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The Bouncy Castle Java APIs for the TLS, including a JSSE provider. The APIs are designed primarily to be used in conjunction with the BC FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.
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package org.bouncycastle.tls;

import java.io.IOException;
import java.io.OutputStream;
import java.util.Enumeration;
import java.util.Hashtable;

import org.bouncycastle.tls.crypto.TlsHash;

/**
 * Buffers input until the hash algorithm is determined.
 */
class DeferredHash
    implements TlsHandshakeHash
{
    protected static final int BUFFERING_HASH_LIMIT = 4;

    protected TlsContext context;

    private DigestInputBuffer buf;
    private Hashtable hashes;
    private boolean forceBuffering;
    private boolean sealed;

    DeferredHash(TlsContext context)
    {
        this.context = context;
        this.buf = new DigestInputBuffer();
        this.hashes = new Hashtable();
        this.forceBuffering = false;
        this.sealed = false;
    }

    private DeferredHash(TlsContext context, Hashtable hashes)
    {
        this.context = context;
        this.buf = null;
        this.hashes = hashes;
        this.forceBuffering = false;
        this.sealed = true;
    }

    public void copyBufferTo(OutputStream output)
        throws IOException
    {
        if (buf == null)
        {
            // If you see this, you need to call forceBuffering() before sealHashAlgorithms()
            throw new IllegalStateException("Not buffering");
        }

        buf.copyTo(output);
    }

    public void forceBuffering()
    {
        if (sealed)
        {
            throw new IllegalStateException("Too late to force buffering");
        }

        this.forceBuffering = true;
    }

    public TlsHandshakeHash notifyPRFDetermined()
    {
        int prfAlgorithm = context.getSecurityParametersHandshake().getPrfAlgorithm();
        if (prfAlgorithm == PRFAlgorithm.tls_prf_legacy)
        {
            checkTrackingHash(Shorts.valueOf(HashAlgorithm.md5));
            checkTrackingHash(Shorts.valueOf(HashAlgorithm.sha1));
        }
        else
        {
            checkTrackingHash(Shorts.valueOf(TlsUtils.getHashAlgorithmForPRFAlgorithm(prfAlgorithm)));
        }
        return this;
    }

    public void trackHashAlgorithm(short hashAlgorithm)
    {
        if (sealed)
        {
            throw new IllegalStateException("Too late to track more hash algorithms");
        }

        checkTrackingHash(Shorts.valueOf(hashAlgorithm));
    }

    public void sealHashAlgorithms()
    {
        if (!sealed)
        {
            sealed = true;
            checkStopBuffering();
        }
    }

    public TlsHandshakeHash stopTracking()
    {
        Hashtable newHashes = new Hashtable();
        int prfAlgorithm = context.getSecurityParametersHandshake().getPrfAlgorithm();
        if (prfAlgorithm == PRFAlgorithm.tls_prf_legacy)
        {
            cloneHash(newHashes, HashAlgorithm.md5);
            cloneHash(newHashes, HashAlgorithm.sha1);
        }
        else
        {
            cloneHash(newHashes, TlsUtils.getHashAlgorithmForPRFAlgorithm(prfAlgorithm));
        }
        return new DeferredHash(context, newHashes);
    }

    public TlsHash forkPRFHash()
    {
        checkStopBuffering();

        TlsHash prfHash;

        int prfAlgorithm = context.getSecurityParametersHandshake().getPrfAlgorithm();
        if (prfAlgorithm == PRFAlgorithm.tls_prf_legacy)
        {
            prfHash = new CombinedHash(context, cloneHash(HashAlgorithm.md5), cloneHash(HashAlgorithm.sha1));
        }
        else
        {
            prfHash = cloneHash(TlsUtils.getHashAlgorithmForPRFAlgorithm(prfAlgorithm));
        }

        if (buf != null)
        {
            buf.updateDigest(prfHash);
        }

        return prfHash;
    }

    public byte[] getFinalHash(short hashAlgorithm)
    {
        TlsHash d = (TlsHash)hashes.get(Shorts.valueOf(hashAlgorithm));
        if (d == null)
        {
            throw new IllegalStateException("HashAlgorithm." + HashAlgorithm.getText(hashAlgorithm) + " is not being tracked");
        }

        d = (TlsHash)d.clone();
        if (buf != null)
        {
            buf.updateDigest(d);
        }

        return d.calculateHash();
    }

    public void update(byte[] input, int inOff, int len)
    {
        if (buf != null)
        {
            buf.write(input, inOff, len);
            return;
        }

        Enumeration e = hashes.elements();
        while (e.hasMoreElements())
        {
            TlsHash hash = (TlsHash)e.nextElement();
            hash.update(input, inOff, len);
        }
    }

    public byte[] calculateHash()
    {
        throw new IllegalStateException("Use fork() to get a definite Digest");
    }

    public Object clone()
    {
        throw new IllegalStateException("attempt to clone a DeferredHash");
    }

    public void reset()
    {
        this.forceBuffering = false;
        this.sealed = false;

        if (buf != null)
        {
            buf.reset();
            return;
        }

        Enumeration e = hashes.elements();
        while (e.hasMoreElements())
        {
            TlsHash hash = (TlsHash)e.nextElement();
            hash.reset();
        }
    }

    protected void checkStopBuffering()
    {
        if (!forceBuffering && sealed && buf != null && hashes.size() <= BUFFERING_HASH_LIMIT)
        {
            Enumeration e = hashes.elements();
            while (e.hasMoreElements())
            {
                TlsHash hash = (TlsHash)e.nextElement();
                buf.updateDigest(hash);
            }

            this.buf = null;
        }
    }

    protected void checkTrackingHash(Short hashAlgorithm)
    {
        if (!hashes.containsKey(hashAlgorithm))
        {
            TlsHash hash = context.getCrypto().createHash(hashAlgorithm.shortValue());
            hashes.put(hashAlgorithm, hash);
        }
    }

    protected TlsHash cloneHash(short hashAlgorithm)
    {
        return cloneHash(Shorts.valueOf(hashAlgorithm));
    }

    protected TlsHash cloneHash(Short hashAlgorithm)
    {
        return (TlsHash)(((TlsHash)hashes.get(hashAlgorithm)).clone());
    }

    protected void cloneHash(Hashtable newHashes, short hashAlgorithm)
    {
        cloneHash(newHashes, Shorts.valueOf(hashAlgorithm));
    }

    protected void cloneHash(Hashtable newHashes, Short hashAlgorithm)
    {
        TlsHash hash = cloneHash(hashAlgorithm);
        if (buf != null)
        {
            buf.updateDigest(hash);
        }
        newHashes.put(hashAlgorithm, hash);
    }
}