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HMACMD5 Constructor (Byte[])

Note: This constructor is new in the .NET Framework version 2.0.

Initializes a new instance of the HMACMD5 class using the supplied key.

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

public HMACMD5 (
	byte[] key
)
public HMACMD5 (
	byte[] key
)
public function HMACMD5 (
	key : byte[]
)

Parameters

key

The secret key for HMACMD5 encryption. The key can be any length, but if it is more than 64 bytes long it will be hashed (using SHA-1) to derive a 64-byte key. Therefore, the recommended size of the secret key is 64 bytes.

Exception typeCondition

ArgumentNullException

The key parameter is a null reference (Nothing in Visual Basic).

HMACMD5 is a type of keyed hash algorithm that is constructed from the MD5 hash function and used as a Hash-based Message Authentication Code (HMAC). The HMAC process mixes a secret key with the message data, hashes the result with the hash function, mixes that hash value with the secret key again, and then applies the hash function a second time. The output hash will be 128 bits in length.

This constructor uses a key you provide to create the HMACMD5 object.

The following code example shows how to encode a file using HMACMD5 and then how to decode the file.

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

public class HMACMD5example
{
	// 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.
		HMACMD5 myhmacMD5 = new HMACMD5(key);
		FileStream inStream = new FileStream(sourceFile, FileMode.Open);
		FileStream outStream = new FileStream(destFile, FileMode.Create);
		// Compute the hash of the input file.
		byte[] hashValue = myhmacMD5.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); 
		myhmacMD5.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. 
		HMACMD5 hmacMD5 = new HMACMD5(key);
		// Create an array to hold the keyed hash value read from the file.
		byte[] storedHash = new byte[hmacMD5.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 = hmacMD5.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: HMACMD5 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 HMACMD5Example
{
    // 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.
        HMACMD5 myhmacMD5 = new HMACMD5(key);
        FileStream inStream = new FileStream(sourceFile, FileMode.Open);
        FileStream outStream = new FileStream(destFile, FileMode.Create);

        // Compute the hash of the input file.
        ubyte hashValue[] = myhmacMD5.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);
        myhmacMD5.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. 
        HMACMD5 hmacMD5 = new HMACMD5(key);

        // Create an array to hold the keyed hash value read from the file.
        ubyte storedHash[] = new ubyte[hmacMD5.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[] = hmacMD5.ComputeHash(inStream);

        // compare the computed hash with the stored value
        for (int i = 0; i < storedHash.length; i++) {
            if (computedHash.get_Item(i) != storedHash.get_Item(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: HMACMD5 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 HMACMD5Example

Windows 98, Windows 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 .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.

.NET Framework

Supported in: 2.0
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