GSM crypto upgrade? (was Re: Newt's phone calls)

[Adam Back <aba@dcs.ex.ac.uk>]

Paul Bradley <paul@fatmans.demon.co.uk> writes:
> Adam Back <aba@dcs.ex.ac.uk> writes:
> > encryption system.  Anyone know how modular the design is, for instance if
> > it would be possible to give a GSM A5 based cell phone a crypto upgrade
> > using published electrical interface standards?  (I want one of those -
> > Nokia phone with IDEA + 2048 bit RSA signatures + DH forward secrecy!)
> 
> My guess is that this would not work.
> 
> Does anyone know if when you use a GSM phone to call a landline 
> number the cellphone<-> base station trafic is encrypted???
> 
> my guess is that only when you call GSM to GSM is the trafic 
> encrypted and even then I would imagine each phone agrees a key with 
> the base station for the network then the trafic between the base 
> stations is cleartext. The only way, if this were the case, would be 
> to write the code so that the headers and other network information 
> like SIM ID number etc... were cleartext or just A5 to the network as 
> standard and only the actual speech data was encrypted under 
> something stronger. This approach could become troublesome, 
> if I have time I`ll get hold of some GSM specifications and look at 
> it more closely.

All you've got to do is super-encrypt the IDEA encrypted traffic with
the standard A5 hardware - the base station won't notice the
difference.

Schematically, standard GSM hardware:

                       +-------------+
      +-----------+    | compress/   |        +------------+
  <-->| A/D & D/A |<-->| decompress  |<------>| A5 enc/dec |<-->
      +-----------+    +-------------+        +------------+


schematically, adding a super-encryption layer:

                       +-------------+
      +-----------+    | compress/   |        +------------+
  <-->| A/D & D/A |<-->| decompress  |<-|  |->| A5 enc/dec |<-->
      +-----------+    +-------------+  |  |  +------------+
                                        v  v
                                  +-------------+
                                  | IDEA/RSA/DH |
                                  +-------------+


So the question was (addressed to anyone who knows anything about the
electrical interfaces used in GSM) about standardisation of electrical
interfaces -- for instace if the electrical interface between the
voice compression/decompression IC and A5 IC were standardised, you
could build a replacement voice codec IC which performed IDEA/RSA/DH
as well as standard the voice codec function, and had the same pin
out.  This IC would allow a wide range of GSM phones to be upgraded
with minimal effort on the part of GSM phone manufacturers -- or even
desoldered and replaced by end users, or crypto hardware company.

However, there are several reasons why it would probably require
proper integration into a GSM phone design:

- keys tied to phone number memories
- display of signature result on screen
- PIN for phone's RSA signature keys
- face to face key exchange
- key revocation
- generating new keys

Adam
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