot of people have already talked about **encryption**. The radio transmission is sent in code; someone monitoring the transmission frequency can intercept the transmission, but have no idea what it says.
But an enemy can still use this information against us – they know something has been transmitted, but not what. They can jam that frequency, stopping us talking (by transmitting another, more powerful signal on the same frequency, which will cause radio interference) and/or they can use the presence of the transmission to find us. These techniques are called **electronic warfare.**
How can we deal with this? First, we can change our transmission frequency rapidly. If we hop through a list of different frequencies in a secret sequence, the listener will have trouble keeping up. We need accurate clocks in our radios for them to synchronise the times of the hops. Now the enemy can’t jam on a specific frequency any more, and their jammer transmitter only has so much output power; if they try to jam lots of frequencies to cover all your potential hops, there’s less power to cover more spectrum and it will be less effective. This is called **frequency agility** and is an **electronic protection measure,** which is **defensive electronic warfare.**
This isn’t in and of itself encryption, so is used in combination with encryption of the message being carried. The older NATO waveform for this is HAVEQUICK and the modern one is SATURN.
We can take this concept further to hide our transmissions below a listener’s ability to pick it up. We spread our transmission over so many frequencies that it is difficult to pick out of background radio noise. In combination with very rapid, pseudo-random frequency changes, if you aren’t a friendly receiver who knows exactly what they’re looking for, it is very hard to know that anything is being transmitted at all. This is called **spread spectrum** and is an example of **low probability of intercept** radio, which is also a form of **electronic warfare**.
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