Experimental Physics and
Industrial Control System

[email protected] (Alan K Biocca) · Tue, 14 Feb 95 14:13:20 PST

Tony Cox writes:

> PGP has a more secure method for doing what we want. You encrypt the file for
> multiple users like this:-
> 
> 	pgp -e R3.12.0.Tar Bob Chip Tony Alan Janet Uncle-Tom-Cobbly ...

First, a few notes here for collaborators not 'up' on this pgp-stuff:
Pgp -e uses public key encryption for a list of recipients.  It actually
encrypts the message using private-key (IDEA algorithm) based encryption,
and then it uses public-key encryption to encrypt the random private key that
it selected for this particular message.  Thus any intended recipient can
decrypt using his/her private key.  Pgp automates all this detail so it
is easy to use.

Using public key encryption is not exactly 'more secure', but different
in the sense of key management.

What Tony is proposing is a good way of handling the problem as well, let's
take a look at the mechanics of each approach:

Private Key System:

  Distributor:

    initial preparation:

    1.  setup ftp-only account with password
    2.  get and install pgp
    3.  tar up the release
    4.  choose passphrase and encrypt
    5.  copy into account
    6.  notify & reveal account/passwd/file/key to recipients (if changed)

    for each new customer:

    1.  verify customer, reveal account/passwd/file/key

  Customer:

    initial prep:

    1.  obtain and install pgp

    for each release:

    1.  convince distributor to give access information (if changed)
    2.  ftp encrypted file
    3.  decrypt

Public Key System:

  Distributor:

    initial preparation:

    1.  setup anon ftp account & directory
    2.  get and install pgp
    3.  tar up the release
    4.  encrypt with current list of recipients
    5.  copy into anon ftp directory
    6.  notifiy recipients

    for each new customer:

    1.  get & verify customer's public key
    2.  add to keyring
    3.  add customer's key id to encryption script
    4.  re-encrypt tarfile (keep copy of unencrypted tarfile or decrypt/encrypt)
    5.  notify customer

  Customer:

    initial preparation:

    1.  obtain and install pgp
    2.  generate a public key - private key pair
    3.  convey public key to distributor
    4.  convince distributor that it is the correct customer's key
    5.  wait for re-encryption

    for each release:

    1.  access anon ftp and obtain encrypted file
    2.  decrypt

Summary:

Effort of public-key encryption is greater at startup for both distributor
and customer but new-key distribution is eliminated.  The hard part in either
case is key management - in one case giving the correct parties the key, in
the other accepting public keys from the correct parties.  The difficulty of
doing these are essentially equivalent.  Even though public keys can be
emailed or posted without loss of security the verification process still 
requires the same kind of secure channel and confidence that passing the
private keys requires.

I like Tony's public-key solution but it appears to be somewhat more work.
The private key system minimizes the distributor's effort for each new 
customer.

The final choice should be by the distributor.

Alan K Biocca

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System