Flac - Apr 2011 - Checking file integrity

Hello all,

I  have  read FLAC stores a hashsum inside its files
to check their integrity. But how to do  it  without
having  to  extract  the file? Is the hashsum calcu-
lated on the extracted or compressed data?

Thanks in advance,
Anton

Anton Shepelev wrote:
> I  have  read FLAC stores a hashsum inside its files
> to check their integrity. But how to do  it  without
> having  to  extract  the file? Is the hashsum calcu-
> lated on the extracted or compressed data?
I'm not sure what you're asking for, but the hash is computed on the raw
sample data.  Raw samples are compressed inside the FLAC file.

To extract the computed hash (MD5):

   metaflac --show-md5sum audio.flac

To extract the raw sample data and do the MD5 hash computation another 
way you can uncompress the FLAC file, convert it to raw with a program 
like sox(1), and recompute the MD5 hash:

   flac --decode audio.flac  (this creates audio.wav)
   sox audio.wav audio.raw   (this creates audio.raw)
   md5sum audio.raw          (this should match the hash above)

For example:

$ metaflac --show-md5sum audio.flac
7853aca9317d3b348b3aad5219fa63c9
$ flac --decode --silent audio.flac
$ sox audio.wav audio.raw
$ md5sum audio.raw
7853aca9317d3b348b3aad5219fa63c9  audio.raw

I hope this helps.

Anton Shepelev wrote:
> I  have  read FLAC stores a hashsum inside its files
> to check their integrity. But how to do  it  without
> having  to  extract  the file? Is the hashsum calcu-
> lated on the extracted or compressed data?
I should have mentioned before that apparently the MD5 hash is computed 
on the uncompressed raw sample data and the easiest way to check the 
FLAC file is with

   flac --verify audio.flac

--verify won't extract the data to a file, but it does seem to run over 
the entire file (as one would expect of such a function).

J.B. Nicholson-Owens:
> I'm not sure what you're asking for
I  have  a huge archive of FLAC files and want auto-
matically to check the integrity thereof, so  as  if
some  file be reported as corrupted I can restore it
from a mirror backup.
> I should have mentioned before that apparently the
> MD5  hash is computed on the uncompressed raw sam-
> ple data and the easiest way  to  check  the  FLAC
> file is with
>
>        flac --verify audio.flac
>
This  doesn't work, and the official FLAC focumenta-
tion says this option  is  only  applicable  to  WAV
files:

    -V, --verify
        Verify  the  encoding process. With this
        option,  flac  will  create  a  parallel
        decoder  that  decodes the output of the
        encoder and compares the result  against
        the  original. It will abort immediately
        with an error if a mismatch  occurs.  -V
        increases the total encoding time but is
        guaranteed to catch any unforseen bug in
        the encoding process.

It seems to have nothing to do with the MD5 hash.

The  procedure  you  have described in your previous
reply is quite complicated for an  automatic  check-
ing.  Why  does FLAC calculate MD5 on the RAW uncom-
pressed data?  If  it  were  using  compressed  data
instead  the checking wouldn't require decompression
and would be quicker: just calculate the hash on the
binary  file  and  compare  it  against  the  stored
value...

Anton