From: Alexander Kjeldaas <astor@guardian.no>
Date: Sun, 13 Dec 1998 15:12:18 -0800 (PST)
To: cypherpunks@toad.com
Subject: cfs' ecb+ofb mode vs. interleaved cbc mode
Message-ID: <19981214001208.B25332@lucifer.guardian.no>
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For fast file-system crypto, you really want to implement the ciphers
so that you encrypt several blocks at a time.  CFS does this by using
a special mode (described as ecb+ofb by schneier).  The CFS
documentation explains:


"At attach time, CFS derives from the passphrase into two DES keys, K1
and K2.  K1 is used to create two (or three) DES pseudorandom stream
ciphers, each 128KB long, S1 and S2.  To encrypt a file block, it is
first XORd against a unique bitstream derived from the inode number of
the file.  The result is then XORd against S1 based on its offset in
the file.  This is then DES ECB encrypted with K2, and the result of
that is XORd against the appropriate position in S2.  The resulting
ciphertext is what is stored.  The cipher is reversed in the obvious
manner.  Filenames are similarly encrypted.

There does not appear to be a feasible attack that allows an
independent search for the two subkeys K1 and K2; in a brute-force
known-plaintext attack, an attacker would have to try all 2**112 key
combinations.  Note that in the single DES mode, the two keys may be
vulnerable to independent exhaustive search under a so-called "linear"
attack, but this attack appears to require a large number of chosen
plaintexts encrypted under the same inode number.  Under most
conditions where the attacker cannot introduce large numbers of chosen
plaintexts, I believe even the single-DES CFS encryption to be very
strong in practice.  Note that this is not the same as a "proof".  In
any event, CFS is always at least as secure as DES or triple DES (as
selected when the directory is created).  If want high security,
select triple DES (now the default); for better performance, use the
hybrid single-DES option."


I'd like to know how this method compares to using an interleaved cbc
mode - for example 8-way interleaved cbc.  The n-way interlaved
cbc-mode works by chaining each n'th block together instead of each
block.  This means you get n more messages.  For a 512-byte block and
a 128-bit block length, this means that each message will be
512/(8*16) = 4 blocks long.  For a 4k block you get messages that are
32 blocks long.  This might or might not be a problem.

With CFS you need 2x the key material - something that is really hard
to get by when you're using 256-bit keys.  It seems to me you'll have
to type in an extremely long password or each of the keys will in
practice be weaker than what the theory tells you.  Another point is
that CFS requires a lot more nonswappable memory than interleaved cbc
mode. This means that interleaved cbc mode initially seems more
attractive for file-system use.  Am I missing something?

astor

-- 
 Alexander Kjeldaas, Guardian Networks AS, Trondheim, Norway
 http://www.guardian.no/