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Suppose I want to encrypt the voice data before sending using a ham radio, for example this unit.

Assume the encryption is simply AES encryption/decryption using predefined fixed key.

I am thinking of the following implementation:

1) Between the radio's transmitter/receiver and its logic unit, I'll place my circuit. 2) Low pass filter the signal then ADC it (with 10-bit accuracy, but this is irrelevant) with sampling frequency identical to that used by the radio. Ideally, this rate will already be higher than the max. freq. so as to avoid aliasing. 3) Apply the AES algorithm to each 128-bit segment of data. 4) DAC.

Going forward, I would implement some digital signature methods, thus signing the hash of the data, every 100 128-bit segment (or any other number). This will add 512-bit of data every 100*128 bits.

My question: 1) Is there any comment on the implementation, from a signal processing / communication point of view? 2) The added 512-bit signature will decrease the bit rate by (12800-512)/12800 => 4.16%. Is this a problem?

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Digital audio data that is transmitted wirelessly is usually encoded in some efficient format. It sounds like you're talking about broadcasting raw PCM samples that are encrypted before transmission. That's certainly unusual.

Unless you are already following some kind of standard that I'm not aware of, you're probably best suited by packing your audio data into the payload some higher-level protocol that includes synchronization data to recover framing, the encryption that you desire, and most likely error correction. Your approach of just encrypting raw PCM samples with no error correction has the unpleasant side effect that a single bit error in the encrypted data will turn your entire block of samples into garbage.

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  • $\begingroup$ I see your point. But how can I know what kind of encoding is being used by these units? hint: units of different vendors are inter operable, meaning that there is something standard... $\endgroup$ – student1 Jul 20 '14 at 17:20
  • $\begingroup$ Consult their specifications? Contact the vendor? $\endgroup$ – Jason R Jul 20 '14 at 17:57
  • $\begingroup$ Nothing is mentioned in the specifications. But I will try that. $\endgroup$ – student1 Jul 20 '14 at 18:01
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  1. The radio you linked to is strictly analog, at least as far as I can tell. The linked page mentions no digital capability of any kind, but references a couple of analog signalling methods.

  2. Encryption is not allowed on amateur radio systems. site: ARRL, the national association for Amateur Radio

If you ignore (2,) you still have to deal with (1.) You are going to have to make something that amounts to the codecs used on old analog modems. You will have to find a way to reduce the data rate of the digital data to something that you can push through a (very) narrow bandwidth. Your original scheme also doesn't allow for any way for your receiver to sychronize itself with the data stream. The info on this amateuer radio site should give you some idea of what you'll have to deal with. Note, that though they do digital transmission of data, they do NOT encrypt it.

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