.
Configuring a public server: When a server is connected to the Internet in any way, security immediately becomes a serious concern. When that server contains customer data, financial data, or any other private data, the concern is even more serious. We say that a server like that must meet Properly Administered Host guidelines. For instance, you should use restricted shells when appropriate, to limit command scope. Secure shells should be used when insecure transport or command login from a not totally secure site is necessary. One-time passwords are another option to consider. Accounts should all have valid login shells, or if used for ftp only, a shell can be set tobin/false. The password files and logs should be regularly reviewed. Umasks should be set appropriately, and accounts should be deleted regularly. Security tools should be used to check typical mistakes like accounts with no or easily guessed passwords. Services like finger, FTP, whois, sendmail, DNS, and WINS should be turned off if not absolutely necessary. On NT, NTFS must be used to format all drives, directories accessible to end-users should not be in the PATH for admins, and all files should be assigned to a valid owner. If FTP is enabled, only anonymous FTP should be allowed and any access by FTP must be limited to read-only. Access via WWW must be limited to read-only. .
Basic Cryptography Terminology and Concepts: Cryptogtraphic systems consist of plaintext, the message before it is encrypted, the ciphertext, the message after it has been encrypted, the algorithm, or mathematical operation to obscure the plaintext so that it is unreadable, and the key, a sequesnce that can encrypt or decrypt the cyphertext. The first type of cryptography is called symmetrical, where the same secret key is used to encrypt and decrypt the message. However, symmetric systems pose problems for transactions on the internet. Communications are often spontaneous, so there is often no opportunity to exchange secret keys before the transmission.