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Introduced checksum tag for relocated 64 kB superblock on overwriteable media.

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Thomas Schmitt
2009-08-17 17:22:31 +02:00
parent 07a67a59e7
commit 868005ed0e
6 changed files with 172 additions and 48 deletions
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107
doc/checksums.txt
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@ -27,8 +27,8 @@ iso_md5_start(), iso_md5_compute(), iso_md5_clone(), iso_md5_end()
Representation in the Image
The checksums are stored as stream recognizable checksum tags and as a compact
array at the end of the session. The latter allows to quickly load many
There may be several stream recognizable checksum tags and a compact array
of MD5 items at the end of the session. The latter allows to quickly load many
file checksums from media with slow random access.
@ -69,26 +69,58 @@ array. (Then follows eventual padding.)
The tags are a single lines of printable text, padded by 0 bytes. They have
the following format:
Tag_id pos=# range_start=# range_size=# [next=#] md5=# self=#\n
Tag_id pos=# range_start=# range_size=# [session_start|next=#] md5=# self=#\n
Tag_id distinguishes the three tag types
"libisofs_sb_checksum_tag_v1" Superblock tag
"libisofs_tree_checksum_tag_v1" Directory tree tag
"libisofs_checksum_tag_v1" Session tag
Tag_id distinguishes the following tag types
"libisofs_rlsb32_checksum_tag_v1" Relocated 64 kB superblock tag
"libisofs_sb_checksum_tag_v1" Superblock tag
"libisofs_tree_checksum_tag_v1" Directory tree tag
"libisofs_checksum_tag_v1" Session tag
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=.
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(.
Example (session starts at at Logical Block Address 32):
Example:
A relocated superblock which points to the last session. Then the first session
which starts at Logical Block Address 32. The following sessions have the same
structure as the first one.
LBA 0:
<... ECMA-119 System Area and Volume Descriptors ...>
libisofs_sb_checksum_tag_v1 pos=50 range_start=32 range_size=18 md5=17471035f1360a69eedbd1d0c67a6aa2 self=52d602210883eeababfc9cd287e28682
<... ECMA-119 Directory Entries ...>
libisofs_tree_checksum_tag_v1 pos=334 range_start=32 range_size=302 md5=41acd50285339be5318decce39834a45 self=fe100c338c8f9a494a5432b5bfe6bf3c
LBA 18:
libisofs_rlsb32_checksum_tag_v1 pos=18 range_start=0 range_size=18 session_start=311936 md5=6fd252d5b1db52b3c5193447081820e4 self=526f7a3c7fefce09754275c6b924b6d9
<... padding up to LBA 32 ...>
LBA 32:
<... First Session: ECMA-119 System Area and Volume Descriptors ...>
libisofs_sb_checksum_tag_v1 pos=50 range_start=32 range_size=18 md5=17471035f1360a69eedbd1d0c67a6aa2 self=52d602210883eeababfc9cd287e28682
<... ECMA-119 Directory Entries (the tree of file names) ...>
LBA 334:
libisofs_tree_checksum_tag_v1 pos=334 range_start=32 range_size=302 md5=41acd50285339be5318decce39834a45 self=fe100c338c8f9a494a5432b5bfe6bf3c
<... Data file payload and checksum array ...>
libisofs_checksum_tag_v1 pos=81554 range_start=32 range_size=81522 md5=8adb404bdf7f5c0a078873bb129ee5b9 self=57c2c2192822b658240d62cbc88270cb
LBA 81554:
libisofs_checksum_tag_v1 pos=81554 range_start=32 range_size=81522 md5=8adb404bdf7f5c0a078873bb129ee5b9 self=57c2c2192822b658240d62cbc88270cb
<... more sessions ...>
LBA 311936:
<... Last Session: ECMA-119 System Area and Volume Descriptors ...>
LBA 311954:
libisofs_sb_checksum_tag_v1 pos=311954 range_start=311936 range_size=18 next=312286 md5=7f1586e02ac962432dc859a4ae166027 self=2c5fce263cd0ca6984699060f6253e62
<... Last Session: tree, tree checksum tag, data payload, session tag ...>
There are five tag parameters. The first three are decimal numbers, the others
are strings of 32 hex digits:
There are several tag parameters. Addresses are given as decimal numbers, MD5
checksums as strings of 32 hex digits.
pos=
gives the block address where the tag supposes itself to be stored.
@ -117,6 +149,11 @@ are strings of 32 hex digits:
uniquely placed directly after the ECMA-119 Volume Descriptor Set Terminator
where no such cleartext is supposed to reside by accident.
Only with relocated 64 kB superblock tag:
session_start=
The start block address (System Area) of the session to which the relocated
superblock points.
md5=
The checksum payload of the tag as lower case hex digits.
@ -135,26 +172,39 @@ are strings of 32 hex digits:
Checking Before Image Tree Loading
In order to check for a trustworthy loadable image tree, read the first 32
blocks from to the session start and look in block 16 to 32 for the superblock
blocks from to the session start and look in block 16 to 32 for a superblock
checksum tag by
iso_util_decode_md5_tag(block, &tag_type, &pos,
&range_start, &range_size, &next_tag, md5, 2);
If it appears and has plausible parameters, then check whether its MD5 matches
the MD5 of the data blocks which were read before.
(Keep the original MD5 context of the data blocks and clone one for obtaining
the MD5 bytes.)
Compute the block into the MD5 checksum after your are done with interpreting
it.
&range_start, &range_size, &next_tag, md5, 0);
If those MD5s match, then compute the checksum block into the kept MD5 context
If a tag of type 2 or 4 appears and has plausible parameters, then check
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:
iso_util_decode_md5_tag(block, &tag_type, &pos,
&range_start, &range_size, &next_tag, md5, 3);
Again, if the parameters match the reading state, the MD5 must match the
MD5 computed from the data blocks which were before.
If so, then the tree is ok and safe to be loaded by iso_image_import().
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.
Checking a Whole Session
@ -175,6 +225,12 @@ read before the tag block.
Checking Single Files in a Loaded Image
An image may consist of many sessions wherein many data blocks may not belong
to files in the directory tree of the most recent session. Checking this
tree and all its data files can ensure that all actually valid data in the
image are trustworthy. This will leave out the trees of the older sessions
and the obsolete data blocks of overwritten or deleted files.
Once the image has been loaded, you can obtain MD5 sums from IsoNode objects
which fulfill
iso_node_get_type(node) == LIBISO_FILE
@ -202,3 +258,8 @@ for reading file content from the loaded image.
iso_image_get_session_md5() gives start LBA and session payload size as of
"isofs.ca" and the session checksum as of the checksum array.
For reading you may use the IsoDataSource object which you submitted
to iso_image_import() when reading the image. If this source is associated
to a libburn drive, then libburn function burn_read_data() can read directly
from it.