iso verification: results - successful but still bewildered

To: Thomas Schmitt <scdbackup@gmx.net>
Cc: debian-user@lists.debian.org
Subject: iso verification: results - successful but still bewildered
From: Andrew F Comly 康大成 <andrew.comly@protonmail.com>
Date: Mon, 15 Aug 2016 01:01:54 -0400
Message-id: <[🔎] mPJugPyxJ3Pzi7QS5PJ15TFByxu9UikGR0DY4goJOsiJcBAPdqvM8DWIWpTHWG9TmJkj1nq8BtgU4DH0Mi3LVOL33_lQ4zpTWTEGsatYOYU=@protonmail.com>
Reply-to: Andrew F Comly 康大成 <andrew.comly@protonmail.com>

scdbackup@gmx.net, debian-user@lists.debian.org

Thomas,

Thank you for your previous assistance concerning the hacked iso issue. Below is my progress. The only remaining issues are found in Retort4-5 below:

Why this overwriting of partition 1 by zeros ?

The partition table will be overwritten by the ISO anyways.

Retort1-->Thanks for this info.

--------------------------------------------

dd if=/media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso of=/dev/sdb bs=1M

...

(For me it runs substantially faster if i add option bs=1M.)

Retort2-->I ran dd again:

[code]

$ dd if=/media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso of=/dev/sdb

dd: failed to open ‘/dev/sdb’: Permission denied

$ ll /dev/sdb

brw-rw----. 1 root disk 8, 16 8月 14 19:45 /dev/sdb

$ sudo chown a:adm /dev/sdb

$ ll /dev/sdb

brw-rw----. 1 a adm 8, 16 8月 14 19:45 /dev/sdb

$ ll /dev/sdb

brw-rw----. 1 a adm 8, 16 8月 14 19:45 /dev/sdb

$ dd if=/media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso of=/dev/sdb

$ dd if=/media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso of=/dev/sdb bs=1M

613+0 records in

613+0 records out

642777088 bytes (643 MB) copied, 105.551 s, 6.1 MB/s

[/code]

--------------------------------------------

Did the command last long enough to be credible ?

Did it report

1255424+0 records in

1255424+0 records out

Retort3--> Again, I burnt the iso to disk again (above and below):

[code]

$ dd if=/media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso of=/dev/sdb bs=1M

613+0 records in

613+0 records out

642777088 bytes (643 MB) copied, 105.551 s, 6.1 MB/s

[/code]

--------------------------------------------

When you write:

$ dd if=debian-8.4.0-amd64-netinst.iso bs=1 skip=32768 count=6 | od -c

...

0000000 001 C D 0 0 1

Your file yielded two blanks on this shell command

dd if="/dev/sdb" bs=2048 skip=16 count=1 |

dd bs=1 count=6 | od -x | head -1 | \

awk '{print $2 " " $3 " " $4}'

Wheras an ISO is supposed to yield (on little-endian machine):

4301 3044 3130

Retort4-->

Do you mean that you want me to try out these commands? Or is the program not usable on an average lubuntu 14.04 Samsung netbook like mine? Is there anything more I can do?

--------------------------------------------

As we learn from the copy-to-stick command, we need to curb to 1255424

blocks of dd''s default size (512 bytes per block). My USB stick is at

/dev/sdc:

$ dd if=/dev/sdc count=1255424 | sha256sum

Retort5--> May I assume that the number "1255424" is the records in/out integer? The first results below I used a count matching the burn of the iso to sdb:

[code]

$ dd if=/dev/sdb count=613 | sha256sum

613+0 records in

613+0 records out

313856 bytes (314 kB) copied, 0.0509401 s, 6.2 MB/s

943ad23096494bdff9a29f99368430285f8ecbe57da49db3d1fcf4be0800643d -

[/code]

The second results below I used a count matching the burn of the iso to sdb:

[code]

$ dd if=/dev/sdb count=1255424 | sha256sum

1255424+0 records in

1255424+0 records out

fd7729e3bdfcb2b1b8793f5a19096fb2eee3cf709c39112ed2da7e7a2d7f6886 -

642777088 bytes (643 MB) copied, 31.813 s, 20.2 MB/s

$ sha256sum /media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso

fd7729e3bdfcb2b1b8793f5a19096fb2eee3cf709c39112ed2da7e7a2d7f6886

[/code]

