random access into an encrypted file?

[Eric Hughes <hughes@soda.berkeley.edu>]

>can the methods recently proposed here work
>for fast "random" access of bytes from the middle of a possibly-large
>file?

The model that has been most discussed recently has been that of
encrypting sectors on the hard disk.  In order to have random access
to files, you have to have random access to sectors.  Therefore, the
encryption mechanism chosen must support random sector access.

This is not difficult, but many of the techniques used for
telecommunication encryption do not work.  In particular, encryption
modes that depend upon some previous state of the encryption machine
do not work well.

Cipher block chaining is a mode of operation for block ciphers that
where the plaintext is xor'd with the previous block of ciphertext
before encryption.  The first block of plaintext, where there is no
previous block, is xor'd with an initial vector, which may be
considered part of the keying material.  Now consider what would
happen if you encrypted your whole disk in CBC mode.  You'd have to
start at the beginning of the disk and decrypt up to the point that
you want to read.  For a bit stream, this is fine, since one is
decrypting the whole thing.

CBC, however, is useful for doing sector encryption.  A DES block is 8
bytes, a sector is typically 512.  I assume here that one has to read
the whole sector out of memory, although with some very clever and not
obviously worthwhile optimizations one could decrypt on demand.  Now
CBC is a reasonable choice ifor in-sector encryption, because you have
to read the whole thing anyway.

Yet CBC requiress an initial vector.  This is where counter mode come
in.  A good block cipher has what is called the avalanche property,
which says that altering any bit of the input alters on average half
of the bits of the output.  (Note: if it altered more than half, the
1's-complement would change by less than half.)  Thus the initial
vectors do not need to change particularly much from one initial
vector to the next.  Hence an integer-valued counter works fine.  For
hard disks the sector number, already present, makes just such a
unique initial vector.

Summary: CBC within sectors, initial vectors provided by the sector
number.

This characterization of keying material works for block ciphers
generally and yields a clean abstraction for the rest of the system.

	algorithm identifier (index or function pointer or link spec)
	plaintext/ciphertext block length
	key length

The rest of the encryption code need only know these values.  Here are
some examples.  Lengths are byte lengths.

	single DES, 8, 8 (64 bits, of which only 56 are used)
	double DES, 8, 16
	triple DES, 8, 24
	IDEA, 8, 16

Nice and clean.

Eric