Checksums

A checksum is a digital signature of a file created by a hashing algorithm. File checksums provide the ability to verify the integrity of files downloaded from the PDS Geosciences Node. We provide MD5 checksum values for all archived files hosted at the PDS Geosciences Node. Checksum files are created for each PDS volume. The files are stored in the data set directory along with the volume directory or in a checksum subdirectory (see example below). The checksum file name is a combination of the volume name and the file creation date (year, month, and day).

Software

MD5Deep [http://md5deep.sourceforge.net/] is a free computer program that is widely used for verifying the integrity of downloaded files. Checksum values are created for all files in a directory tree and then compared with checksum values produced at an earlier time. This file verification software ultimately determines whether or not the files were downloaded correctly. If a failed comparison is identified, the corresponding file should be downloaded again.

Volume Verification Example

• First the contents of the MSL APXS EDR data volume and the corresponding MD5 checksum file are downloaded to the local c:\temp directory.
  
  ftp://pds-geosciences.wustl.edu/msl/msl-m-apxs-2-edr-v1/
  

• Assuming MD5Deep is installed, the MD5 file contents are compared to the calculated values for the downloaded PDS volume. The command below uses MD5Deep to compare the checksum values and output a list of files that failed including the name, checksum value and file locations.

• Pattern:
  c:\temp>md5deep -X \\checksum_file_path \checksum_name.md5 -r \\location_of_original_files

• Actual example:
  c:\temp>md5deep -X c:\temp\mslapx_0xxx_130311.md5 -r c:\temp\mslapx_0xxx

• Success Output:
  The program will complete without producing any messages if all the corresponding checksum values match. The command prompt will be ready for the next command.
  
  c:\temp>

• Failure Output:
  If MD5Deep identifies checksum mismatches, it will output the names of mismatched files and their calculated MD5 checksums.
  
  8eaf3ba7261e9679b459bd0cd26f0a0f  c:\temp\mslapx_0xxx\errata.txt
  a43ab52ce4b92dadbcad1cb7ce206dfe  c:\temp\mslapx_0xxx\label\labinfo.txt

Individual File Verification Example

• To verify the integrity of a single file, first use MD5Deep to calculate the checksum value for the file. Then manually compare this value to the corresponding checksum in the provided MD5 checksum file.

• First, a file checksum must be created for the target file.
  
  Pattern:
  c:\temp>md5deep \\location_of_file\fileName.ext
  
  Actual example:
  c:\temp>md5deep c:\temp\mslapx_0xxx\aareadme.txt
  
  a686bf4058c123282847e8416f9ee70c  c:\temp\mslapx_0xxx\aareadme.txt

• Then open the provided MD5 checksum file with any text editor. Locate the file in question, and confirm the checksums match. They are not case sensitive.