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We ve described a message digest as the foundation of a pseudo-random number generator or password-based encryption, and now as a representative of a larger message. Another use for a message digest is to check data integrity, which is the term used to describe what the HMAC does. If you re concerned that the information may be altered, you send the data along with a check. If the message was altered, the check will also be different. Of course, you must ensure that the check value cannot be altered to match any changes in the message. If the check value shows no alterations, the data has been shown to have integrity. Integrity is a word for honest, sound, and steadfast. When used in relationship to data, it may seem pretentious, but it does describe data that you can count on, at least in terms of its authenticity.
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In our example, the HMAC seems to serve as a signature. Daniel can know that the data came from Pao-Chi and that no one tampered with it in transit. But HMAC has some shortcomings. The first is the statement, Daniel can know that the data came from Pao-Chi. Maybe he can know it came from Pao-Chi, but can anyone else After all, to verify that the data came from Pao-Chi, the recipient must know what the key is to create the appropriate HMAC. Daniel knows what the shared secret key is, but no one else does. Daniel could write a bogus message (say, setting the number of presses to eight) and create the correct HMAC. So from anyone else s point of view, the message may have come from Pao-Chi or Daniel; no one else can know for sure who signed it. The second drawback is that for someone other than Pao-Chi or Daniel to verify the signature, the correspondents must reveal the secret key. Now this third party has access to the key and can also create messages that appear genuine. Usually, HMACs are used only to verify that contents have not been altered in transit. They are meant to be used as an on-the-fly check and not as a permanent record. For that reason, you need another way to create unique, verifiable signatures, and that way is to encrypt the digest with the signer s private RSA key. It works like this. Pao-Chi digests the message and then encrypts the digest with his private key. He sends Daniel the message along with the encrypted digest, which serves as the signature. Daniel separates the two components and digests the message he received. He has a message in his possession and knows the digest that will produce it (he just computed it). He must determine whether the message he now has is the same message Pao-Chi sent. If Daniel knew what Pao-Chi computed as a digest, he could make that determination. Well, he has Pao-Chi s digest it s the signature. So Daniel uses Pao-Chi s public key to decrypt the signature. That s the value Pao-Chi signed (see Figure 5-10). Is it the same answer Daniel got If it is, he knows that the data was not altered in transit and that Pao-Chi is vouching for the contents. Notice something powerful about the digital signature: Each chunk of data has its own signature. This means that no single digital signature is associated with an individual or key pair. Each signature is unique to the data signed and the keys used. When an individual signs two messages with the same key, the signatures will be different. Moreover, when two people with different keys sign the same data, they will produce different signatures. As a result, someone cannot take a valid signature and append
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