Re: [Bug-tar] Add an option to tar, --use-encrypt-program

[Matt Olson]

Hi Paul,

Some have hacked use of tar to allow encryption by calling a wrapper script for openssl using --use-compress-program option.   Example:

/bin/tar -c -v --use-compress-program /usr/local/bin/do_crypt.sh -f /dev/nst0 /data

This works fine.  The problem is when using tape spanning (-M / --multi-volume). 

/bin/tar -c -v --use-compress-program -M --tape-length=1006632960 /usr/local/bin/do_crypt.sh -f /dev/nst0 -f /dev/nst1 /data

Results in error: 
/bin/tar: Cannot use multi-volume compressed archives

This makes perfect sense.  When using compression, it is unknowable in advance at what point in tarring the source data "tape-length" will be reached.  In other words, it is not possible to predict what the compression ratio will be in advance. 

Hence this tape spanning scenario creates a problem when attempting to use an encryption program instead of a compression program. 

Proposed solution is to create an option like --use-compress-program which streams the data through an external program, yet allows use of -M option. 

Does that make sense? 

Since my last e-mail, I pulled down the source for tar and started looking through it.  It's not quite as straight forward as I had assumed since --use-compress-program is essentially an override of -z (gzip), -j (bzip2), etc.  I was hoping to re-use the --use-compress-program code and just remove the -M check. 

--Matt

--

Matt Olson
Director of Platform Services

KAVI CORPORATION
503.813.9383
www.kavi.com

On 01/19/2011 11:54 PM, Paul Eggert wrote:

Thanks for the suggestion.  Could you explain that exception in
a bit more detail?  Also, what should happen if --use-encrypt-program
and --use-compress-program are both specified?