I'm having difficulty finding documentation for the over-the-air protocols used by encrypted public safety radios. It seems that most systems are based on Project 25, previously known as APCO Project 25, now simply called P25, and that this encryption standard is based on AES-256.

What's not clear to me:

  1. Is P25 the state-of-the-art, it it an aspirational system that hasn't been deployed yet, or is it legacy?
  2. Is everything done with hard-coded symmetric keys, or is there public key to exchange the AES keys?

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