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MAC stands for message authentication checksum (or message authentication code), and H stands for hash or hash-based function, so an HMAC
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If the data does not match the digest, you know that something went wrong
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The Digital Signature
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(pronounced aitch mac ) is a hash-based message authentication algorithm. A checksum is an algorithm that checks data by summing it. Suppose you had a column of numbers (say, in an accountant s ledger). If the correct numbers are there, the sum of the column is a specific value. Later, to check that the ledger is still correct, you don t compare each number individually; rather, you find the sum of the column. If the second sum matches the first sum, the check passes. Of course, if someone can change one number, it s easy also to change the sum at the bottom of the ledger so that it matches the change in the single number. It would also be easy to change another number in the column to offset the first change. A MAC is a way to detect changes in the data or in the sum. To detect changes in the data, a MAC can be based on a digest, block cipher, or stream cipher (see 2). To detect changes in the actual checksum, the MAC uses a key. Most HMACs work this way. Two parties share a secret key ( 4 shows how that s done), and then each digest the key and message. The digest depends on the message and the key, so an attacker would have to know what the key is to alter the message and attach a correct checksum. For example, suppose Pao-Chi sends Daniel message 1 shown earlier (the message instructing him to ship four units to Satomi). Pao-Chi uses an HMAC so that Daniel can verify that the data did not change. Using a key exchange algorithm (RSA, DH, ECDH), the two agree on a 128-bit key. Pao-Chi uses SHA-1 to digest the key and message as one chunk of data. The result is as follows. (The two vertical lines || indicate concatenation; see also Figure 5-8.)
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Pao-Chi s HMAC result (SHA-1 digest of key || message 1): 60 c4 65 a8 a4 9d 35 6a 68 36 f8 f0 56 3d d2 7f 7e 26 35 b2
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We haven t told you what the key is, so you can t verify that the result we present is the actual result of an HMAC. If you want to know what the key is, you can figure it out. Put together a chunk of data a key candidate followed by the message and then digest it. Is it the same result given here No Try another key, and another, and so on until you find the correct one. It s a 128-bit key. Now Pao-Chi sends Daniel the message and the HMAC result together. Suppose that Satomi intercepts the transmission and tries to get Daniel
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Figure 5-8
5
The HMAC algorithm digests the key and the data (in that order) to produce a value
to ship five presses instead of four by substituting message 2 for PaoChi s. After replacing the message, she sends it to Daniel. If she failed to replace the HMAC result, Daniel would digest the key and fake message and get the following (see Figure 5-9).
Daniel s HMAC result (SHA1 digest of key || message 2): a8 32 3b 8d f3 6b 3e e1 08 bb 6b 0b f0 cc a5 5b 26 d4 d1 41
Figure 5-9 Daniel digests the correct key but the wrong message, so he knows that something is wrong
The Digital Signature
The digested message is not the same as Pao-Chi s. (Daniel knows what Pao-Chi got for an HMAC; that s part of the message.) So Daniel knows that what Pao-Chi digested and what he digested are not the same. Something maybe the key, maybe the actual message, maybe even the HMAC value was changed. Daniel doesn t know exactly what was changed, but that doesn t matter. He knows something went wrong. He contacts PaoChi again, and they start over. Another possibility is for Satomi to substitute message 2 for message 1 and substitute the HMAC. But the problem is that Satomi can t know what the correct HMAC value should be. To demonstrate this, suppose Satomi substitutes six presses for four presses. Here s the SHA-1 digest.
Daniel, I sold 6 presses to Satomi. Ship immediately. SHA-1 digest: 66 05 40 8c 24 6e 05 f8 00 20 f4 72 14 08 bc 22 53 b2 eb d2
If Satomi substitutes this digest, Daniel will still know something is wrong because that s not the value he s going to get. He s not digesting the message; rather, he s digesting the key and the message. So what should Satomi use
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