[902] in Cypherpunks
random access into an encrypted file?
daemon@ATHENA.MIT.EDU (Mark Edward Zimmerman)
Sun Jun 6 08:14:49 1993
From: zimm@alumni.cco.caltech.edu (Mark Edward Zimmerman)
Date: Sun, 6 Jun 93 04:50:30 PDT
To: cypherpunks@toad.com
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!)
