Integrity and Security:Encryption and Authentication

Encryption and Authentication

The various provisions that a database system may make for authorization may still not provide sufficient protection for highly sensitive data. In such cases, data may be stored in encrypted form. It is not possible for encrypted data to be read unless the reader knows how to decipher (decrypt) them. Encryption also forms the basis of good schemes for authenticating users to a database.

Encryption Techniques

There are a vast number of techniques for the encryption of data. Simple encryption techniques may not provide adequate security, since it may be easy for an unauthorized user to break the code. As an example of a weak encryption technique, consider the substitution of each character with the next character in the alphabet. Thus,

Perryridge

becomes

Qfsszsjehf

If an unauthorized user sees only “Qfsszsjehf,” she probably has insufficient information to break the code. However, if the intruder sees a large number of encrypted branch names, she could use statistical data regarding the relative frequency of characters to guess what substitution is being made (for example, E is the most common letter in English text, followed by T, A, O, N, I and so on).

A good encryption technique has the following properties:

• It is relatively simple for authorized users to encrypt and decrypt data.

• It depends not on the secrecy of the algorithm, but rather on a parameter of the algorithm called the encryption key.

• Its encryption key is extremely difficult for an intruder to determine.

One approach, the Data Encryption Standard (DES), issued in 1977, does both a substitution of characters and a rearrangement of their order on the basis of an encryption key. For this scheme to work, the authorized users must be provided with the encryption key via a secure mechanism. This requirement is a major weakness, since the scheme is no more secure than the security of the mechanism by which the encryption key is transmitted. The DES standard was reaffirmed in 1983, 1987, and again in 1993. However, weakness in DES was recongnized in 1993 as reaching a point where a new standard to be called the Advanced Encryption Standard (AES), needed to be selected. In 2000, the Rijndael algorithm (named for the inventors

V. Rijmen and J. Daemen), was selected to be the AES. The Rijndael algorithm was chosen for its significantly stronger level of security and its relative ease of implementation on current computer systems as well as such devices as smart cards. Like the DES standard, the Rijndael algorithm is a shared-key (or, symmetric key) algorithm in which the authorized users share a key.

Public-key encryption is an alternative scheme that avoids some of the problems that we face with the DES. It is based on two keys; a public key and a private key. Each user Ui has a public key Ei and a private key Di. All public keys are published: They can be seen by anyone. Each private key is known to only the one user to whom the key belongs. If user U1 wants to store encrypted data, U1 encrypts them using public key E1. Decryption requires the private key D1.

Because the encryption key for each user is public, it is possible to exchange information securely by this scheme. If user U1 wants to share data with U2, U1 encrypts the data using E2, the public key of U2. Since only user U2 knows how to decrypt the data, information is transferred securely.

For public-key encryption to work, there must be a scheme for encryption that can be made public without making it easy for people to figure out the scheme for decryption. In other words, it must be hard to deduce the private key, given the public key. Such a scheme does exist and is based on these conditions:

• There is an efficient algorithm for testing whether or not a number is prime.

• No efficient algorithm is known for finding the prime factors of a number.

For purposes of this scheme, data are treated as a collection of integers. We create a public key by computing the product of two large prime numbers: P1 and P2. The private key consists of the pair (P1, P2). The decryption algorithm cannot be used successfully if only the product P1P2 is known; it needs the individual values P1 and P2. Since all that is published is the product P1P2, an unauthorized user would need to be able to factor P1P2 to steal data. By choosing P1 and P2 to be sufficiently large (over 100 digits), we can make the cost of factoring P1P2 prohibitively high (on the order of years of computation time, on even the fastest computers).

The details of public-key encryption and the mathematical justification of this technique’s properties are referenced in the bibliographic notes.

Although public-key encryption by this scheme is secure, it is also computation- ally expensive. A hybrid scheme used for secure communication is as follows: DES keys are exchanged via a public-key – encryption scheme, and DES encryption is used on the data transmitted subsequently.

