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Symmetric-Key Cryptography
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If you want to encrypt something, follow these steps. 1. Select a symmetric algorithm and a PRNG. You should choose an encryption scheme that is not susceptible to attacks on the algorithm. It should also allow key sizes big enough to thwart a brute-force attack. If you need to reuse your cryptographic keys, choose a block cipher. If you need to guarantee interoperability with other cryptographic programs or products, choose AES. Otherwise, you might want to choose a stream cipher for performance reasons. 2. Collect your seed value and feed it to the PRNG. Make sure that your seed contains enough entropy to thwart a brute-force attack. It s best to combine several seeds, including user input. 3. Using the PRNG, generate a key. Choose a key size that requires a brute-force attack that is so time-consuming that it is unfeasible. Currently, the most popular key size is 128 bits. 4. Apply the symmetric algorithm, which will work with the key to encrypt your plaintext. 5. Save and protect your key. The next chapter talks about how to protect keys. To recover the data you encrypted, follow these steps. 1. Retrieve your key. 2. Apply the symmetric algorithm, which will work with the key to decrypt your plaintext.
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Real-World Example: Oracle Databases
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How do people and companies use symmetric-key cryptography today Here is one example. Most companies store volumes of sensitive information in databases. A database is a software package that stores data in a systematic way and enables users to easily and quickly find what they re looking for. For example, a company may have personnel files containing names, addresses, salaries, and Social Security numbers of all employees.
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A hospital may keep medical records of hundreds of patients. An e-commerce company might store credit card numbers and customers purchasing histories. The owners of the databases may want to make sure that only the appropriate people have access to the information. One way to protect the data is to encrypt it. If attackers break into the database, they still can t read the sensitive material. Oracle sells a database product, Oracle 8i, release 8.1.6, that comes with an encryption package. If you are a developer using the database, and you want to encrypt the elements before storing them, you generate some random or pseudo-random bytes to be used as the key and then call on the package to perform the encryption. The calls to the encryption function are PL/SQL, which are standard database language conventions. For instance, to encrypt the data, you would add a line of code that looks something like this.
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dbms_obfuscation_toolkit.DESEncrypt(input_string => plaintext, key => keyData, encrypted_string => ciphertext);
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And that s it. Well, you also need to save the key somewhere (not in the same location). The next chapter talks about how to do that. If your application was using SQL, it would now have the opportunity to store the data in the clear (plaintext) or encrypted (ciphertext). This line shows that you are using DES, but Triple DES is also available. When your program needs to retrieve data, you recall it from the database, recover your key, and make something like the following call:
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dbms_obfuscation_toolkit.DESDecrypt(input_string => ciphertext, key => keyData, decrypted_string => plaintext);
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Thanks to Mary Ann Davidson and Kristy Browder of Oracle for providing this example.
CHAPTER
Symmetric-Key Management
Symmetric-key encryption can keep your secrets safe, but because you need your keys to recover encrypted data, you must also keep them safe. The process of keeping all your keys safe and available for use is known as key management. This chapter is about managing symmetric keys. In 2, Symmetric-Key Cryptography, Pao-Chi generated a random or pseudo-random key, and used it to encrypt data. If he wants to decrypt the data, he must use the same key. This means he has to either memorize the key or store it somewhere. Memorizing it isn t practical, so he must store it so that he can recall it when he wants to, but no one else can. Right now you re probably asking, If there s some place Pao-Chi can keep his key safe, why doesn t he just put his sensitive information there as well The answer is that it s easier to protect a small key than many megabytes worth of information. In fact, some of the key storage solutions you ll see in this chapter are small devices designed in part to protect keys. So the idea is to use symmetric-key crypto to protect the megabytes of information and some other technique to protect the 16 bytes (or so) of keys.
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