Compression method detection

Good morning!

1. Is there any software which tries to determine which compression
method is used in some file? Such utility can contains a lot of
unpacking libraries (arj,zip,gzip,lzo,compress,rar,etc) and try to use
various unpacking functions with the different file offsets.. It can
also analyze "randomness" of the file to guess which compression
method is used and so on..

2. Does the world need such utility?

Best regards,
Vadim.

vk@tvcom.ru (Vadim Kolontsov) wrote in message news:<f09e4ffd.0411100153.1cc29ce5@posting.g
oogle.com>...
> Good morning!
>
>   1. Is there any software which tries to determine which compression
> method is used in some file? Such utility can contains a lot of
> unpacking libraries (arj,zip,gzip,lzo,compress,rar,etc) and try to use
> various unpacking functions with the different file offsets.. It can
> also analyze "randomness" of the file to guess which compression
> method is used and so on..
>
>   2. Does the world need such utility?

No, and no, IMO.  The reasons are 1. many "rippers" already exist that
can detect and extract archive formats; 2. it is possible with some
routines to "decompress" any file -- you get gibberish of course, but
no error is generated; 3. analyzing "randomness" to determine the
archive format and/or compression method is pretty much impossible
without knowing the characteristics of the source material beforehand.

trixter@despammed.com (Jim Leonard) wrote in message news:<f93aad69.0411101152.7fd095af@pos
ting.google.com>...
> No, and no, IMO.  The reasons are 1. many "rippers" already exist that
> can detect and extract archive formats; 2. it is possible with some
> routines to "decompress" any file -- you get gibberish of course, but
> no error is generated;

1. As far as I know, such rippers tries to find specific signatures of
archivers (headers and so on). They can't find just compressed stream
with unknown header. Am I wrong?

2. a) Well, it seems that such situtation is possible, but rare
enough. "Wrong" stream usually brokes decompression routine.
b) Human [almost] always can determine if decompressed output is a
garbage or not.

Best regards,
Vadim.

vk@tvcom.ru (Vadim Kolontsov) wrote in message news:<f09e4ffd.0411110248.74a0e245@posting.g
oogle.com>...
> trixter@despammed.com (Jim Leonard) wrote in message news:<f93aad69.041110
1152.7fd095af@posting.google.com>... 
>
> 1. As far as I know, such rippers tries to find specific signatures of
> archivers (headers and so on). They can't find just compressed stream
> with unknown header. Am I wrong?

You are not wrong, but usually the entire archive structure is
necessary for proper decompression of the files.  For example, in a
"solid" stream (like the kind RAR produces), how would you know if you
were finished with one file and beginning another?

> 2. a) Well, it seems that such situtation is possible, but rare
> enough. "Wrong" stream usually brokes decompression routine.

It depends on the decompression routine.

The main point is that you would need something to compare
decompressed output against in order to determine if you had the right
method, and because you won't know that beforehand, it would be pretty
pointless to try.  Just my opinion, you're welcome to do whatever you
want :-)