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package com.wangshanhai.power.otp;

import org.apache.commons.codec.binary.Base32;

import java.security.SecureRandom;

/**
 * A secret generator to generate OTP secrets
 *
 * @author Bastiaan Jansen
 */
public class SecretGenerator {

    private SecretGenerator() {}

    /**
     * Default amount of bits for secret generation
     */
    public static final int DEFAULT_BITS = 160;

    private static final SecureRandom random = new SecureRandom();
    private static final Base32 encoder = new Base32();

    /**
     * Generate an OTP base32 secret with default amount of bits
     *
     * @return generated secret
     */
    public static byte[] generate() {
        return generate(DEFAULT_BITS);
    }

    /**
     * Generate an OTP base32 secret
     *
     * @param bits length, this should be greater than or equal to the length of the HMAC algorithm type:
     *             SHA1: 160 bits
     *             SHA256: 256 bits
     *             SHA512: 512 bits
     * @return generated secret
     */
    public static byte[] generate(final int bits) {
        if (bits <= 0)
            throw new IllegalArgumentException("Bits must be greater than or equal to 0");

        byte[] bytes = new byte[bits / Byte.SIZE];
        random.nextBytes(bytes);

        return encoder.encode(bytes);
    }
}