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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;

class Hex
{
    protected final byte[] encodingTable =
    {
        (byte)'0', (byte)'1', (byte)'2', (byte)'3', (byte)'4', (byte)'5', (byte)'6', (byte)'7',
        (byte)'8', (byte)'9', (byte)'a', (byte)'b', (byte)'c', (byte)'d', (byte)'e', (byte)'f'
    };

    /*
     * set up the decoding table.
     */
    protected final byte[] decodingTable = new byte[128];

    protected Hex()
    {
        for (int i = 0; i < decodingTable.length; i++)
        {
            decodingTable[i] = (byte)0xff;
        }

        for (int i = 0; i < encodingTable.length; i++)
        {
            decodingTable[encodingTable[i]] = (byte)i;
        }

        decodingTable['A'] = decodingTable['a'];
        decodingTable['B'] = decodingTable['b'];
        decodingTable['C'] = decodingTable['c'];
        decodingTable['D'] = decodingTable['d'];
        decodingTable['E'] = decodingTable['e'];
        decodingTable['F'] = decodingTable['f'];
    }

    private static final Hex encoder = new Hex();

    public static byte[] decodeStrict(String data)
    {
        try
        {
            return encoder.decodeStrict(data, 0, data.length());
        }
        catch (Exception e)
        {
            throw new IllegalStateException("exception decoding Hex string: " + e.getMessage(), e);
        }
    }

    byte[] decodeStrict(String str, int off, int len) throws IOException
    {
        if (null == str)
        {
            throw new NullPointerException("'str' cannot be null");
        }
        if (off < 0 || len < 0 || off > (str.length() - len))
        {
            throw new IndexOutOfBoundsException("invalid offset and/or length specified");
        }
        if (0 != (len & 1))
        {
            throw new IOException("a hexadecimal encoding must have an even number of characters");
        }

        int resultLen = len >>> 1;
        byte[] result = new byte[resultLen];

        int strPos = off;
        for (int i = 0; i < resultLen; ++i)
        {
            byte b1 = decodingTable[str.charAt(strPos++)];
            byte b2 = decodingTable[str.charAt(strPos++)];

            int n = (b1 << 4) | b2;
            if (n < 0)
            {
                throw new IOException("invalid characters encountered in Hex string");
            }

            result[i] = (byte)n;
        }
        return result;
    }

}