Cryptography in the database: The last line of defense

By Kevin Kenan 07 Dec 2005 | SearchOracle.com

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The following is an excerpt from the new book Cryptography in the Database: The Last Line of Defense (2006) by Kevin Keenan and published by Addison-Wesley. Kevin Kenan presents a start-to-finish blueprint and execution plan for designing and building—or selecting and integrating—a complete database cryptosystem. Kenan systematically shows how to eliminate weaknesses, overcome pitfalls, and defend against attacks that can compromise data even if it's been protected by strong encryption. For more information or to purchase the book, click here.

What is cryptography?

Cryptography is the art of "extreme information security." It is extreme in the sense that once treated with a cryptographic algorithm, a message (or a database field) is expected to remain secure even if the adversary has full access to the treated message. The adversary may even know which algorithm was used. If the cryptography is good, the message will remain secure.

This is in contrast to most information security techniques, which are designed to keep adversaries away from the information. Most security mechanisms prevent access and often have complicated procedures to allow access to only authorized users. Cryptography assumes that the adversary has full access to the message and still provides unbroken security. That is extreme security.

A more popular conception of cryptography characterizes it as the science of "scrambling" data. Cryptographers invent algorithms that take input data, called plaintext, and produce scrambled output. Scrambling, used in this sense, is much more than just moving letters around or exchanging some letters for others. After a proper cryptographic scrambling, the output is typically indistinguishable from a random string of data. For instance, a cryptographic function might turn "Hello, whirled!" into 0x397B3AF517B6892C.

While simply turning a message into a random sequence of bits may not seem useful, you'll soon see that cryptographic hashes, as such functions are known, are very important to modern computer security. Cryptography, though, offers much more.

Many cryptographic algorithms, but not all, are easily reversible if you know a particular secret. Armed with that secret, a recipient could turn 0x397B3AF517B6892C back into "Hello, whirled!" Anyone who did not know the secret would not be able to recover the original data. Such reversible algorithms are known as ciphers, and the scrambled output of a cipher is ciphertext. The secret used to unscramble ciphertext is called a key. Generally, the key is used for both scrambling, called encryption, and unscrambling, called decryption. A fundamental principle in cryptography, Kerckhoffs' Principle, states that the security of a cipher should depend only on keeping the key secret. Even if everything else about the cipher is known, so long as the key remains secret, the plaintext should not be recoverable from the ciphertext.

The opposite of Kerckhoffs' Principle is security through obscurity. Any cryptographic system where the cipher is kept secret depends on security through obscurity. Given the difficulty that even professional cryptographers have in designing robust and efficient encryption systems, the likelihood of a secret cipher providing better security than any of the well-known and tested ciphers is vanishingly small. Plus, modern decompilers, disassemblers, debuggers, and other reverse-engineering tools ensure that any secret cipher likely won't remain secret for long.

Cryptographic algorithms can be broadly grouped into three categories: symmetric cryptography, asymmetric (or public-key) cryptography, and cryptographic hashing. Each of these types has a part to play in most cryptographic systems, and we next consider each of them in turn.

Symmetric Cryptography

Symmetric key cryptography is so named because the cipher uses the same key for both encryption and decryption. Two famous ciphers, Data Encryption Standard (DES) and Advanced Encryption Standard (AES), both use symmetric keys. Because symmetric key ciphers are generally much faster than public-key ciphers, they are suitable for encrypting small and large data items.

Modern symmetric ciphers come in two flavors. Block ciphers encrypt a chunk of several bits all at once, while stream ciphers generally encrypt one bit at a time as the data stream flows past. When a block cipher must encrypt data longer than the block size, the data is first broken into blocks of the appropriate size, and then the encryption algorithm is applied to each. Several modes exist that specify how each block is handled. The modes enable an algorithm to be used securely in a variety of situations. By selecting an appropriate mode, for instance, a block cipher can even be used as stream cipher.

The chief advantage of a stream cipher for database cryptography is that the need for padding is avoided. Given that block ciphers operate on a fixed block size, any blocks of data smaller than that size must be padded. Stream ciphers avoid this, and when the data stream ends, the encryption ends.We'll return to block and stream ciphers in the algorithm discussion in Chapter 4 "Cryptographic Engines and Algorithms."

The primary drawback of symmetric key ciphers is key management. Because the same key is used for both encryption and decryption, the key must be distributed to every entity that needs to work with the data. Should an adversary obtain the key, not only is the confidentiality of the data compromised, but integrity is also threatened given that the key can be used to encrypt as well as decrypt. The risks posed by losing control of the key make distributing and storing the key difficult. How can the key be moved securely to all the entities that need to decrypt the data? Encrypting the key for transmission would make sense, but what key would be used to encrypt the key, and how would you get the key-encrypting key to the destination?

Once the key is at the decryption location, how should it be secured so that an attacker can't steal it? Again, encryption offers a tempting solution, but then you face the problem of securing the key used to encrypt the original key. We'll look at these problems in more detail in Chapter 5 "Keys: Vaults, Manifests, and Managers." In terms of the key distribution problem, cryptographers have devised an elegant solution using public-key cryptography, which we examine next.

To read the rest of the chapter, click here.

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