Having secure communications, and eavesdropping on opponent's communications, is a good thing in black-trenchcoat SR campaigns.
My PC group has largely relied on Mind Net, but it has limited range, it requires sustaining focus or sustaining penalties (or tasking a Spirit of Man to cast and sustain it), and it's not so good for transmitting detailed tactical maps and other high-density information.
Radio doesn't have these limitations but in SR4, decryption is easy.

So combine magic and tech. Set up a comm channel, and use a spirit to communicate your key to the other person or people. Low-end: use a watcher spirit. High-end: use a spirit with metaplanar shortcut. Darn hard to intercept!

I think that it's nearly impossible to do with magic; however with GM approval a technomancer MIGHT be able to us the same mechanics of using resonant metaplanar quests for covering datatrail to conceal the team comunications. However I point out that by RAW there's nothing that suggest the possibility of doing it so this would be an houserule.

I really don't understand what you are suggesting.

You want a spirit to carry the 128bit key to your transmissions back and forth? That only works if you consider the encryption tech in SR to be using the same tech we use today, but it is all abstracted out, so you can't make that assumption. Decryption is not dependent on the means used to encrypt a signal, so any sniffer still just needs to succeed at a decryption test to know what you data you are transmitting.

Basically most anyone you run against has the same or more resources than the PCs, so if the team devises a means to communicate 100% securely (short of designing an entirely new spell or tech), there is no reason some if not most of the corps/mobs/runner teams you encounter would not also use the same technique, so you can no longer listen in on them either.

By cannon (if you want, I'll look up page numbers when I'm not as sleep deprived) encryption keys are described as being anything the user wants, from a string of characters to a song. A spirit can definitely relay a short phrase, symbol, or whatever that serves as the key to decrypt the transmissions.

But yes, the moment you pull this rabbit out of your hat, you GM will have all your enemies doing it as well--that's how it goes with any strategy in SR.

Winternight used the spirits-as-secured-couriers thing prior to the System Failure campaign so there is in-game precedent.