Secure voice software issues

[hfinney at shell.portal.com]

A couple of comments on the cryptophone idea.

First, there has seemed to be general agreement in our earlier discussions
of this concept that the hard part is compressing the voice to the point
where it can go over commonly-available modems.  The government-standard
CELP algorithm is too slow for general-purpose home computers.  You need
an algorithm that can operate in real time and compress intelligibly down
to about 13K bits per second.  It has to be either able to compress and
decompress simultaneously or else you need some switching logic to decide
which person is talking and which is listening at each moment, with both
sides reversing roles in synchrony.

Second, Diffie-Hellman key exchange will probably take about as long as
an RSA decryption with similar modulus sizes.  So speed would not seem to
be a reason to choose DH over RSA for key exchange.  If PGP is slow on
your machine, DH will be, too.

Hal Finney
hfinney at shell.portal.com