random access into an encrypted file?

[zimm@alumni.cco.caltech.edu (Mark Edward Zimmerman)]

I'm enjoying the discussion of encrypting file systems, but have a
perhaps-naive question: can the methods recently proposed here work
for fast "random" access of bytes from the middle of a possibly-large
file?

Specifically, over the years I have written some free-text
information-retrieval programs which build complete inverted indices
to every word in a chosen text file (which may be many megabytes long,
limited by disk space, not by RAM) --- and in order to fetch and
display text quickly from an arbitrary point in the file, my programs
do a lot of fseek() operations.  If a file is encrypted under various
schemes, I wonder how long it would take to fetch byte 100,000,000?
Could it cause me some performance problems?  :-)

Just thought I'd raise the issue....  BTW, if anybody wants to work
with large text files, the stuff I've done is all free under GNU GPL;
for nicest user interface, see Mac version which hides behind
HyperCard (in INFO-MAC archive at sumex-aim.stanford.edu, under
directory info-mac/card with a name beginning "freetext", I think).
Generic command-line C code to build indices is "qndxr.c" in various
archives, and the generic command-line browser is "brwsr.c".  See
description in THE DIGITAL WORD, eds. Landow & Delany, MIT Press,
1993, pps. 53-68, for more details.  Briefly, the programs let you
scroll around in alphabetized word lists, generate key-word-in-context
displays and do simple proximity filtering, and retrieve chunks of
text on demand, very fast.  Index-building is 15-20 MB/hour on an
older Mac II-class machine, 60-80 MB/hour on a Sparcstation, etc.

Best,  ^z  (no relation!)