pre-encrypted files

Of all the changes presented in the Microsoft STLP proposal, I found
the support for pre-encrypted files to be the most useful.
Pre-encrypted files were considered for SSL 3.0, but we felt that there
was not time to finish the design without holding back the rest of the

There are two main reasons why a special mechanism for dealing with
pre-encrypted files is interesting. One is to keep plain-text versions
of the file off the disk of the distributing site. This has the
advantage of protecting the file from those that manage to get
unauthorized read access to the contents.

The second reason for special-casing pre-encrypted files is that there
may be a performance advantage for the distributor of the file if the
identical contents needs to be made available to multiple recipients.
It is this latter reason that seems to motivate the desire to include
this in the TLS proposals.

There are several issues to keep in mind while folding this support
into the existing SSL specification. None of them are compelling
reasons not to do the work.

The record layering model will end up being reflected all the way up to
how the pre-encrypted file is stored on the disk.  SSL records are
limited to 16K bytes and each fragment is separately MACed.

The STLP proposal suggests a particularly weak MAC algorithm merely to
allow the MACs to be (mostly) precomputed for the pre-encrypted files.
Whatever protocol this group ends up with will be used to support more
protocols than merely HTTP. The correct MAC algorithm to use is HMAC
which will soon (if not already) be an RFC. The (potentially) weaker
integrity requirements for some files (say images) that are broadcast
many times, does not allow us to weaken that integrity check for all
data for all higher level protocols.

If I have time, I'll say more about MACs in a subsequent message.

The STLP proposal had the new encryption key arriving in the
ChangeCipherSpec message. This is a mistake. The ChangeCipherSpec
message is NOT a handshake message; it sits directly on top of the
record layer so that there is explicit separation of the different
sub-protocols. It also means that there can be a number of changes
staged for triggering at a single point.

The STLP proposal also used the security of the current stream to
protect the new encryption key. This leads to problems if the stream is
currently at a less secure mode (null encryption or 40-bit encryption)
than the required security of the new data. You do have to be careful
not to increase the entropy of a current key above that allowed by
U.S.  export laws. (This is extremely unfortunate for an international
protocol, but that is the reality of the situation.)

The STLP proposal includes a stacking method to recover an earlier key
value. There is no indication of the stack limit that implementations
must support.

A more straightforward approach would be to add another SSL handshake
message: set write key. Since the two ends already share a 128-bit
master key, the originating end could use a new salt together with the
current master key to compute a hash and an XOR offset from the desired
new encryption key. This could be used by either end and would take
effect when the next ChangeCipherSpec message was sent. It has the
advantage of not leaking a subsequent key when the previous was
compromised. (Keeping the appropriate (according to U.S. export rules)
amount of entropy in the new key is left as an exercise to the reader.)

It might be tempting to try and transmit a constant (for a file across
all users) MAC key using the same method used for the encryption key,
but this would be a mistake.  An earlier recipient of the file would
then have enough information to successfully pull off a MITM attack
against a later recipient.

There may be a need to be able to set the compression method as well as
the encryption key.  You could either assume that the compression
method will not change for a particular stream (meaning that the server
may have to keep multiple copies of the pre-encrypted file ready) or
you could introduce another handshake message that would allow the
server to choose among the compression methods originally proposed by
the browser.

Philip L. Karlton
Principal Curmudgeon
Netscape Communications

     They that can give up essential liberty to obtain a little
     temporary safety deserve neither liberty nor safety.
		- Benjamin Franklin

Received on Tuesday, 28 May 1996 05:59:47 UTC