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Ganymed SSH2 for Java is a library which implements the SSH-2 protocol in pure Java

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package com.trilead.ssh2.crypto.digest;

/**
 * HMAC.
 * 
 * @author Christian Plattner, [email protected]
 * @version $Id: HMAC.java,v 1.1 2007/10/15 12:49:57 cplattne Exp $
 */
public final class HMAC implements Digest
{
	Digest md;
	byte[] k_xor_ipad;
	byte[] k_xor_opad;

	byte[] tmp;

	int size;

	public HMAC(Digest md, byte[] key, int size)
	{
		this.md = md;
		this.size = size;

		tmp = new byte[md.getDigestLength()];

		final int BLOCKSIZE = 64;

		k_xor_ipad = new byte[BLOCKSIZE];
		k_xor_opad = new byte[BLOCKSIZE];

		if (key.length > BLOCKSIZE)
		{
			md.reset();
			md.update(key);
			md.digest(tmp);
			key = tmp;
		}

		System.arraycopy(key, 0, k_xor_ipad, 0, key.length);
		System.arraycopy(key, 0, k_xor_opad, 0, key.length);

		for (int i = 0; i < BLOCKSIZE; i++)
		{
			k_xor_ipad[i] ^= 0x36;
			k_xor_opad[i] ^= 0x5C;
		}
		md.update(k_xor_ipad);
	}

	public final int getDigestLength()
	{
		return size;
	}

	public final void update(byte b)
	{
		md.update(b);
	}

	public final void update(byte[] b)
	{
		md.update(b);
	}

	public final void update(byte[] b, int off, int len)
	{
		md.update(b, off, len);
	}

	public final void reset()
	{
		md.reset();
		md.update(k_xor_ipad);
	}

	public final void digest(byte[] out)
	{
		digest(out, 0);
	}

	public final void digest(byte[] out, int off)
	{
		md.digest(tmp);

		md.update(k_xor_opad);
		md.update(tmp);

		md.digest(tmp);

		System.arraycopy(tmp, 0, out, off, size);

		md.update(k_xor_ipad);
	}
}