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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.crypto;

import java.io.IOException;

import org.bouncycastle.tls.AlertDescription;
import org.bouncycastle.tls.TlsFatalAlert;
import org.bouncycastle.tls.TlsUtils;

public abstract class TlsCryptoUtils
{
    // "tls13 "
    private static final byte[] TLS13_PREFIX = new byte[]{ 0x74, 0x6c, 0x73, 0x31, 0x33, 0x20 };

    public static TlsSecret hkdfExpandLabel(TlsSecret secret, short hashAlgorithm, String label, byte[] context, int length)
        throws IOException
    {
        int labelLength = label.length();
        if (labelLength < 1)
        {
            throw new TlsFatalAlert(AlertDescription.internal_error);
        }

        int contextLength = context.length;
        int expandedLabelLength = TLS13_PREFIX.length + labelLength;

        byte[] hkdfLabel = new byte[2 + (1 + expandedLabelLength) + (1 + contextLength)];

        // uint16 length
        {
            TlsUtils.checkUint16(length);
            TlsUtils.writeUint16(length, hkdfLabel, 0);
        }

        // opaque label<7..255>
        {
            TlsUtils.checkUint8(expandedLabelLength);
            TlsUtils.writeUint8(expandedLabelLength, hkdfLabel, 2);

            System.arraycopy(TLS13_PREFIX, 0, hkdfLabel, 2 + 1, TLS13_PREFIX.length);

            int labelPos = 2 + (1 + TLS13_PREFIX.length);
            for (int i = 0; i < labelLength; ++i)
            {
                char c = label.charAt(i);
                hkdfLabel[labelPos + i] = (byte)c;
            }
        }

        // context
        {
            TlsUtils.writeOpaque8(context, hkdfLabel, 2 + (1 + expandedLabelLength));
        }

        return secret.hkdfExpand(hashAlgorithm, hkdfLabel, length);
    }
}