public String hmacSHA256(String message, byte[] key) {

        // Start by getting an object to generate SHA-256 hashes with.
        MessageDigest sha256 = null;
        try {
            sha256 = MessageDigest.getInstance("SHA-256");
        } catch (NoSuchAlgorithmException e) {
            throw new java.lang.AssertionError(
                    this.getClass().getName()
                    + ".hmacSHA256(): SHA-256 algorithm not found!");
        }

        // Hash the key if necessary to make it fit in a block (see RFC 2104).
        if (key.length > 64) {
            sha256.update(key);
            key = sha256.digest();
            sha256.reset();
        }

        // Pad the key bytes to a block (see RFC 2104).
        byte block[] = new byte[64];
        for (int i = 0; i < key.length; ++i) block[i] = key[i];
        for (int i = key.length; i < block.length; ++i) block[i] = 0;

        // Calculate the inner hash, defined in RFC 2104 as
        // SHA-256(KEY ^ IPAD + MESSAGE)), where IPAD is 64 bytes of 0x36.
        for (int i = 0; i < 64; ++i) block[i] ^= 0x36;
        sha256.update(block);
        try {
            sha256.update(message.getBytes("UTF-8"));
        } catch (UnsupportedEncodingException e) {
            throw new java.lang.AssertionError(
                    "ITunesU.hmacSH256(): UTF-8 encoding not supported!");
        }
        byte[] hash = sha256.digest();
        sha256.reset();

        // Calculate the outer hash, defined in RFC 2104 as
        // SHA-256(KEY ^ OPAD + INNER_HASH), where OPAD is 64 bytes of 0x5c.
        for (int i = 0; i < 64; ++i) block[i] ^= (0x36 ^ 0x5c);
        sha256.update(block);
        sha256.update(hash);
        hash = sha256.digest();

        // The outer hash is the message signature...
        // convert its bytes to hexadecimals.
        char[] hexadecimals = new char[hash.length * 2];
        for (int i = 0; i < hash.length; ++i) {
            for (int j = 0; j < 2; ++j) {
                int value = (hash[i] >> (4 - 4 * j)) & 0xf;
                char base = (value < 10) ? ('0') : ('a' - 10);
                hexadecimals[i * 2 + j] = (char)(base + value);
            }
        }

        // Return a hexadecimal string representation of the message signature.
        return new String(hexadecimals);

    }