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1.4. Confidentiality
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The goal of confidentiality is to keep the contents of a transient communication or data on temporary or persistent storage secret. If Alice and Bob want to exchange some information that they do not want Eve to see, the challenge is to make sure that Eve is not able to understand that information, even if Eve can see the bits that are being transferred over the network. Suppose Eve is an eavesdropper who may be able to listen in on the contents of Alice and Bob s secret conversations. If Alice and Bob are communicating over a network, then Eve is able to see the bits the zeros and ones that make up Alice and Bob s conversation go back and forth over the wires (or over the air, in the case Alice and Bob are using a wireless network). A real-world Eve might employ various existing software tools to eavesdrop. On an Ethernet network that uses a hub (as opposed to a switch), for instance, each computer is capable of actually seeing all the network traffic that is generated and received by any other computer. A computer s operating system is typically responsible for only allowing applications running on that computer to access traffic that is directed to or from that computer, and filtering out traffic that originates or is destined for other computers on the same network. However, if a user has root or administrator privileges on a computer, that user can use a software package such as Ethereal, tcpdump, or dsniff to access network traffic. These software packages are run in a promiscuous mode, in which the operating system provides the software access to all traffic on the network instead of providing filtered traffic that is just directed to or from the computer on which it is running. While such packages exist to help network administrators and engineers debug problems, they can be used for eavesdropping. Attackers may not have administrator privileges, but can obtain them by first getting access to some account, and then exploiting software vulnerabilities in the operating system to gain such privileges. Usually, some kind of encryption technology is used to achieve confidentiality. Most encryption technologies use a key to encrypt the communication between Alice and Bob. A key is a secret sequence of bits that Alice and Bob know (or share) that is not known to potential attackers.4 A key may be derived from a password that is known to both Alice and Bob. An encryption algorithm will take the key as input, in addition to the message that Alice wants to transfer to Bob, and will scramble the message in a way that is mathematically dependent on the key. The message is scrambled such that when Eve sees the scrambled communication, she will not be able to understand its contents. Bob can use the key to unscramble the message by computing the mathematical inverse of the encryption algorithm. If Alice and Bob use good encryption technology and keep the key secret, then Eve will not be able to understand their communication.
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4. In this chapter, we use the term key to refer to a secret key. In some encryption schemes (covered in 13), some keys can be made public.
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CHAPTER 1 s SECURITY GOALS
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1.5. Message/Data Integrity
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When Alice and Bob exchange messages, they do not want a third party such as Mallory to be able to modify the contents of their messages. Mallory has capabilities similar to Eve, but Eve is a passive eavesdropper while Mallory is an active eavesdropper. Though Eve is able to see the zeros and ones go by, she is unable to modify them. Eve therefore cannot modify any part of the conversation. On the other hand, Mallory has the ability to modify, inject, or delete the zeros and ones, and thus change the contents of the conversation a potentially more significant kind of attack. Mallory is sometimes referred to as a man in the middle. Alice and Bob can use an integrity check to detect if an active eavesdropper like Mallory has modified the messages in an attempt to corrupt or disrupt their conversation. That is, Alice and Bob want to protect the message integrity of their conversation. One approach that they can take to ensure message integrity is to add redundancy to their messages. Consider a hypothetical scenario in which Alice wants to send an I owe you (IOU) message such as I, Alice, owe you, Bob, $1.00, and Mallory has the ability to change only one character in the message. If Mallory wants Alice to be in more debt to Bob, she could change the message to I, Alice, owe you, Bob, $1000 by changing the dot to a zero. On the other hand, if Mallory wants to cheat Bob out of his dollar, she could change the message to I, Alice, owe you, Bob, $0.00. Assuming Mallory can only change a single character in a message, Alice could add redundancy to her message by repeating the dollar amount twice so that Bob could detect tampering. For example, if Alice sends the message I, Alice, owe you, Bob, $1.00. Confirm, $1.00, then Mallory would not be able to change both of the dollar values in the message, and Bob would be able to detect tampering by Mallory. If Mallory changes one of the amounts in the message, Bob will see a mismatch between the two dollar amounts and discard the message. In this manner, redundancy can be used to provide message integrity. While Mallory may not be able to tamper with Alice s IOU if she uses redundancy, she may still be able to conduct a denial-of-service attack. If Mallory changes one of the dollar amounts in the IOU each time Alice tries to send it to Bob, and Bob is forced to discard the message each time because of the mismatched dollar amounts, Bob will never receive the IOU he rightly deserves! (Denial-of-service attacks are discussed further in Section 1.7.) Unfortunately, a real-world active eavesdropper will typically have the power to change much more than a single character in a message, and the simple approach of repeating the dollar amount will not work. In addition, repeating information more than once requires extra communications bandwidth and is not terribly efficient. In networking communications protocols, approaches such as CRCs (cyclic redundancy checks) can be used to achieve integrity and detect when bits in a message have been lost or altered due to inadvertent communications failures. These techniques compute short codes that are functions of the message being sent. Alice can attach a short code to the message such that if the message or code are modified, Bob can determine whether they were tampered with. However, while CRCs are sufficient to detect inadvertent communications failures, they are typically not good enough to deal with adversaries such as Mallory. If Mallory knows that a CRC is being used, and she has no restrictions on how many bytes she can modify, she can also change the short code to match her modified message.
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