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Further enhancements of new API call iso_util_decode_md5_tag().

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Thomas Schmitt
2009-08-16 22:03:51 +02:00
parent 955471a064
commit 07a67a59e7
3 changed files with 45 additions and 33 deletions
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57
doc/checksums.txt
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@ -132,12 +132,40 @@ are strings of 32 hex digits:
Usage at Read Time
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
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.
If those 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().
Checking a Whole Session
In order to check the trustworthyness of a whole session, read from its start
up to the session tag. Read the blocks and submit each single one of them to
In order to check the trustworthyness of a whole session, continue reading
and checksumming after the tree was verified.
iso_util_decode_md5_tag(block, &pos, &range_start, &range_size, md5, 1);
Read and checksum the blocks. When reaching block address next_tag (from the
tree tag) submit this block to
iso_util_decode_md5_tag(block, &tag_type, &pos,
&range_start, &range_size, &next_tag, md5, 1);
If this returns 1, then check whether the returned parameters pos, range_start,
and range_size match the state of block reading, and whether the returned
@ -145,29 +173,10 @@ bytes in parameter md5 match the MD5 computed from the data blocks which were
read before the tag block.
Checking before Image Tree Loading
In order to check for a trustworthy loadable image tree, read the first 32
blocks to the session start and look in block 16 to 32 for the superblock
checksum tag by
iso_util_decode_md5_tag(block, &pos, &range_start, &range_size, md5, 2);
If one appears and has plausible parameters, then check whether its MD5 matches
the MD5 of the data blocks read before.
(Keep the original MD5 context of the data blocks and clone one for obtaining
the MD5 bytes.)
If those MD5s match, then compute the checksum block into the kept MD5 context
and go on with searching for the tree checksum tag. This can be found in a
read-in block by:
iso_util_decode_md5_tag(block, &pos, &range_start, &range_size, md5, 3)
Again, if the parameters match the reading state, the MD5 must match the
MD5 computed from the data blocks before.
If so, then the tree is ok and safe to be loaded by iso_image_import().
Checking Single Files in a Loaded Image
The image has to be loaded, so you can obtain IsoNode objects which yield
Once the image has been loaded, you can obtain MD5 sums from IsoNode objects
which fulfill
iso_node_get_type(node) == LIBISO_FILE
The recorded checksum can be obtained by