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From: kent@BBN.COM (Steve Kent)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Authenticated SMTP, anyone done one?
Message-ID: <9106180335.AA16085@ucbvax.Berkeley.EDU>
Date: 17 Jun 91 19:00:34 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 52

Larry,

	A few comments in response to our recent message about PEM and
what facilities it provides:

	- PEM operates in conjunction with SMTP and RFC 822 mail.  It
is compatible with both and thus can be used to protect any RFC 822
message that can be sent via SMTP mail relays.  Use of PEM is strictly
voluntary; only Internet users who want the security services of PEM
are expected to make use of it and, as you observed, it takes two to
tango.

	- Your description of PEM security services and mechanisms was
a bit short and not quite accurate.  PEM uses DES, not RSA, to encrypt
messages, but encryption is optional.  PEM uses computes a message
integrity check (MIC) on each message and encrypts it to provide
message integrity and authenticity.  The integrity and authentication
functions are not optional, i.e., they are always employed.  The
version of PEM that we expect to be widely used employs RSA keys both
to encrypt the MIC (for authenticity, integrity and a basis for
non-repudiation) and to encrypt the DES key in support of
confidentiality (secrecy).  A key management infrastructure, based on
the CCITT/ISO Directory authentication standards (X.509) is used by
PEM to support distribution of the RSA keys.

	- A sender of a PEM-protected message can ensure that the
message content is decipherable only by the intended recipients and
that each recipient can verify the message inetgrity and the sender
ID.  PEM does not address directly more subtle confidentiality issues,
e.g., traffic analysis, though "double-enveloping" can be employed to
mitigate this threat as well.

	I hope this clarifies the issue of "what PEM does."  As for
timing, those of use working on the PEM standards are behind schedule,
as noted.  Three RFCs were issued almost 2 years ago and we are now
trying to get revised standards out this summer.  Several independent
implementations of PEM are now undergoing testing on the Internet and
arrangements for the key management infrastructure are progressing.
The latter work includes a numer of activities, e.g., license
agreements with RSA to enable a PEM freeware implementation to be made
available and interoperable with product-level PEM implementations in
the Internet.  The bottom line on RSA licenses for PEM is that they
are expected to be quite cheap, as little $2.50 per-user (maybe even
less for students), if the keys are locally managed (by the school,
company, or whatever organization with which the user is affiliated).

	I recommed you review the existing RFCs (1113-1115), or the
Internet-Drafts which will replace them, for more info on PEM.

Steve Kent
Chair
Internet Privacy and Security Research Group