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GNU Info (libc.info)Cryptographic FunctionsDES Encryption and Password Handling ************************************ On many systems, it is unnecessary to have any kind of user authentication; for instance, a workstation which is not connected to a network probably does not need any user authentication, because to use the machine an intruder must have physical access. Sometimes, however, it is necessary to be sure that a user is authorized to use some service a machine provides--for instance, to log in as a particular user id (Note: Users and Groups). One traditional way of doing this is for each user to choose a secret "password"; then, the system can ask someone claiming to be a user what the user's password is, and if the person gives the correct password then the system can grant the appropriate privileges. If all the passwords are just stored in a file somewhere, then this file has to be very carefully protected. To avoid this, passwords are run through a "one-way function", a function which makes it difficult to work out what its input was by looking at its output, before storing in the file. The GNU C library already provides a one-way function based on MD5 and for compatibility with Unix systems the standard one-way function based on the Data Encryption Standard. It also provides support for Secure RPC, and some library functions that can be used to perform normal DES encryption.
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