Class using RSA to encrypt, decrypt, compare MD5 hash, read and write file

MahoaRSA.javav

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package onthi;

import java.math.BigInteger;
import java.security.InvalidKeyException;
import java.security.MessageDigest;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.util.Base64;
import javax.crypto.Cipher;

/**
 *
 * @author Linh
 */
public class MaHoaRSA {

    private Cipher cipher;

    public String maHoa(String chuoi_goc, PrivateKey khoa) throws Exception {
        String chuoi_da_mahoa = "";

        cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.ENCRYPT_MODE, khoa);
        chuoi_da_mahoa = Base64.getEncoder().encodeToString(cipher.doFinal(chuoi_goc.getBytes()));

        return chuoi_da_mahoa;
    }

    public String giaiMa(String chuoi_daduoc_mahoa, PublicKey khoa) throws Exception {
        String chuoi_daduoc_giaima;
        cipher = Cipher.getInstance("RSA");
        cipher.init(Cipher.DECRYPT_MODE, khoa);
        chuoi_daduoc_giaima = new String(cipher.doFinal(Base64.getDecoder().decode(chuoi_daduoc_mahoa)));
        return chuoi_daduoc_giaima;
    }
    
    public String bamMD5(String chuoi_can_bam) throws Exception{
        String chuoi_daduoc_bam;
        
        MessageDigest md = MessageDigest.getInstance("MD5");
        md.update(chuoi_can_bam.getBytes(), 0, chuoi_can_bam.length());
        chuoi_daduoc_bam = new BigInteger(1, md.digest()).toString(16);
        
        return chuoi_daduoc_bam;
    }
}