KeyedHashAlgorithm Class

Definition

Namespace:

System.Security.Cryptography

Assemblies:

netstandard.dll, System.Security.Cryptography.dll

Source:

KeyedHashAlgorithm.cs

Represents the abstract class from which all implementations of keyed hash algorithms must derive.

C#

public abstract class KeyedHashAlgorithm : System.Security.Cryptography.HashAlgorithm

Inheritance

Object

HashAlgorithm

KeyedHashAlgorithm

Derived

System.Security.Cryptography.HMAC

System.Security.Cryptography.MACTripleDES

Examples

The following code example demonstrates how to derive from the KeyedHashAlgorithm class.

C#

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)
        {
            hash1.TransformBlock(rgbInner, 0, 64, rgbInner, 0);
            bHashing = true;
        }
        hash1.TransformBlock(rgb, ib, cb, rgb, ib);
    }

    protected override byte[] HashFinal()
    {
        if (!bHashing)
        {
            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;
    }
}

Remarks

Hash functions map binary strings of an arbitrary length to small binary strings of a fixed length. A cryptographic hash function has the property that it is computationally infeasible to find two distinct inputs that hash to the same value. Small changes to the data result in large, unpredictable changes in the hash.

A keyed hash algorithm is a key-dependent, one-way hash function used as a message authentication code. Only someone who knows the key can verify the hash. Keyed hash algorithms provide authenticity without secrecy.

Hash functions are commonly used with digital signatures and for data integrity. The HMACSHA1 class is an example of a keyed hash algorithm.

Due to collision problems with SHA1, Microsoft recommends a security model based on SHA256 or better.

Constructors

KeyedHashAlgorithm()

Initializes a new instance of the KeyedHashAlgorithm class.

Fields

HashSizeValue	Represents the size, in bits, of the computed hash code. (Inherited from HashAlgorithm)
HashValue	Represents the value of the computed hash code. (Inherited from HashAlgorithm)
KeyValue	The key to use in the hash algorithm.
State	Represents the state of the hash computation. (Inherited from HashAlgorithm)

Properties

CanReuseTransform	Gets a value indicating whether the current transform can be reused. (Inherited from HashAlgorithm)
CanTransformMultipleBlocks	When overridden in a derived class, gets a value indicating whether multiple blocks can be transformed. (Inherited from HashAlgorithm)
Hash	Gets the value of the computed hash code. (Inherited from HashAlgorithm)
HashSize	Gets the size, in bits, of the computed hash code. (Inherited from HashAlgorithm)
InputBlockSize	When overridden in a derived class, gets the input block size. (Inherited from HashAlgorithm)
Key	Gets or sets the key to use in the hash algorithm.
OutputBlockSize	When overridden in a derived class, gets the output block size. (Inherited from HashAlgorithm)

Methods

Clear()	Releases all resources used by the HashAlgorithm class. (Inherited from HashAlgorithm)
ComputeHash(Byte[], Int32, Int32)	Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm)
ComputeHash(Byte[])	Computes the hash value for the specified byte array. (Inherited from HashAlgorithm)
ComputeHash(Stream)	Computes the hash value for the specified Stream object. (Inherited from HashAlgorithm)
ComputeHashAsync(Stream, CancellationToken)	Asynchronously computes the hash value for the specified Stream object. (Inherited from HashAlgorithm)
Create()	Obsolete. Creates an instance of the default implementation of a keyed hash algorithm.
Create(String)	Obsolete. Creates an instance of the specified implementation of a keyed hash algorithm.
Dispose()	Releases all resources used by the current instance of the HashAlgorithm class. (Inherited from HashAlgorithm)
Dispose(Boolean)	Releases the unmanaged resources used by the KeyedHashAlgorithm and optionally releases the managed resources.
Equals(Object)	Determines whether the specified object is equal to the current object. (Inherited from Object)
GetHashCode()	Serves as the default hash function. (Inherited from Object)
GetType()	Gets the Type of the current instance. (Inherited from Object)
HashCore(Byte[], Int32, Int32)	When overridden in a derived class, routes data written to the object into the hash algorithm for computing the hash. (Inherited from HashAlgorithm)
HashCore(ReadOnlySpan<Byte>)	Routes data written to the object into the hash algorithm for computing the hash. (Inherited from HashAlgorithm)
HashFinal()	When overridden in a derived class, finalizes the hash computation after the last data is processed by the cryptographic hash algorithm. (Inherited from HashAlgorithm)
Initialize()	Resets the hash algorithm to its initial state. (Inherited from HashAlgorithm)
MemberwiseClone()	Creates a shallow copy of the current Object. (Inherited from Object)
ToString()	Returns a string that represents the current object. (Inherited from Object)
TransformBlock(Byte[], Int32, Int32, Byte[], Int32)	Computes the hash value for the specified region of the input byte array and copies the specified region of the input byte array to the specified region of the output byte array. (Inherited from HashAlgorithm)
TransformFinalBlock(Byte[], Int32, Int32)	Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm)
TryComputeHash(ReadOnlySpan<Byte>, Span<Byte>, Int32)	Attempts to compute the hash value for the specified byte array. (Inherited from HashAlgorithm)
TryHashFinal(Span<Byte>, Int32)	Attempts to finalize the hash computation after the last data is processed by the hash algorithm. (Inherited from HashAlgorithm)

Applies to

See also

• Cryptographic Services