JavaCard implementation of HMAC SHA256

jc_HmacSha256.java

private short computeHmacSha256(byte[] key, short key_offset, short key_length, 
									byte[] message, short message_offset, short message_length,
									byte[] mac, short mac_offset){
    	
    short BLOCKSIZE=64; 
    short HASHSIZE=32;
    	
		// compute inner hash
		for (short i=0; i<key_length; i++){
			hmacBuffer[i]= (byte) (key[(short)(key_offset+i)] ^ (0x36));
		}
		Util.arrayFillNonAtomic(hmacBuffer, key_length, (short)(BLOCKSIZE-key_length), (byte)0);
		
		short retLen = Util.arrayCopyNonAtomic(message, message_offset, hmacBuffer, BLOCKSIZE, message_length);
		
		sha256.reset();
		sha256.doFinal(hmacBuffer, (short)0, (short)(BLOCKSIZE+message_length), hmacBuffer, BLOCKSIZE); // copy hash result to data buffer!
		
		// compute outer hash
		for (short i=0; i<key_length; i++){
			hmacBuffer[i]= (byte) (key[(short)(key_offset+i)] ^ (0x5c));
		}
  
		Util.arrayFillNonAtomic(hmacBuffer, key_length, (short)(BLOCKSIZE-key_length), (byte)0);
		// previous hash already copied to correct offset in scratch
		sha256.reset();
		sha256.doFinal(hmacBuffer, (short)0, (short)(BLOCKSIZE+HASHSIZE), mac, mac_offset);
		
		return HASHSIZE;
}