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HMACSHA384 Class

Updated: May 2011

Computes a Hash-based Message Authentication Code (HMAC) using the SHA384 hash function.

Namespace:  System.Security.Cryptography
Assembly:  mscorlib (in mscorlib.dll)

[ComVisibleAttribute(true)]
public class HMACSHA384 : HMAC

HMACSHA384 is a type of keyed hash algorithm that is constructed from the SHA-384 hash function and used as a Hash-based Message Authentication Code (HMAC). The HMAC process mixes a secret key with the message data and hashes the result. The hash value is mixed with the secret key again, and then hashed a second time. The output hash is 384 bits long.

An HMAC can be used to determine whether a message sent over a nonsecure channel has been tampered with, provided that the sender and receiver share a secret key. The sender computes the hash value for the original data and sends both the original data and the hash value as a single message. The receiver recalculates the hash value on the received message and checks that the computed HMAC matches the transmitted HMAC.

If the original and computed hash values match, the message is authenticated. If they do not match, either the data or the hash value has been changed. HMACs provide security against tampering because knowledge of the secret key is required to change the message and reproduce the correct hash value.

HMACSHA384 accepts all key sizes and produces a hash sequence that is 384 bits long.

.NET Framework 2.0 Considerations

In the .NET Framework version 2.0, the HMACSHA384 class produced results that were not consistent with other implementations of HMAC-SHA-384. The .NET Framework version 2.0 Service Pack 1 updates this class. However, the HMAC values it produces are inconsistent with the output of the .NET Framework 2.0 implementation of the class. To enable .NET Framework 2.0 SP1 applications to interact with .NET Framework 2.0 applications, the .NET Framework 2.0 SP1 introduces the following four changes to the HMACSHA384 class:

  • The ProduceLegacyHmacValues Boolean property supports the earlier implementation. When you set this property to true, the HMACSHA384 object produces values that match the values produced by the .NET Framework 2.0.

  • In some applications, it may be expensive or difficult to change the code. For these situations, .NET Framework 2.0 SP1 provides a configuration switch, legacyHMACMode, for the application’s .config file. This switch causes all HMAC objects created in the application to use the .NET Framework 2.0 calculation.

    <configuration>
        <runtime>
        <legacyHMACMode enabled="1" />
        </runtime>
    </configuration>
    
  • To help debug any issues that arise when upgrading to the .NET Framework 2.0 SP1, the first time an instance of the HMACSHA384 class is created, a warning about the implementation changes is sent to the event log and to any attached debugger. If you set the legacyHMACMode configuration switch to use the .NET Framework 2.0 calculation, this message is not generated.

  • The .NET Framework 2.0 SP1 also introduces a second configuration switch, legacyHMACWarning, that lets you manually suppress the warning message for your application.

    <configuration>
        <runtime>
           <legacyHMACWarning enabled="0" />
        </runtime>
    </configuration>
    

Note that these four changes affect only the HMACSHA384 and HMACSHA512 classes, and not the SHA256Managed class.

The following example shows how to sign a file by using the HMACSHA384 object, and then how to verify the file.

using System;
using System.IO;
using System.Security.Cryptography;

public class HMACSHA384example
{

    public static void Main(string[] Fileargs)
    {
        string dataFile;
        string signedFile;
        //If no file names are specified, create them. 
        if (Fileargs.Length < 2)
        {
            dataFile = @"text.txt";
            signedFile = "signedFile.enc";

            if (!File.Exists(dataFile))
            {
                // Create a file to write to. 
                using (StreamWriter sw = File.CreateText(dataFile))
                {
                    sw.WriteLine("Here is a message to sign");
                }
            }

        }
        else
        {
            dataFile = Fileargs[0];
            signedFile = Fileargs[1];
        }
        try
        {
            // Create a random key using a random number generator. This would be the 
            //  secret key shared by sender and receiver.
            byte[] secretkey = new Byte[64];
            //RNGCryptoServiceProvider is an implementation of a random number generator. 
            using (RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider())
            {
                // The array is now filled with cryptographically strong random bytes.
                rng.GetBytes(secretkey);

                // Use the secret key to sign the message file.
                SignFile(secretkey, dataFile, signedFile);

                // Verify the signed file
                VerifyFile(secretkey, signedFile);
            }
        }
        catch (IOException e)
        {
            Console.WriteLine("Error: File not found", e);
        }

    }  //end main 
    // Computes a keyed hash for a source file and creates a target file with the keyed hash 
    // prepended to the contents of the source file.  
    public static void SignFile(byte[] key, String sourceFile, String destFile)
    {
        // Initialize the keyed hash object. 
        using (HMACSHA384 hmac = new HMACSHA384(key))
        {
            using (FileStream inStream = new FileStream(sourceFile, FileMode.Open))
            {
                using (FileStream outStream = new FileStream(destFile, FileMode.Create))
                {
                    // Compute the hash of the input file.
                    byte[] hashValue = hmac.ComputeHash(inStream);
                    // Reset inStream to the beginning of the file.
                    inStream.Position = 0;
                    // Write the computed hash value to the output file.
                    outStream.Write(hashValue, 0, hashValue.Length);
                    // Copy the contents of the sourceFile to the destFile. 
                    int bytesRead;
                    // read 1K at a time
                    byte[] buffer = new byte[1024];
                    do
                    {
                        // Read from the wrapping CryptoStream.
                        bytesRead = inStream.Read(buffer, 0, 1024);
                        outStream.Write(buffer, 0, bytesRead);
                    } while (bytesRead > 0);
                }
            }
        }
        return;
    } // end SignFile 


    // Compares the key in the source file with a new key created for the data portion of the file. If the keys  
    // compare the data has not been tampered with. 
    public static bool VerifyFile(byte[] key, String sourceFile)
    {
        bool err = false;
        // Initialize the keyed hash object.  
        using (HMACSHA384 hmac = new HMACSHA384(key))
        {
            // Create an array to hold the keyed hash value read from the file.
            byte[] storedHash = new byte[hmac.HashSize / 8];
            // Create a FileStream for the source file. 
            using (FileStream inStream = new FileStream(sourceFile, FileMode.Open))
            {
                // Read in the storedHash.
                inStream.Read(storedHash, 0, storedHash.Length);
                // Compute the hash of the remaining contents of the file. 
                // The stream is properly positioned at the beginning of the content,  
                // immediately after the stored hash value.
                byte[] computedHash = hmac.ComputeHash(inStream);
                // compare the computed hash with the stored value 

                for (int i = 0; i < storedHash.Length; i++)
                {
                    if (computedHash[i] != storedHash[i])
                    {
                        err = true;
                    }
                }
            }
        }
        if (err)
        {
            Console.WriteLine("Hash values differ! Signed file has been tampered with!");
            return false;
        }
        else
        {
            Console.WriteLine("Hash values agree -- no tampering occurred.");
            return true;
        }

    } //end VerifyFile

} //end class

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

Date

History

Reason

May 2011

Updated the code example.

Information enhancement.

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