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Chapter: Fundamentals of Database Systems : Additional Database Topics: Security and Distribution : Database Security

Encryption and Public Key Infrastructures

1. The Data Encryption and Advanced Encryption Standards 2. Symmetric Key Algorithms 3. Public (Asymmetric) Key Encryption 4. Digital Signatures 5. Digital Certificates

Encryption and Public Key Infrastructures


The previous methods of access and flow control, despite being strong control measures, may not be able to protect databases from some threats. Suppose we communicate data, but our data falls into the hands of a nonlegitimate user. In this situation, by using encryption we can disguise the message so that even if the transmission is diverted, the message will not be revealed. Encryption is the conversion of data into a form, called a ciphertext, which cannot be easily understood by unauthorized persons. It enhances security and privacy when access controls are bypassed, because in cases of data loss or theft, encrypted data cannot be easily understood by unauthorized persons.


With this background, we adhere to following standard definitions:

·        Ciphertext: Encrypted (enciphered) data.


·        Plaintext (or cleartext): Intelligible data that has meaning and can be read or acted upon without the application of decryption.


·        Encryption: The process of transforming plaintext into ciphertext.


·        Decryption: The process of transforming ciphertext back into plaintext.


            Encryption consists of applying an encryption algorithm to data using some prespecified encryption key. The resulting data has to be decrypted using a decryption key to recover the original data.


1. The Data Encryption and Advanced Encryption Standards


The Data Encryption Standard (DES) is a system developed by the U.S. government for use by the general public. It has been widely accepted as a cryptographic standard both in the United States and abroad. DES can provide end-to-end encryption on the channel between sender A and receiver B. The DES algorithm is a careful and complex combination of two of the fundamental building blocks of encryption: substitution and permutation (transposition). The algorithm derives its strength from repeated application of these two techniques for a total of 16 cycles. Plaintext (the original form of the message) is encrypted as blocks of 64 bits. Although the key is 64 bits long, in effect the key can be any 56-bit number. After questioning the adequacy of DES, the NIST introduced the Advanced Encryption Standard (AES). This algorithm has a block size of 128 bits, compared with DES’s 56-block size, and can use keys of 128, 192, or 256 bits, compared with DES’s 56-bit key. AES introduces more possible keys, compared with DES, and thus takes a much longer time to crack.


2. Symmetric Key Algorithms


A symmetric key is one key that is used for both encryption and decryption. By using a symmetric key, fast encryption and decryption is possible for routine use with sensitive data in the database. A message encrypted with a secret key can be decrypted only with the same secret key. Algorithms used for symmetric key encryption are called secret-key algorithms. Since secret-key algorithms are mostly used for encrypting the content of a message, they are also called content-encryption algorithms.


The major liability associated with secret-key algorithms is the need for sharing the secret key. A possible method is to derive the secret key from a user-supplied password string by applying the same function to the string at both the sender and receiver; this is known as a password-based encryption algorithm. The strength of the symmetric key encryption depends on the size of the key used. For the same algorithm, encrypting using a longer key is tougher to break than the one using a shorter key.


3. Public (Asymmetric) Key Encryption

In 1976, Diffie and Hellman proposed a new kind of cryptosystem, which they called public key encryption. Public key algorithms are based on mathematical functions rather than operations on bit patterns. They address one drawback of symmetric key encryption, namely that both sender and recipient must exchange the common key in a secure manner. In public key systems, two keys are used for encryption/decryption. The public key can be transmitted in a non-secure way, whereas the private key is not transmitted at all. These algorithms—which use two related keys, a public key and a private key, to perform complementary operations (encryption and decryption)—are known as asymmetric key encryption algorithms. The use of two keys can have profound consequences in the areas of confidentiality, key distribution, and authentication. The two keys used for public key encryption are referred to as the public key and the private key. The private key is kept secret, but it is referred to as a private key rather than a secret key (the key used in conventional encryption) to avoid confusion with conventional encryption. The two keys are mathematically related, since one of the keys is used to perform encryption and the other to perform decryption. However, it is very difficult to derive the private key from the public key.