Authentication

Authentication refers to the task of verifying the identity of a person/software connecting to a database. The simplest form of authentication consists of a secret pass- word which must be presented when a connection is opened to a database.

Password-based authentication is used widely by operating systems as well as databases. However, the use of passwords has some drawbacks, especially over a network. If an eavesdropper is able to “sniff” the data being sent over the network, she may be able to find the password as it is being sent across the network. Once the eavesdropper has a user name and password, she can connect to the database, pretending to be the legitimate user.

A more secure scheme involves a challenge-response system. The database sys- tem sends a challenge string to the user. The user encrypts the challenge string using a secret password as encryption key, and then returns the result. The database system can verify the authenticity of the user by decrypting the string with the same secret password, and checking the result with the original challenge string. This scheme ensures that no passwords travel across the network.

Public-key systems can be used for encryption in challenge – response systems. The database system encrypts a challenge string using the user’s public key and sends it to the user. The user decrypts the string using her private key, and returns the result to the database system. The database system then checks the response. This scheme has the added benefit of not storing the secret password in the database, where it could potentially be seen by system administrators.

Another interesting application of public-key encryption is in digital signatures to verify authenticity of data; digital signatures play the electronic role of physical signatures on documents. The private key is used to sign data, and the signed data can be made public. Anyone can verify them by the public key, but no one could have generated the signed data without having the private key. Thus, we can authenticate the data; that is, we can verify that the data were indeed created by the person who claims to have created them.

Furthermore, digital signatures also serve to ensure nonrepudiation. That is, in case the person who created the data later claims she did not create it (the electronic equivalent of claiming not to have signed the check), we can prove that that person must have created the data (unless her private key was leaked to others).

Summary

• Integrity constraints ensure that changes made to the database by authorized users do not result in a loss of data consistency.

• In earlier chapters, we considered several forms of constraints, including key declarations and the declaration of the form of a relationship (many to many, many to one, one to one). In this chapter, we considered several additional forms of constraints, and discussed mechanisms for ensuring the maintenance of these constraints.

• Domain constraints specify the set of possible values that may be associated with an attribute. Such constraints may also prohibit the use of null values for particular attributes.

• Referential-integrity constraints ensure that a value that appears in one relation for a given set of attributes also appears for a certain set of attributes in another relation.

• Domain constraints, and referential-integrity constraints are relatively easy to test. Use of more complex constraints may lead to substantial overhead. We saw two ways to express more general constraints. Assertions are declarative expressions that state predicates that we require always to be true.

• Triggers define actions to be executed automatically when certain events occur and corresponding conditions are satisfied. Triggers have many uses, such as implementing business rules, audit logging, and even carrying out actions outside the database system. Although triggers were added only lately to the SQL standard as part of SQL:1999, most database systems have long imple- mented triggers.

• The data stored in the database need to be protected from unauthorized access, malicious destruction or alteration, and accidental introduction of inconsistency.

• It is easier to protect against accidental loss of data consistency than to protect against malicious access to the database. Absolute protection of the database from malicious abuse is not possible, but the cost to the perpetrator can be made sufficiently high to deter most, if not all, attempts to access the database without proper authority.

• A user may have several forms of authorization on parts of the database. Authorization is a means by which the database system can be protected against malicious or unauthorized access.

• A user who has been granted some form of authority may be allowed to pass on this authority to other users. However, we must be careful about how authorization can be passed among users if we are to ensure that such authorization can be revoked at some future time.

• Roles help to assign a set of privileges to a user according to on the role that the user plays in the organization.

• The various authorization provisions in a database system may not provide sufficient protection for highly sensitive data. In such cases, data can be encrypted. Only a user who knows how to decipher (decrypt) the encrypted data can read them. Encryption also forms the basis for secure authentication of users.

Review Terms

Suppose that the view is materialized, that is, the view is computed and stored. Write active rules to maintain the view, that is, to keep it up to date on insertions to and deletions from depositor or account. Do not bother about updates.

