How to Compute a Digest of a File

When you compute a digest of a file, you can check to see that the file has not been tampered with by comparing digest outputs. A digest does not alter the original file.

1. List the available digest algorithms.

   % digest -l md5 sha1 sha256 sha384 sha512

2. Compute the digest of the file and save the digest listing.

   Provide an algorithm with the digest command.

   % digest -v -a algorithm input-file > digest-listing

   -v

   Displays the output in the following format:

   algorithm (input-file) = digest

   -a algorithm

   Is the algorithm to use to compute a digest of the file. Type the algorithm as the algorithm appears in the output of Step 1.

   input-file

   Is the input file for the digest command.

   digest-listing

   Is the output file for the digest command.

Example 14–8 Computing a Digest With the MD5 Mechanism

In the following example, the digest command uses the MD5 mechanism to compute a digest for an email attachment.

% digest -v -a md5 email.attach >>  $HOME/digest.emails.05.07
% cat  ~/digest.emails.05.07
md5 (email.attach) = 85c0a53d1a5cc71ea34d9ee7b1b28b01

When the -v option is not used, the digest is saved with no accompanying information:

% digest -a md5 email.attach >>  $HOME/digest.emails.05.07
% cat  ~/digest.emails.05.07
85c0a53d1a5cc71ea34d9ee7b1b28b01

Example 14–9 Computing a Digest With the SHA1 Mechanism

In the following example, the digest command uses the SHA1 mechanism to provide a directory listing. The results are placed in a file.

% digest -v -a sha1 docs/* > $HOME/digest.docs.legal.05.07
% more ~/digest.docs.legal.05.07
sha1 (docs/legal1) = 1df50e8ad219e34f0b911e097b7b588e31f9b435
sha1 (docs/legal2) = 68efa5a636291bde8f33e046eb33508c94842c38
sha1 (docs/legal3) = 085d991238d61bd0cfa2946c183be8e32cccf6c9
sha1 (docs/legal4) = f3085eae7e2c8d008816564fdf28027d10e1d983