It is important to make sure your files stay intact and unaltered when you transfer. It can also be important to show that your files are intact and unaltered, especially if you are using video for evidence.
Hashes, also known as checksums, are a way to check if your files have transferred intact. Hashes are also valuable metadata for evidentiary video, because they can be used to show whether your files are tampered with over time. It is therefore a good idea to capture hashes as early as possible in the video lifecycle, such as when you first offload videos from your camera.
A hash is an alphanumeric string of characters that is created by running a hash function algorithm (such as MD5 or SHA-1) on a file. The resulting hash value will be the same every time you run the algorithm on the file, so long as the file is unchanged. If the file is altered in any way, the resulting hash value will be different.
Run a hash algorithm on a file as soon as you acquire it, and again whenever you want to check that it is intact. You may want to check a file, for example, after you have transferred it from one location to another, if your file has been stored for a long time, or if you want to know whether someone has altered it.
There are many free and commercially available software applications you can use to compute hashes (see “Try This” below). In the near future, some cameras will allow you to compute and embed hashes in the video file at the point of creation.
Try This: BASIC
Karen’s Hasher is a Windows GUI tool for computing and verifying hashes.
Try This: ADVANCED
MD5 is a command-line tool for computing MD5 checksums that comes pre-installed on Macs.
Try This: ADVANCED
Microsoft File Checksum Integrity Verifier is a Windows command-line tool for computing MD5 and SHA1 hashes.
Try This: ADVANCED
md5deep and hashdeep are command-line tools for computing and comparing multiple checksums for entire directories of files.
Try This: ADVANCED
sha1sum is a command-line tool for computing and checking SHA-1 checksums that is part of the GNU Core Utilities.