Condividi tramite


Estensione della classe KeyedHashAlgorithm

Le classi di crittografia fornite da .NET Framework sono estremamente estensibili. In questa sezione viene fornito un esempio di estensione delle classi di crittografia mediante la creazione di una classe di algoritmi hash con chiavi che implementa l'algoritmo hash MD5.

Un algoritmo hash con chiavi è una funzione hash unidirezionale dipendente da chiavi utilizzata come funzione MAC (Message Authentication Code, codice di autenticazione dei messaggi). In .NET Framework sono disponibili due livelli di classi di algoritmi hash con chiavi di questo tipo, HMACSHA1 e MACTripleDES, che derivano dalla classe astratta KeyedHashAlgorithm. Se si desidera implementare una classe di algoritmi hash con chiavi che implementi un algoritmo hash diverso da quello fornito da .NET Framework, è preferibile creare una nuova classe derivata da KeyedHashAlgorithm e impiegare l'algoritmo hash preferito.

Nell'esempio riportato di seguito viene creata una nuova classe di algoritmi hash con chiavi tramite l'algoritmo hash MD5. Poiché questa classe eredita dalla classe KeyedHashAlgorithm, può essere utilizzata con altre classi di crittografia gestite quale CryptoStream.

Imports System
Imports System.Security.Cryptography
 _

Public Class TestHMACMD5

    Private Shared Sub PrintByteArray(ByVal arr() As [Byte])
        Dim i As Integer
        Console.WriteLine(("Length: " + arr.Length.ToString()))
        For i = 0 To arr.Length - 1
            Console.Write("{0:X}", arr(i))
            Console.Write("    ")
            If (i + 9) Mod 8 = 0 Then
                Console.WriteLine()
            End If
        Next i
        If i Mod 8 <> 0 Then
            Console.WriteLine()
        End If
    End Sub 

    Public Shared Sub Main()
        ' Create a key.
        Dim key1 As Byte() = {&HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB, &HB}
        ' Pass the key to the constructor of the HMACMD5 class.  
        Dim hmac1 As New HMACMD5(key1)

        ' Create another key.
        Dim key2 As Byte() = System.Text.Encoding.ASCII.GetBytes("KeyString")
        ' Pass the key to the constructor of the HMACMD5 class.  
        Dim hmac2 As New HMACMD5(key2)

        ' Encode a string into a byte array, create a hash of the array,
        ' and print the hash to the screen.
        Dim data1 As Byte() = System.Text.Encoding.ASCII.GetBytes("Hi There")
        PrintByteArray(hmac1.ComputeHash(data1))

        ' Encode a string into a byte array, create a hash of the array,
        ' and print the hash to the screen.
        Dim data2 As Byte() = System.Text.Encoding.ASCII.GetBytes("This data will be hashed.")
        PrintByteArray(hmac2.ComputeHash(data2))
    End Sub 
End Class 
 _

Public Class HMACMD5
    Inherits KeyedHashAlgorithm
    Private hash1 As MD5
    Private hash2 As MD5
    Private bHashing As Boolean = False

    Private rgbInner(64) As Byte
    Private rgbOuter(64) As Byte


    Public Sub New(ByVal rgbKey() As Byte)
        HashSizeValue = 128
        ' Create the hash algorithms.
        hash1 = MD5.Create()
        hash2 = MD5.Create()
        ' Get the key.
        If rgbKey.Length > 64 Then
            KeyValue = hash1.ComputeHash(rgbKey)
            ' No need to call Initialize; ComputeHash does it automatically.
        Else
            KeyValue = CType(rgbKey.Clone(), Byte())
        End If
        ' Compute rgbInner and rgbOuter.
        Dim i As Integer = 0
        For i = 0 To 63
            rgbInner(i) = &H36
            rgbOuter(i) = &H5C
        Next i
        i = 0
        For i = 0 To KeyValue.Length - 1
            rgbInner(i) = rgbInner(i) Xor KeyValue(i)
            rgbOuter(i) = rgbOuter(i) Xor KeyValue(i)
        Next i
    End Sub 


    Public Overrides Property Key() As Byte()
        Get
            Return CType(KeyValue.Clone(), Byte())
        End Get
        Set(ByVal Value As Byte())
            If bHashing Then
                Throw New Exception("Cannot change key during hash operation")
            End If
            If value.Length > 64 Then
                KeyValue = hash1.ComputeHash(value)
                ' No need to call Initialize; ComputeHash does it automatically.
            Else
                KeyValue = CType(value.Clone(), Byte())
            End If
            ' Compute rgbInner and rgbOuter.
            Dim i As Integer = 0
            For i = 0 To 63
                rgbInner(i) = &H36
                rgbOuter(i) = &H5C
            Next i
            For i = 0 To KeyValue.Length - 1
                rgbInner(i) ^= KeyValue(i)
                rgbOuter(i) ^= KeyValue(i)
            Next i
        End Set
    End Property


    Public Overrides Sub Initialize()
        hash1.Initialize()
        hash2.Initialize()
        bHashing = False
    End Sub 


    Protected Overrides Sub HashCore(ByVal rgb() As Byte, ByVal ib As Integer, ByVal cb As Integer)
        If bHashing = False Then
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0)
            bHashing = True
        End If
        hash1.TransformBlock(rgb, ib, cb, rgb, ib)
    End Sub 


