Generate HMAC_SHA256 Signature in JavaCard Applet

Question

I am trying to sign a message which contains in inBuffer byte array using my own derived key S (also byte array). The snippet of the function from javacard (jc) applet module is given below. I am using javacard2.2.2 library for developing jc applet. I am using android application for sending process request. I am reciving return code '6A81' which means 'function not supported'. Now, I have no clue that how to proceed as I failed to understand that it is mentioning about HMAC_SHA256 not supported or I am making some mistake in the function. Please help.

    Signature m_sessionMAC = null;
    HMACKey keyType = null;
    Sign = new byte[64];

    bytesRead = apdu.setIncomingAndReceive();

    // Create HMAC Key Used in Mac
    m_sessionMAC = Signature.getInstance(Signature.ALG_HMAC_SHA_256, false);

    // Create HMAC Key Used in Mac
    keyType = (HMACKey) KeyBuilder.buildKey(KeyBuilder.TYPE_HMAC, KeyBuilder.LENGTH_HMAC_SHA_256_BLOCK_64, false); 
    keyType.setKey(S,(short) 0, (short) S.length);
    m_sessionMAC.init(keyType, Signature.MODE_SIGN);

    //Generate Signature on inBuffer (received data to sign)
    echoOffset = m_sessionMAC.sign(inBuffer, ISO7816.OFFSET_CDATA, ISO7816.OFFSET_LC, Sign , (short)0); 
    Util.arrayCopyNonAtomic(Sign, ( short ) 0, inBuffer, ( short ) 0, echoOffset); 
    apdu.setOutgoingAndSend( ( short ) 0, (short) echoOffset );

Please help me in this regards or also provide any pointers for implementing HMAC_SHA256 or HMAC_SHA1 symmetric crypto. in javacard applet.

Thank you in advance.

Answer 1

Most cryptographic algorithms are optional for a JavaCard. Therefore it may be that your card does not support Signature.ALG_HMAC_SHA_256. But HMAC algorithm isn't very complex therefore you should check if your card supports MessageDigest.ALG_SHA_256.

If it is supported you can follow RFC2104 and implement HMAC yourself:

K = HMAC key of length 32
ipad = the byte 0x36 repeated 32 times
opad = the byte 0x5C repeated 32 times.

To compute HMAC over the data `text' we perform

H(K XOR opad, H(K XOR ipad, text))

You can test your implementation by comparing your result with the test vectors noted in RFC 4231

Answer 2

In addition to Robert answer I'd like to highlight that you have to check CryptoException when you are calling getInstance() method. As Robert already mentioned algorithms could be optional hence it is good practice to check before like:

try {
    Signature.getInstance(Signature.ALG_HMAC_SHA_256, false);
} catch (CryptoException e) {
    if (e.getReason() == CryptoException.NO_SUCH_ALGORITHM) {
        // Do something to treat algorithm absebce
    }
}