Programming .NET Security : Cryptography Explained (part 2)

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2. Integrity

Integrity becomes an issue when Alice wants to send a message to Bob but is concerned that Eve will tamper with the message and change the contents. In this case, Alice does not care if Eve can read the message—she only wants to make sure that Bob can detect any changes made by Eve.

Alice takes the message that she has written and creates a cryptographic hash code, which is a numeric code based on the contents of the message that acts as a "fingerprint" to identify the message. Changing even the smallest portion of the message results in a different hash code. Alice sends the message and the hash code to Bob.

Bob takes the message that he has received and creates his own hash code using the same technique that Alice used. If his code matches the one that Alice sent, Bob can assume that Eve has not modified the message—if the hash codes do not match, then Bob can assume that Eve has altered the message en route. Figure 4 illustrates the use of hash codes.

Figure 4. Alice and Bob use hash codes to check the integrity of their messages

Eve will be able to change the message without detection if she changes the hash code as well. Eve creates a new message and a new hash code, both of which she sends to Bob. When Bob creates a hash code for the message, it will match the hash code he has received, leading him to think that the message has not been tampered with.

The solution to this problem is to use a "keyed" hash code, which uses the contents of the message and a secret key to create the hash code, as illustrated in Figure 5. Eve can still modify the message, but she can no longer create a valid hash code, because she lacks the key used to create the original code.

Figure 5. Keyed hash codes are created using the contents of the message and a secret key.

Unless Eve is able to discover the key, she will be unable to create hash codes that will fool Bob; if Eve acquires the key, she will be able to generate valid hash codes for her own messages, rendering Alice and Bob's efforts to ensure message integrity useless. As with confidentiality, the issue of integrity becomes one of protecting secret keys from Eve.

3. Authentication

The goal of authentication is to allow Bob to establish that Alice is the author of a message. For our purposes, this means that Alice should be able to create a "digital signature" for the message and that Bob should be able to check the signature to ensure that it is valid. In addition, Eve should not be able to create and sign messages as though she were Alice.

Digital signatures rely on asymmetric encryption techniques, although they are applied differently than we discussed earlier. Alice creates a pair of keys, one of which she makes public and one of which she keeps private. To sign the message, Alice creates a cryptographic hash code of the message that she wants to send to Bob, as discussed earlier. Alice then signs the hash code using her private key, as shown in Figure 6. This creates a digital signature that is unique to the combination of the document and Alice's private key.

Figure 6. Alice uses public key cryptography to create a digital signature

When Bob receives the message, he verifies the signature using Alice's public key. If the signature is valid, then Alice has "signed" the message, and Bob can assume that Eve has not forged the message.

Because digitally signing a document requires the creation of a hash code, it also addresses the issue of integrity that we discussed earlier. If Eve modifies the message that Alice has sent, then Alice's signature will not be valid. Bob will generate his own hash code for the message, and it will not be the same as the one Alice signed. Figure 7 shows the protocol for message signatures.

Figure 7. Alice uses a digital signature to authenticate a message

Eve is unable to forge messages from Alice, because she does not know Alice's secret key and is unable to create digital signatures that will fool Bob. However, as with the areas of confidentiality and integrity, if Eve is able to acquire the secret key, she will be able to sign messages as though she were Alice.

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