Adding encryption to a ham radio 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?
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.