Introduction+to+Computer+Security_Part9 - 19. Cryptography...

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19. Cryptography 229 decrypt the data. When public key cryptography is used for encryption, any party may use any other party's public key to encrypt a message; however, only the party with the corresponding private key can decrypt, and thus read, the message. Since secret key encryption is typically much faster, it is normally used for encrypting larger amounts of data. 19.2.2 Integrity In computer systems, it is not always possible for humans to scan information to determine if data has been erased, added, or modified. Even if scanning were possible, the individual may have no way of knowing what the correct data should be. For example, "do" may be changed to "do not," or $1,000 may be changed to $10,000. It is therefore desirable to have an automated means of detecting both intentional and unintentional modifications of data. While error detecting codes have long been used in communications protocols (e.g., parity bits), these are more effective in detecting (and correcting) unintentional modifications. They can be defeated by adversaries. Cryptography can effectively detect both intentional and unintentional modification; however, cryptography does not protect files from being modified. Both secret key and public key cryptography can be used to ensure integrity. Although newer public key methods may offer more flexibility than the older secret key method, secret key integrity verification systems have been successfully integrated into many applications. When secret key cryptography is used, a message authentication code (MAC) is calculated from and appended to the data. To verify that the data has not been modified at a later time, any party with access to the correct secret key can recalculate the MAC. The new MAC is compared with the original MAC, and if they are identical, the verifier has confidence that the data has not been modified by an unauthorized party. FIPS 113, Computer Data Authentication , specifies a standard technique for calculating a MAC for integrity verification. Public key cryptography verifies integrity by using of public key signatures and secure hashes. A secure hash algorithm is used to create a message digest. The message digest, called a hash, is a
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IV. Technical Controls Sometimes a secure hash is used for integrity verification. However, this can be defeated if the hash is not 136 stored in a secure location, since it may be possible for someone to change the message and then replace the old hash with a new one based on the modified message. Electronic signatures rely on the secrecy of the keys and the link or binding between the owner of the key 137 and the key itself. If a key is compromised (by theft, coercion, or trickery), then the electronic originator of a message may not be the same as the owner of the key. Although the binding of cryptographic keys to actual people is a significant problem, it does not necessarily make electronic signatures less secure than written signatures. Trickery and coercion are problems for written signatures as well. In addition, written signatures are
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Introduction+to+Computer+Security_Part9 - 19. Cryptography...

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