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Chapter 6 . Securing Linux 217 To work with passwords for groups, you can use the grpconv command to convert passwords in /etc/groups to shadowed group passwords in /etc/gshadow. If you change passwords or group passwords and something breaks (you are unable to log in to the accounts), you can use the pwunconv and grpunconv commands, respectively, to reverse password conversion. Using the shadow password file and picking good passwords are a great start toward securing your system. You may have noticed by now that security is not just a one-time job. It is an ongoing process, as much about policies as programs. Keep reading to learn more. Using Encryption Techniques The previous sections told you how to lock the doors to your Linux system to deny access to crackers. The best lock is useless, however, if you are mugged in your own driveway and have your keys stolen. Likewise, the best computer security can be for naught if you are sending passwords and other critical data unprotected across the Internet. A savvy cracker can use a tool called a protocol analyzer or a network sniffer to peek at the data flowing across a network and pick out passwords, credit card data, and other juicy bits of information. The cracker does this by breaking into a poorly protected system on the same network and running software, or by gaining physical access to the same network and plugging in his or her own equipment. You can combat this sort of theft by using encryption. The two main types of encryption in use today are symmetric cryptography and public-key cryptography. Symmetric Cryptography Symmetric cryptography, also called private-key cryptography, uses a single key to both encrypt and decrypt a message. This method is generally inappropriate for securing data that will be used by a third party because of the complexity of secure key exchange. Symmetric cryptography is generally useful for encrypting data for one s own purposes. A classic use of symmetric cryptography is for a personal password vault. Anyone who has been using the Internet for any amount of time has accumulated a quantity of user names and passwords for accessing various sites and resources. A personal password vault lets you store this access information in an encrypted form. The end result is that you only have to remember one password to unlock all of your access information.
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