NAME

fixity - Verifying a file is unaltered or corrupted

AVAILABILITY

SUNWsamfs

DESCRIPTION

File fixity is a term used by digital preservationists and archivists when referring to the contents of a digital file being fixed or unchanged. Fixity checking verifies that a digital file has not been altered or corrupted and does not ensure the preservation of a digital file. Using the capabilities of Oracle HSM, files can be identified with a fixity attribute that validates a file's contents when a file is archived or stages and can be use to detect corrupted files allowing the corrective actions to be taken.

OPERATION

A file has a fixity attribute that is set using the ssum command or using the sam_ssum (3x) API. Using either of these methods cause the file's message digest to be calculated with the specified algorithm. After the calculation the message-digest is stored with the file and the file is made immutable. The file also has its generate, use and validated attributes set. Once the fixity attribute is set it will never be removed by Oracle HSM. The fixity status and its message-digest can be examined using the sls (1) command.

If the file has a known message-digest value it can be specified with either of the the command or the API. The supplied value is compared against the calculated and, if they do not match, the command or API function fails and fixity attribute is not set.

Every archive copy of a file with the fixity attribute has its message-digest calculated as is written to archive media. This value is compared to the value associated with the file. If the values do not match the archive copy is unusable.

EXAMPLE

Here is an example using the ssum command to set fixity: with a SHA-256 message-digest algorithm:

      ssum -F -a SHA-256 ∕qfs1∕archive∕document.png

If a file has a known SHA-256 message-digest value, the following example shows how it can be used to validate the fixity:

      ssum -F -a SHA-256 -h 5ff20a4d0ed4a85b61147c76a8f54730 ∕qfs1∕archive∕paper.doc

ADDITIONAL CAPABILITIES

Oracle HSM has additional capabilities that extend the preservation of files.

WORM-FS

Files with the fixity attribute can be placed into an Oracle HSM WORM file system. This extends the data preservation by ensuring that files cannot be altered or removed even if the immutability attribute is removed. See sam_worm (5) for more information.

DATA VERIFICATION

The ssum and sam_ssum() API function provide a data verification option. In addition to forcing the generation and use of message-digests for archiving and staging, this option prevents the release of a file until all archive copies have been created and their message-digest values verified. See ssum (1) command for more information.

DATA INTEGRITY AND VALIDATION

When Oracle HSM archiver is configured to tape media for copies, Data Integrity and Validation (DIV) can by used on capable tape drives. While the message-digest is calculated for the file every block written to the tape media has a separate checksum generated and sent to the tape drive to validate the block. See defaults.conf (4) for additional information.

LIMITATIONS

The fixity support in Oracle HSM is not without exposures:

If the file is not in a WORM file system a user with super-user priviledges can remove the immutability attribute making the file modifiable.

If for any reason a file with the fixity attribute is altered or corrupted by removing its immutability, it will not be detected until Oracle HSM attempts to archive the file. It may be advantageous to use the sfind command with the ssum_F option to locate files with the fixity attribute and audit their contents at regular intervals. Using this command with the ssum_v option will find fixity files without a validated message-digest.

A user with super-user priviledges could alter the extended attribute containing the message-digest value causing the file to appear to be corrupted.