A public key encryption scheme, or infrastructure, has six ingredients:


        1. Plaintext. This is the data or readable message that is fed into the algorithm as input.


        2. Encryption algorithm. This algorithm performs various transformations on the plaintext.


        3. and 4. Public and private keys. These are a pair of keys that have been selected so that if one is used for encryption, the other is used for decryption. The exact transformations performed by the encryption algorithm depend on the public or private key that is provided as input. For example, if a message is encrypted using the public key, it can only be decrypted using the private key.


        5. Ciphertext. This is the scrambled message produced as output. It depends on the plaintext and the key. For a given message, two different keys will produce two different ciphertexts.


        6. Decryption algorithm. This algorithm accepts the ciphertext and the matching key and produces the original plaintext.


As the name suggests, the public key of the pair is made public for others to use, whereas the private key is known only to its owner. A general-purpose public key cryptographic algorithm relies on one key for encryption and a different but related key for decryption. The essential steps are as follows:


·        Each user generates a pair of keys to be used for the encryption and decryption of messages.


·        Each user places one of the two keys in a public register or other accessible file. This is the public key. The companion key is kept private.


·        If a sender wishes to send a private message to a receiver, the sender encrypts the message using the receiver’s public key.

·        When the receiver receives the message, he or she decrypts it using the receiver’s private key. No other recipient can decrypt the message because only the receiver knows his or her private key.


The RSA Public Key Encryption Algorithm. One of the first public key schemes was introduced in 1978 by Ron Rivest, Adi Shamir, and Len Adleman at MIT and is named after them as the RSA scheme. The RSA scheme has since then reigned supreme as the most widely accepted and implemented approach to public key encryption. The RSA encryption algorithm incorporates results from number the-ory, combined with the difficulty of determining the prime factors of a target. The RSA algorithm also operates with modular arithmetic—mod n.


Two keys, d and e, are used for decryption and encryption. An important property is that they can be interchanged. n is chosen as a large integer that is a product of two large distinct prime numbers, a and b, n = a × b. The encryption key e is a randomly chosen number between 1 and n that is relatively prime to (a – 1) × (b – 1). The plaintext block P is encrypted as Pe where Pe = P mod n. Because the exponentiation is performed mod n, factoring Pe to uncover the encrypted plaintext is difficult. However, the decrypting key d is carefully chosen so that (Pe)d mod n = P. The decryption key d can be computed from the condition that d × e = 1 mod ((a – 1) × (b – 1)). Thus, the legitimate receiver who knows d simply computes (Pe)d mod n = P and recovers P without having to factor Pe.


4. Digital Signatures


A digital signature is an example of using encryption techniques to provide authentication services in electronic commerce applications. Like a handwritten signature, a digital signature is a means of associating a mark unique to an individual with a body of text. The mark should be unforgettable, meaning that others should be able to check that the signature comes from the originator.


A digital signature consists of a string of symbols. If a person’s digital signature were always the same for each message, then one could easily counterfeit it by simply copying the string of symbols. Thus, signatures must be different for each use. This can be achieved by making each digital signature a function of the message that it is signing, together with a timestamp. To be unique to each signer and counterfeit-proof, each digital signature must also depend on some secret number that is unique to the signer. Thus, in general, a counterfeitproof digital signature must depend on the message and a unique secret number of the signer. The verifier of the signature, however, should not need to know any secret number. Public key tech-niques are the best means of creating digital signatures with these properties.


5. Digital Certificates

A digital certificate is used to combine the value of a public key with the identity of the person or service that holds the corresponding private key into a digitally signed statement. Certificates are issued and signed by a certification authority (CA). The entity receiving this certificate from a CA is the subject of that certificate. Instead of requiring each participant in an application to authenticate every user, third-party authentication relies on the use of digital certificates.


The digital certificate itself contains various types of information. For example, both the certification authority and the certificate owner information are included. The following list describes all the information included in the certificate:


        The certificate owner information, which is represented by a unique identi-fier known as the distinguished name (DN) of the owner. This includes the owner’s name, as well as the owner’s organization and other information about the owner.


        The certificate also includes the public key of the owner.


        The date of issue of the certificate is also included.


        The validity period is specified by ‘Valid From’ and ‘Valid To’ dates, which are included in each certificate.


        Issuer identifier information is included in the certificate.


        Finally, the digital signature of the issuing CA for the certificate is included. All the information listed is encoded through a message-digest function, which creates the digital signature. The digital signature basically certifies that the association between the certificate owner and public key is valid.

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