    Protected Overrides Function HashFinal() As Byte()
        If bHashing = False Then
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0)
            bHashing = True
        End If
        ' Finalize the original hash.
        hash1.TransformFinalBlock(New Byte(0) {}, 0, 0)
        ' Write the outer array.
        hash2.TransformBlock(rgbOuter, 0, 64, rgbOuter, 0)
        ' Write the inner hash and finalize the hash.
        hash2.TransformFinalBlock(hash1.Hash, 0, hash1.Hash.Length)
        bHashing = False
        Return hash2.Hash
    End Function 
End Class 
using System;
using System.Security.Cryptography;

public class TestHMACMD5 
{
    static private void PrintByteArray(Byte[] arr) 
    {
        int i;
        Console.WriteLine("Length: " + arr.Length);
        for (i=0; i<arr.Length; i++) 
        {
            Console.Write("{0:X}", arr[i]);
            Console.Write("    ");
            if ( (i+9)%8 == 0 ) Console.WriteLine();
        }
        if (i%8 != 0) Console.WriteLine();
    }
    public static void Main() 
    {
        // Create a key.
        byte[] key1 = {0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b};
        // Pass the key to the constructor of the HMACMD5 class.  
        HMACMD5 hmac1 = new HMACMD5(key1);

        // Create another key.
        byte[] key2 = System.Text.Encoding.ASCII.GetBytes("KeyString");     
        // Pass the key to the constructor of the HMACMD5 class.  
        HMACMD5 hmac2 = new HMACMD5(key2);

        // Encode a string into a byte array, create a hash of the array,
        // and print the hash to the screen.
        byte[] data1 = System.Text.Encoding.ASCII.GetBytes("Hi There");
        PrintByteArray(hmac1.ComputeHash(data1));

        // Encode a string into a byte array, create a hash of the array,
        // and print the hash to the screen.
        byte[] data2 = System.Text.Encoding.ASCII.GetBytes("This data will be hashed.");
        PrintByteArray(hmac2.ComputeHash(data2));
    }
}
public class HMACMD5 : KeyedHashAlgorithm 
{
    private MD5            hash1;
    private MD5            hash2;
    private bool            bHashing = false;

    private byte[]          rgbInner = new byte[64];
    private byte[]          rgbOuter = new byte[64];

    public HMACMD5 (byte[] rgbKey) 
    {
        HashSizeValue = 128;
        // Create the hash algorithms.
        hash1 = MD5.Create();
        hash2 = MD5.Create();
        // Get the key.
        if (rgbKey.Length > 64) 
        {
            KeyValue = hash1.ComputeHash(rgbKey);
            // No need to call Initialize; ComputeHash does it automatically.
        }
        else 
        {
            KeyValue = (byte[]) rgbKey.Clone();
        }
        // Compute rgbInner and rgbOuter.
        int i = 0;
        for (i=0; i<64; i++) 
        { 
            rgbInner[i] = 0x36;
            rgbOuter[i] = 0x5C;
        }
        for (i=0; i<KeyValue.Length; i++) 
        {
            rgbInner[i] ^= KeyValue[i];
            rgbOuter[i] ^= KeyValue[i];
        }        
    }    

    public override byte[] Key 
    {
        get { return (byte[]) KeyValue.Clone(); }
        set 
        {
            if (bHashing) 
            {
                throw new Exception("Cannot change key during hash operation");
            }
            if (value.Length > 64) 
            {
                KeyValue = hash1.ComputeHash(value);
                // No need to call Initialize; ComputeHash does it automatically.
            }
            else 
            {
                KeyValue = (byte[]) value.Clone();
            }
            // Compute rgbInner and rgbOuter.
            int i = 0;
            for (i=0; i<64; i++) 
            { 
                rgbInner[i] = 0x36;
                rgbOuter[i] = 0x5C;
            }
            for (i=0; i<KeyValue.Length; i++) 
            {
                rgbInner[i] ^= KeyValue[i];
                rgbOuter[i] ^= KeyValue[i];
            }
        }
    }
    public override void Initialize() 
    {
        hash1.Initialize();
        hash2.Initialize();
        bHashing = false;
    }
    protected override void HashCore(byte[] rgb, int ib, int cb) 
    {
        if (bHashing == false) 
        {
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0);
            bHashing = true;                
        }
        hash1.TransformBlock(rgb, ib, cb, rgb, ib);
    }

    protected override byte[] HashFinal() 
    {
        if (bHashing == false) 
        {
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0);
            bHashing = true;                
        }
        // Finalize the original hash.
        hash1.TransformFinalBlock(new byte[0], 0, 0);
        // Write the outer array.
        hash2.TransformBlock(rgbOuter, 0, 64, rgbOuter, 0);
        // Write the inner hash and finalize the hash.
        hash2.TransformFinalBlock(hash1.Hash, 0, hash1.Hash.Length);
        bHashing = false;
        return hash2.Hash;
    }        
}

Nell'esempio precedente la classe KeyedHashAlgorithm viene estesa per utilizzare l'algoritmo hash MD5 in una classe denominata HMACMD5. Al momento dell'esecuzione, vengono create due istanze della classe HMACMD5, passando chiavi diverse a ciascun costruttore. Viene quindi generato un hash dei valori delle due stringhe e sulla console verrà visualizzato quanto segue:

Length: 16
92    94    72    7A    36    38    BB    1C
13    F4    8E    F8    15    8B    FC    9D
Length: 16
1    10    3A    9    C2    10    6E    E1
1    13    9B    7D    DA    EA    A3    EC

Vedere anche

Concetti

Servizi di crittografia

Altre risorse

Attività di crittografia