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HMACSHA512 Class

Computes a Hash-based Message Authentication Code (HMAC) using the SHA512 hash function.

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

[ComVisibleAttribute(true)] 
public class HMACSHA512 : HMAC
/** @attribute ComVisibleAttribute(true) */ 
public class HMACSHA512 extends HMAC
ComVisibleAttribute(true) 
public class HMACSHA512 extends HMAC
Not applicable.

HMACSHA512 is a type of keyed hash algorithm that is constructed from the SHA-512 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 512 bits in length.

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 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.

HMACSHA512 accepts keys of any size, and produces a hash sequence of length 512 bits.

.NET Framework 2.0 Considerations

In the Microsoft .NET Framework version 2.0, the HMACSHA512 class produced results that were not consistent with other implementations of HMAC-SHA-512. The 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 applications to interact with .NET Framework 2.0 applications, the introduces the following four changes to the HMACSHA512 class:

  • The ProduceLegacyHmacValues Boolean property supports the earlier implementation. When you set this property to true, the HMACSHA512 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, 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 , the first time an instance of the HMACSHA512 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 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 encode a file using the HMACSHA512 object and then how to decode the file.

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

public class HMACSHA512example
{
	// Computes a keyed hash for a source file, creates a target file with the keyed hash
	// prepended to the contents of the source file, then decrypts the file and compares
	// the source and the decrypted files.
	public static void EncodeFile(byte[] key, String sourceFile, String destFile)
	{
		// Initialize the keyed hash object.
		HMACSHA512 myhmacsha512 = new HMACSHA512(key);
		FileStream inStream = new FileStream(sourceFile, FileMode.Open);
		FileStream outStream = new FileStream(destFile, FileMode.Create);
		// Compute the hash of the input file.
		byte[] hashValue = myhmacsha512.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); 
		myhmacsha512.Clear();
		// Close the streams
		inStream.Close();
		outStream.Close();
		return;
	} // end EncodeFile


	// Decrypt the encoded file and compare to original file.
	public static bool DecodeFile(byte[] key, String sourceFile)
	{
		// Initialize the keyed hash object. 
		HMACSHA512 hmacsha512 = new HMACSHA512(key);
		// Create an array to hold the keyed hash value read from the file.
		byte[] storedHash = new byte[hmacsha512.HashSize/8];
		// Create a FileStream for the source file.
		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 = hmacsha512.ComputeHash(inStream);
		// compare the computed hash with the stored value
		for (int i =0; i < storedHash.Length; i++)
		{
			if (computedHash[i] != storedHash[i])
			{
				Console.WriteLine("Hash values differ! Encoded file has been tampered with!");
				return false;
			}
		}
		Console.WriteLine("Hash values agree -- no tampering occurred.");
		return true;
	} //end DecodeFile

	private const string usageText = "Usage: HMACSHA512 inputfile.txt encryptedfile.hsh\nYou must specify the two file names. Only the first file must exist.\n";
	public static void Main(string[] Fileargs)
	{
		//If no file names are specified, write usage text.
		if (Fileargs.Length < 2)
		{
			Console.WriteLine(usageText);
		}
		else
		{
			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.
				RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
				// The array is now filled with cryptographically strong random bytes.
				rng.GetBytes(secretkey); 

				// Use the secret key to encode the message file.
				EncodeFile(secretkey, Fileargs[0], Fileargs[1]);

				// Take the encoded file and decode
				DecodeFile(secretkey, Fileargs[1]);
			}
			catch (IOException e)
			{
				Console.WriteLine("Error: File not found",e);
			}
		} //end if-else

	}  //end main
} //end class

import System.*;
import System.IO.*;
import System.Security.Cryptography.*;

public class HMACSHA512Example
{
    // Computes a keyed hash for a source file, creates a target file with the 
    // keyed hash prepended to the contents of the source file, then decrypts 
    // the file and compares the source and the decrypted files.
    public static void EncodeFile(ubyte key[], String sourceFile,
        String destFile)
    {
        // Initialize the keyed hash object.
        HMACSHA512 myhmacsha512 = new HMACSHA512(key);
        FileStream inStream = new FileStream(sourceFile, FileMode.Open);
        FileStream outStream = new FileStream(destFile, FileMode.Create);

        // Compute the hash of the input file.
        ubyte hashValue[] = myhmacsha512.ComputeHash(inStream);

        // Reset inStream to the beginning of the file.
        inStream.set_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
        ubyte buffer[] = new ubyte[1024];

        do {
            // Read from the wrapping CryptoStream.
            bytesRead = inStream.Read(buffer, 0, 1024);
            outStream.Write(buffer, 0, bytesRead);
        } while (bytesRead > 0);
        myhmacsha512.Clear();

        // Close the streams
        inStream.Close();
        outStream.Close();
        return;
    } // end EncodeFile

    // Decrypt the encoded file and compare to original file.
    public static boolean DecodeFile(ubyte key[], String sourceFile)
    {
        // Initialize the keyed hash object. 
        HMACSHA512 hmacsha512 = new HMACSHA512(key);

        // Create an array to hold the keyed hash value read from the file.
        ubyte storedHash[] = new ubyte[hmacsha512.get_HashSize() / 8];

        // Create a FileStream for the source file.
        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.
        ubyte computedHash[] = hmacsha512.ComputeHash(inStream);

        // compare the computed hash with the stored value
        for (int i = 0; i < storedHash.length; i++) {
            if (computedHash[i] != storedHash[i]) {
                Console.WriteLine("Hash values differ! Encoded file has been " 
                    + " tampered with!");
                return false;
            }
        }
        Console.WriteLine("Hash values agree -- no tampering occurred.");
        return true;
    } //end DecodeFile


    private static String usageText = "Usage: HMACSHA512 inputfile.txt " 
        + "encryptedfile.hsh\nYou must specify the two file names. Only the " 
        + "first file must exist.\n";

    public static void main(String[] fileargs)
    {
        //If no file names are specified, write usage text.
        if (fileargs.length < 2) {
            Console.WriteLine(usageText);
        }
        else {
            try {
                // Create a random key using a random number generator. This 
                // would be the secret key shared by sender and receiver.
                ubyte secretKey[] = new ubyte[64];

                // RNGCryptoServiceProvider is an implementation of a random
                // number generator.
                RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();

                // The array is now filled with cryptographically strong
                // random bytes.
                rng.GetBytes(secretKey);

                // Use the secret key to encode the message file.
                EncodeFile(secretKey, fileargs[0], fileargs[1]);

                // Take the encoded file and decode
                DecodeFile(secretKey, fileargs[1]);
            }
            catch (IOException e) {
                Console.WriteLine("Error: File not found", e);
            }
        } //end if-else
    } //end main
} //end class HMACSHA512Example

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

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

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