RFC4226 HOTP generator

HotpUtil.java

import org.apache.commons.codec.digest.HmacUtils;

import java.nio.ByteBuffer;

public class HotpUtil {

    private static final int LONG_BYTE_SIZE = Long.SIZE / Byte.SIZE;

    private static final int MAX_DIGITS = 9;

    private static final int[] MODULO_OPERANDS = new int[]{
            1,
            10,
            100,
            1000,
            10000,
            100000,
            1000000,
            10000000,
            100000000,
            1000000000,
    };

    public static int generate(byte[] key, long factor, int digits) {
        if (digits < 1 || digits > MAX_DIGITS) {
            throw new IllegalArgumentException("digits must be 1-" + MAX_DIGITS);
        }

        byte[] hash = HmacUtils.hmacSha1(key, ByteBuffer.allocate(LONG_BYTE_SIZE).putLong(factor).array());

        // hash to integer
        int offset = hash[hash.length - 1] & 0xf;

        int binary = ((hash[offset] & 0x7f) << 24) |
                ((hash[offset + 1] & 0xff) << 16) |
                ((hash[offset + 2] & 0xff) << 8) |
                (hash[offset + 3] & 0xff);

        return binary % MODULO_OPERANDS[digits];
    }
}