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Figure 11-8 Decrypting a message
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If Melissa wants Mike to send encrypted e-mail to her, she must generate her own key pair and send Mike the public key In a typical public-key cryptography setup, everyone has their own private key plus a copy of the public keys for anyone with whom they wish to communicate securely (Figure 11-9)
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Figure 11-9
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Lots of keys
The only problem with all these keys is the chance that someone pretending to be someone else might pass out a public key Thus, there s a strong desire by the recipients to know who is passing out a key This issue falls under the banner of nonrepudiation
Nonrepudiation
Within networking, nonrepudiation simply means that the receiver of information has a very high degree of confidence that the sender of a piece of information truly is who the receiver thinks it should be Nonrepudiation takes place all over a network Is this truly the person who sent in the user name and password to log into my Windows domain Is this really the eBaycom Web site I m entering my credit card number into Did this public key really come from Mike Meyers As a result, nonrepudiation comes in a number of forms, but most of them use a very clever little bit of mathematical magic called a hash
11: Securing TCP/IP
Hash
In computer security, a hash (or more accurately, a cryptographic hash function) is a mathematical function that you run on a string of binary digits of any length that results in a value of some fixed length (often called a checksum or a digest) A cryptographic hash function is a one-way function One-way means the hash is practically irreversible You should not be able to re-create the data even if you know the hashing algorithm and the checksum A cryptographic hash function should also have a unique checksum for any two different input streams (Figure 11-10)
Figure 11-10 A hash at work
Hash
Cryptographic hash functions have a huge number of uses, but one of the most common is for files Let s say I m sharing a file on my Web site I m worried that an evil hacker might alter that file, so I run a hash on the file and supply you with both the file and the checksum Message-Digest algorithm version 5 everybody just calls it MD5 is arguably the most popular hashing function for this type of work Figure 11-11 shows an example of this, a program called NetMD5
Figure 11-11 File and MD5
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This is only one example of how to use hashes It s also about the only example in which you actually see hashes at work MD5 is a very popular cryptographic hash, but it s not the only one The other hash you ll see from time to time is called Secure Hash Algorithm (SHA) There are two versions of SHA: SHA-1 and SHA-2 Many encryption and authentication schemes also use hashes Granted, you won t actually see the hashes as they re used, but trust me: hashes are everywhere For example, some SMTP servers use a special form of MD5, called Challenge-Response Authentication Mechanism-Message Digest 5 (CRAM-MD5), as a tool for server authentication (See the discussion of CHAP later in this chapter for details on how challenge-response works) Now that you understand hashes, let s return to public-key cryptography and see how digital signatures make public-key cryptography even more secure EXAM TIP Look for CRAM-MD5 to show up on the CompTIA Network+ exam as a tool for server authentication
Digital Signatures
As mentioned earlier, public-key cryptography suffers from the risk that you might be getting a message or a public key from someone who isn t who they say they are To avoid this problem we add a digital signature A digital signature is another string of ones and zeroes that can only be generated by the sender, usually by doing something mathematically complex (part of the algorithms always include some hashing) to the message and the private key The person with the matching public key does something to the digital signature using the public key to verify it came from the intended sender Digital signatures are very popular with e-mail users Figure 11-12 shows an e-mail message both encrypted and digitally signed in Mozilla Thunderbird using a special Thunderbird add-on called OpenPGP You ll read more about the PGP family of authentication/encryption tools later in this chapter
Digital signatures are great, but what happens when you want to do business with someone you do not know Before you enter a credit card number to buy that new USB3 Blu-ray Disc player, wouldn t you like to know that the Web site you are doing business with truly is eBay To address that need, the industry came up with the idea of certificates A certificate is a standardized type of digital signature that usually includes the digital signature of a third party, a person or a company that guarantees that who is passing out this certificate truly is who they say they are As you might imagine, certificates are incredibly common with secure Web pages When you go to eBay to sign in, your browser redirects to a secure Web page These are easy to identify by the lock icon at the bottom of the screen or in the address bar (Figure 11-13) or the https:// used (instead of http://) in the address bar EXAM TIP If you see https:// or a small lock icon, you are most likely on a secure Web site
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