[asterisk-dev] GSM Encryption

[Benny Amorsen]

sab sab <sabin_098765 at yahoo.com> writes:

> Thank you for your reply, but I don't want to use a smart phone to do this
> or the data connection of the phone. Basically this is were I want to get:
> wifi network ->wifi adapter raspberry pi -> asterisk raspberry pi
> ("routing")-> ENCRYPTION(personal flavour) ->(chan_mobile)bluetooth
> adapter raspberry pi->phone with bluetooth ->voice connection
> Can this be done?

It doesn't look like you have much luck getting people to understand
your topology :)

If I understand you correctly, you have that setup at both ends of the
connection, right? And you want to encrypt the voice going through the
GSM/whatever network.

This is possible, but very, very tricky. First of all, if you want
encryption you generally cannot handle codec conversions. You need to
send the exact bitstream to the phone that you then expect to see at the
other end of the network, so you need to send in exactly the codec used
by the GSM network. This codec can change during a call, and you have no
way to detect that (other than your decryption suddenly failing). You
also have to handle dropped and malformed packets, which is almost
impossible.

I do not believe it is a viable strategy. You could perhaps invent a
2400bps modem protocol which would sometimes work over voice-GSM, but
the end result would sound awful.

Ten years ago you could just make a HSCD ("modem") call instead and get
57.6kbps through (potentially at the cost of 4 simultaneous calls, but
usually less). You may be able to find a provider and phone which still
supports that. It gives you a bog-standard serial connection so you can
even do PPP and just use SRTP.


/Benny