tyler's glossary of useful terms

Copyright ©2003-2004 Tyler Blessing

CUPS backend

A software plugin for CUPS, the Common Unix Printing System, used for transfering filtered print job data from a host computer to a destination device, which ultimately produces printed output. The typical CUPS installation includes a default backend for each of the common printing protocols, such as USB, IPP, LPD, etc...

MD5

MD5 is an algorithm used to produce a fixed-length (128 bit) "message digest" for any arbitrarily long "message". The algorithm is described in RFC 1321, which conjectures, "that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest."

By this conjecture, the integrity of a file downloaded over the Internet can be verified by calulating its MD5 digest and comparing the result to the MD5 digest calculated by the distributor of the file. Identical MD5 digests indicate (with very high odds) that the file was not damaged or altered during the download.

An MD5 digest is sometimes (inaccurately) referred to as a "checksum".

MD5 on Mac OS X (UDIF-MD5)

On Mac OS X (versions 10.3 and later), the MD5 digest of a "disk image" file (UDIF-MD5) may be calculated using the "Disk Utility" application, which is typically located in the folder "/Applications/Utilities/"

Follow these steps to calculate the UDIF-MD5 digest for a disk image file:

  1. Double-click the disk image file to be verified.
  2. Open the "Disk Utility" application.
  3. Select the disk image file* from the left-side list in the "Disk Utility" window
    * Verify that you selected the disk image file (ending in ".dmg") and NOT the mounted disk image!
  4. From the menubar select "Images -> Checksum -> UDIF-MD5".

Disk Utility will calculate the UDIF-MD5 digest of the disk image file and display the result.

SHA-1

The standard implementation of the Secure Hash Algorithm, which produces a 160-bit "message digest" for any "message" smaller than 264 bits (2 Exabytes). SHA-1 was designed by the United States National Security Agency (NSA); its implemenation is described in RFC 3174.

Variants of SHA-1 include SHA-256, SHA-384, and SHA-512, which are named according to the bit-length of the respective output "message digest" (by this naming convention, SHA-1 would be named SHA-160).

The SHA-1 design was based upon principles similar to those used for the design of the MD5 algorithm.

SHA-1 usage on Mac OS X

On Mac OS X, the SHA-1 digest of any file smaller than 2 Exabytes (2,147,483,648 GB) can be calculated by making use of the built-in openssl toolset. Direct use of openssl, in general, requires using the "Terminal" application, which is typically located in the folder "/Applications/Utilities/"

Follow these steps to calculate the SHA-1 digest for a file:

  1. Open the "Terminal" application.
  2. Enter the following text in the Terminal window (do not include the quotes, but be sure to include the "space" after "sha1"): "/usr/bin/openssl sha1 ".
  3. In the "Finder", locate the file to be verified then drag and drop it onto the open "Terminal" window.
  4. Press the "return" key.

openssl will calculate the SHA-1 digest for the file and display the result.