Quite incredible, with the 'magic' number of "1255424" and using sha256sum the "/dev/sdb" and the original iso MATCH!!! But not the above command using the "records in/out integer" of 613(M), which is coincidentally the size of the downloaded iso on the local disk. Where does the number 1255424 come from? I notice that 613 M = 613 000 000 and if you break this down into 512 K than 613M / 512K = 1197265.6 which is near the magic number, is this not important? At this point I still didn't know where the 'magic number' came from. At this time I thought that sha256sum wasn't my target checksum, so I checked both /dev/sdb and the original iso with sha512sum:

[code]

$ dd if=/dev/sdb count=1255424 | sha512sum && wait && sha512sum /media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso

1255424+0 records in

1255424+0 records out

642777088 bytes (643 MB) copied, 124.684 s, 5.2 MB/s

3365649694bf623d63f37853582d3fe0f7aa774821e2533d2dc79f5d763df1751e20da98da013ccc9bd3257159362434462bd7363caff3a590e75701b81e751c -

3365649694bf623d63f37853582d3fe0f7aa774821e2533d2dc79f5d763df1751e20da98da013ccc9bd3257159362434462bd7363caff3a590e75701b81e751c /media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-lxde-CD-1.iso

[/code]

It looks like the sha512sum of the /dev/sdb and the original iso MATCH ALSO!!!

But may I understand the 'magic number' better?

Sincerely,

Andrew

P.S. - Is there any forum (like linuxquestions.org) that I could post this on? I am new to debian and doubt the email thing is as quick as the email mailing list.

-------- Original Message --------
Subject: Re: iso hacked suspicious: answers
Local Time: August 10, 2016 5:23 PM
UTC Time: August 10, 2016 5:23 PM
From: scdbackup@gmx.net
To: debian-user@lists.debian.org

Hi,

Andrew F Comly wrote:
> E) Burn iso to media
> dd if=/dev/zero of=/dev/sdb1 bs=1M;
> wait;

Why this overwriting of partition 1 by zeros ?
The partition table will be overwritten by the ISO anyways.

(And on what asynchronous processes are you waiting to end ?)

> ddif=/media/LG/AC/bckup/Install/2_Bootable-OS/Debian/i386/debian-8.5.0-i386-l
xde-CD-1.iso of=/dev/sdb

Blanks matter. :))

Assumed that you did put a blank between "dd" and "/media", the command
would be ok for copying the ISO onto a USB stick at /dev/sdb.
(For me it runs substantially faster if i add option bs=1M.)

Did the command last long enough to be credible ?
Did it report
1255424+0 records in
1255424+0 records out
?

> sudo sha256sum /dev/sdb

As we learn from the copy-to-stick command, we need to curb to 1255424
blocks of dd's default size (512 bytes per block). My USB stick is at
/dev/sdc:

$ dd if=/dev/sdc count=1255424 | sha256sum
...
fd7729e3bdfcb2b1b8793f5a19096fb2eee3cf709c39112ed2da7e7a2d7f6886 -

$ fgrep fd7729e3bdfcb2b1b8793f5a19096fb2eee3cf709c39112ed2da7e7a2d7f6886 SHA256SUMS
fd7729e3bdfcb2b1b8793f5a19096fb2eee3cf709c39112ed2da7e7a2d7f6886 debian-8.5.0-i386-lxde-CD-1.iso

Testing with ./check_debian_iso (which counts blocks of 2048 bytes each):

$ ./check_debian_iso SHA512SUMS debian-8.5.0-i386-lxde-CD-1.iso /dev/sdc
Piping 313856 blocks of '/dev/sdc' through 'sha256sum'
to verify checksum list item 'debian-8.5.0-i386-lxde-CD-1.iso'.
313856+0 records in
313856+0 records out
642777088 bytes (643 MB) copied, 31.7854 s, 20.2 MB/s
Ok: '/dev/sdc' matches 'debian-8.5.0-i386-lxde-CD-1.iso' in 'SHA256SUMS'

Have a nice day :)

Thomas

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