Make a list of security concerns for a bank. For each item on your list, state whether this concern relates to physical security, human security, operating- system security, or database security.

Using the relations of our sample bank database, write an SQL expression to define the following views:

a. A view containing the account numbers and customer names (but not the balances) for all accounts at the Deer Park branch.

b. A view containing the names and addresses of all customers who have an account with the bank, but do not have a loan.

c. A view containing the name and average account balance of every customer of the Rock Ridge branch.

For each of the views that you defined in Exercise 6.10, explain how updates would be performed (if they should be allowed at all). Hint: See the discussion of views in Chapter 3.

In Chapter 3, we described the use of views to simplify access to the database by users who need to see only part of the database. In this chapter, we described the use of views as a security mechanism. Do these two purposes for views ever conflict? Explain your answer.

What is the purpose of having separate categories for index authorization and resource authorization?

Database systems that store each relation in a separate operating-system file may use the operating system’s security and authorization scheme, instead of defining a special scheme themselves. Discuss an advantage and a disadvantage of such an approach.

What are two advantages of encrypting data stored in the database?

Perhaps the most important data items in any database system are the pass- words that control access to the database. Suggest a scheme for the secure storage of passwords. Be sure that your scheme allows the system to test passwords supplied by users who are attempting to log into the system.

Bibliographical Notes

Discussions of integrity constraints in the relational model are offered by Hammer and McLeod [1975], Stonebraker [1975], Eswaran and Chamberlin [1975], Schmid and Swenson [1975] and Codd [1979]. The original SQL proposals for assertions and trig- gers are discussed in Astrahan et al. [1976], Chamberlin et al. [1976], and Chamberlin et al. [1981]. See the bibliographic notes of Chapter 4 for references to SQL standards and books on SQL.

Discussions of efficient maintenance and checking of semantic-integrity assertions are offered by Hammer and Sarin [1978], Badal and Popek [1979], Bernstein et al.

[1980a], Hsu and Imielinski [1985], McCune and Henschen [1989], and Chomicki [1992]. An alternative to using run-time integrity checking is certifying the correctness of programs that access the database. Sheard and Stemple [1989] discusses this approach.

Active databases are databases that support triggers and other mechanisms that permit the database to take actions on occurrence of events. McCarthy and Dayal [1989] discuss the architecture of an active database system based on the event– condition–action formalism. Widom and Finkelstein [1990] describe the architecture of a rule system based on set-oriented rules; the implementation of the rule system on the Starburst extensible database system is presented in Widom et al. [1991]. Con- sider an execution mechanism that allows a nondeterministic choice of which rule to execute next. A rule system is said to be confluent if, regardless of the rule chosen, the final state is the same. Issues of termination, nondeterminism, and confluence of rule systems are discussed in Aiken et al. [1995].

Security aspects of computer systems in general are discussed in Bell and La Padula [1976] and by US Dept. of Defense [1985]. Security aspects of SQL can be found in the SQL standards and textbooks on SQL referenced in the bibliographic notes of Chapter 4.Stonebraker and Wong [1974] discusses the Ingres approach to security, which involves modification of users’ queries to ensure that users do not access data for which authorization has not been granted. Denning and Denning [1979] survey database security.

Database systems that can produce incorrect answers when necessary for security maintenance are discussed in Winslett et al. [1994] and Tendick and Matloff [1994].

Work on security in relational databases includes that of Stachour and Thuraisingham [1990], Jajodia and Sandhu [1990], and Qian and Lunt [1996]. Operating-system security issues are discussed in most operating-system texts, including Silberschatz and Galvin [1998].

Stallings [1998] provides a textbook description of cryptography. Daemen and Rijmen [2000] present the Rijndael algorithm. The Data Encryption Standard is presented by US Dept. of Commerce [1977]. Public-key encryption is discussed by Rivestet al. [1978]. Other discussions on cryptography include Diffie and Hellman [1979], Simmons [1979], Fernandez et al. [1981], and Akl [1983].