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Lesson 3: Encrypting and Decrypting Data
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5. Retrieve the KeyedHashAlgorithm.Hash byte array, which contains the hash value. The following console application demonstrates how to create a HMACSHA1 hash by calculating the hash of the file specified in args[1] by using a password specified in args[0] to generate a secret key:
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' VB Sub Main(ByVal args As String()) ' Step 1: Create a secret key Dim saltValueBytes As Byte() = System.Text.Encoding.ASCII.GetBytes("This is my sa1t") Dim key As Rfc2898DeriveBytes = _ New Rfc2898DeriveBytes(args(0), saltValueBytes) Dim secretKey As Byte() = key.GetBytes(16) ' Step 2: Create the hash algorithm object Dim myHash As HMACSHA1 = New HMACSHA1(secretKey) ' Step 3: Store the data to be hashed in a byte array Dim file As FileStream = _ New FileStream(args(1), FileMode.Open, FileAccess.Read) Dim reader As BinaryReader = New BinaryReader(file) ' Step 4: Call the HashAlgorithm.ComputeHash method myHash.ComputeHash(reader.ReadBytes(CType(file.Length, Integer))) ' Step 5: Retrieve the HashAlgorithm.Hash byte array Console.WriteLine(System.Convert.ToBase64String(myHash.Hash)) End Sub // C# byte[] saltValueBytes = Encoding.ASCII.GetBytes("This is my sa1t"); Rfc2898DeriveBytes passwordKey = new Rfc2898DeriveBytes(args[0], saltValueBytes); byte[] secretKey = passwordKey.GetBytes(16); // Step 2: Create the hash algorithm object HMACSHA1 myHash = new HMACSHA1(secretKey); // Step 3: Store the data to be hashed in a byte array FileStream file = new FileStream(args[1], FileMode.Open, FileAccess.Read); BinaryReader reader = new BinaryReader(file); // Step 4: Call the HashAlgorithm.ComputeHash method myHash.ComputeHash(reader.ReadBytes((int)file.Length)); // Step 5: Retrieve the HashAlgorithm.Hash byte array Console.WriteLine(Convert.ToBase64String(myHash.Hash));
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If either the file contents or the password changes, the computed hash will also change. This ensures that both the sender and recipient used the same password to
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generate the hash, which prevents an attacker from modifying the hash. Consider the following console output, which creates a new text file, computes the hash repeatedly, and then modifies the file. After either the file or the password (and key) is modified, the hash also changes:
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C:\>echo Hello, World! > HashThis.txt C:\>KeyedHashExample SomePassword HashThis.txt t04kYA9Z2ki+JbzUqe7llE6EjN4= C:\>KeyedHashExample SomePassword HashThis.txt t04kYA9Z2ki+JbzUqe7llE6EjN4= C:\>KeyedHashExample NotSomePassword HashThis.txt TFNPh9TspBobOvixy1yJ0fX/+vo= C:\>echo Hello, again. >> HashThis.txt C:\>KeyedHashExample SomePassword HashThis.txt yW6K6G7diJEV3bV2nNttgtcCM0o=
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Either HMACSHA1 or MACTripleDES can be used for the previous example. However, whereas HMACSHA1 accepts a secret key of any length, MACTripleDES accepts only secret keys of 8, 16, or 24 bytes.
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A digital signature is a value that can be appended to electronic data to prove that it was created by someone who possesses a specific private key. Public-key algorithms can also be used to form digital signatures. Digital signatures authenticate the identity of a sender (if you trust the sender's public key) and help protect the integrity of data. A signature can be verified by anyone because the sender s public key can be publicly accessible and is typically included in the digital signature format.
IMPORTANT The difference between digital signatures and encryption
Digital signatures do not protect the secrecy of the data being signed. To protect the secrecy of the file, you must encrypt it.
Digital Signature Classes in the .NET Framework
The .NET Framework provides two classes for generating and verifying digital signatures: DSACryptoServiceProvider and RSACryptoServiceProvider. These classes use different
Lesson 3: Encrypting and Decrypting Data
algorithms but provide similar functionality. Each implements the following four methods for use with digital signatures:
SignHash Generates a digital signature based on the hash of a file. SignData Generates a digital signature by first generating the hash for a file, and
then generating a signature based on the hash.
VerifyHash
Verifies a digital signature based on the hash of a file.
VerifyData Verifies a digital signature given the entire file s contents.
Digital signatures provide separate methods for signing and verifying data, whereas hashes do not provide separate methods for verification. The reason that hash algorithms do not need a separate method for signing and verifying is that the recipient can easily re-create the hash and then compare the hash she generated with the hash the sender provided. However, digital signatures use asymmetric encryption. Therefore, the recipient cannot regenerate the signature without the sender s private key, although the signature can be verified by using the sender s public key. The VerifyData and VerifyHash methods use the public sender s public key; the SignData and SignHash methods use the sender s private key.
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