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Updated description of libisofs checksum processing

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and added it to release tarball.
This commit is contained in:
Thomas Schmitt 2009-08-30 15:55:36 +02:00
parent d9a11a3b8d
commit 9cfa55345e
2 changed files with 30 additions and 18 deletions
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1
Makefile.am
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@ -261,6 +261,7 @@ EXTRA_DIST = \
doc/susp_aaip_2_0.txt \
doc/susp_aaip_isofs_names.txt \
doc/zisofs_format.txt \
doc/checksums.txt \
libisofs/aaip-os-dummy.c \
libisofs/aaip-os-linux.c \
libisofs/aaip-os-freebsd.c

47
doc/checksums.txt
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@ -3,7 +3,7 @@
by Thomas Schmitt - mailto:scdbackup@gmx.net
Libburnia project - mailto:libburn-hackers@pykix.org
16 Aug 2009
26 Aug 2009
MD5 is a 128 bit message digest with a very low probability to be the same for
@ -67,7 +67,7 @@ The tree checksum tag is written after the ECMA-119 directory entries.
The session checksum tag is written after all payload including the checksum
array. (Then follows eventual padding.)
The tags are a single lines of printable text, padded by 0 bytes. They have
The tags are single lines of printable text, padded by 0 bytes. They have
the following format:
Tag_id pos=# range_start=# range_size=# [session_start|next=#] md5=# self=#\n
@ -81,14 +81,13 @@ Tag_id distinguishes the following tag types
A relocated superblock may appear at LBA 0 of an image which was produced for
being stored in a disk file or on overwriteable media (e.g. DVD+R, BD-RE).
Typically there is a first session recorded with a superblock at LBA 32 and
the next session may follow shortly after its session tag. (There may be a gap
of padding, often 150 blocks, and aligning to the next address that is
divisible by 32.) Normally no session starts after the address given by
parameter session_start=.
the next session may follow shortly after its session tag. (Typically at the
next block address which is divisible by 32.) Normally no session starts after
the address given by parameter session_start=.
Session oriented media like CD-R[W], DVD+R, BD-R will have no relocated
superblock but rather bear a table-of-content on media level (to be inquired
by MMC commands(.
by MMC commands).
Example:
@ -183,11 +182,11 @@ whether its MD5 matches the MD5 of the data blocks which were read before.
With tag type 2:
Keep the original MD5 context of the data blocks and clone
one for obtaining the MD5 bytes.
If the MD5s match, then compute the checksum block into the kept MD5 context
and go on with reading and computing for the tree checksum tag. This will be
found at block address next_tag, verified and parsed by:
Keep the original MD5 context of the data blocks and clone one for obtaining
the MD5 bytes.
If the MD5s match, then compute the checksum block and all folowing ones into
the kept MD5 context and go on with reading and computing for the tree checksum
tag. This will be found at block address next_tag, verified and parsed by:
iso_util_decode_md5_tag(block, &tag_type, &pos,
&range_start, &range_size, &next_tag, md5, 3);
@ -200,14 +199,12 @@ With tag type 4:
End the MD5 context and start a new context for the session which you will
read next.
You may look for the first session by starting to read at LBA 32, or you may
look for the last session by starting to read at the address given by parameter
session_start=. The former is suitable for a check of the whole image, the
latter is the shortest way to ensure that the tree of the last session is
not corrupted.
Then look for the actual session by starting to read at the address given by
parameter session_start= which is returned by iso_util_decode_md5_tag() as
next_tag. Go on by looking for tag type 2 and follow above prescription.
Checking a Whole Session
Checking the Data Part of the Session
In order to check the trustworthyness of a whole session, continue reading
and checksumming after the tree was verified.
@ -224,6 +221,20 @@ bytes in parameter md5 match the MD5 computed from the data blocks which were
read before the tag block.
Checking All Sessions
If the media is sequentially recordable, obtain a table of content and check
the first track of each session as prescribed above in Checking Before Image
Tree Loading and in Checking the Data Part of the Session.
With disk files or overwriteable media, look for a relocated superblock tag
but do not hop to address next_tag (given by session_start=). Instead look at
LBA 32 for the first session and check it as prescribed above.
After reaching its end, round up the read address to the next multiple of 32
and check whether it is smaller than session_start= from the super block.
If so, expect another session to start there.
Checking Single Files in a Loaded Image
An image may consist of many sessions wherein many data blocks may not belong