20.3.docx - 20.3 PASSWORD MANAGEMENT 20-1 20.3 PASSWORD MANAGEMENT Password Protection The front line of defense against intruders is the password

20.3.docx - 20.3 PASSWORD MANAGEMENT 20-1 20.3 PASSWORD...

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20.3 / PASSWORD MANAGEMENT 20-1 20.3 PASSWORD MANAGEMENT Password Protection The front line of defense against intruders is the password system. Virtually all multiuser systems require that a user provide not only a name or identifier (ID) but also a password. The password serves to authenticate the ID of the individual logging on to the system. In turn, the ID provides security in the following ways: The ID determines whether the user is authorized to gain access to a system. In some systems, only those who already have an ID filed on the system are allowed to gain access. The ID determines the privileges accorded to the user.A few users may have supervisory or “superuser” status that enables them to read files and perform functions that are especially protected by the operating system. Some systems have guest or anonymous accounts, and users of these accounts have more limited privileges than others. The ID is used in what is referred to as discretionary access control. For example, by listing the IDs of the other users, a user may grant permission to them to read files owned by that user. T HE V ULNERABILITY OF P ASSWORDS To understand the nature of the threat to password-based systems, let us consider a scheme that is widely used on UNIX, in which passwords are never stored in the clear. Rather, the following procedure is employed (Figure 20.4a). Each user selects a password of up to eight printable characters in length. This is converted into a 56-bit value (using 7-bit ASCII) that serves as the key input to an encryption routine. The encryption routine, known as crypt(3),is based on DES.The DES algorithm is modified using a 12-bit “salt”value. Typically, this value is related to the time at which the password is assigned to the user. The modified DES algorithm is exercised with a data input consisting of a 64-bit block of zeros.The output of the algorithm then serves as input for a second encryption.This process is repeated for a total of 25 encryptions.The resulting 64-bit output is then translated into an 11-character sequence. The hashed password is then stored, together with a plaintext copy of the salt, in the password file for the corresponding user ID. This method has been shown to be secure against a variety of cryptanalytic attacks [WAGN00]. The salt serves three purposes: It prevents duplicate passwords from being visible in the password file.Even if two users choose the same password, those passwords will be assigned at different times. Hence, the “extended” passwords of the two users will differ. It effectively increases the length of the password without requiring the user to remember two additional characters. Hence, the number of possible passwords is increased by a factor of 4096, increasing the difficulty of guessing a password.
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