Private vs Public keys and uploading

In a message in the 'Revised IGC Approvals...' thread
Paul Remde wrote:

But essentially I believe it is a typical private
key
system which relies on only trusted parties knowing

the secret key. These trusted parties include
anyone writing software to upload flights (which
I
suspect is the reason they would never give me
the specs for writing mac software),

Use of a private key system does not require that the
upload protocol be secret. The EW loggers use private
keys but their protocols are published. Only the actual
method of computing the security code to go in the
IGC file G record is secret.

Anybody can write an uploader for the EW loggers -
all they have to know is that the security code is
an eight byte value. Only somebody who wanted to write
a version of VALI-EWA for another system would have
to know the 'secret'.

Ed Davies.