Java convert Hexadecimal to Base64

Question

I am working on the Matasano CryptoChallenge, and the first one is to create a Hex to Base 64 converter. I honestly don't know how to continue from here. My code:

public class HexToBase64 {

public static void main(String[] args) {
//        String hex  = "49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d";
    String hex = "DA65A";
    convertHexTo64(hex);
}

public static String convertHexTo64(String hex) {
    //convert each letter in the hex string to a 4-digit binary string to create a binary representation of the hex string
    StringBuilder binary = new StringBuilder();
    for (int i = 0; i < hex.length(); i++) {
        int dec = Integer.parseInt(hex.charAt(i) + "", 16);
        StringBuilder bin = new StringBuilder(Integer.toBinaryString(dec));
        while(bin.length() < 4){
            bin.insert(0,'0');
        }
        binary.append(bin);
    }
    //now take 6 bits at a time and convert to a single b64 digit to create the final b64 representation
    StringBuilder b64 = new StringBuilder();
    for (int i = 0; i < binary.length(); i++) {
        String temp = binary.substring(i, i+5);
        int dec = Integer.parseInt(temp, 10);
        //convert dec to b64 with the lookup table here then append to b64
    }

    return b64.toString();
}
}

So after I separate the binary 6 bits at a time and convert to decimal, how do I map the decimal number to the corresponding digit in b64? Would a Hashmap/Hashtable implementation be efficient?

Also, this algorithm displays how I would go about doing the conversion by hand. Is there a better way of doing this? I am looking for a way to convert that will take a reasonable amount of time, so time, and implicitly efficiency, is relevant.

Thank you for your time

EDIT: And the page also mentions that "Always operate on raw bytes, never on encoded strings. Only use hex and base64 for pretty-printing." What does that mean exactly?

Do you want to do it manually or want to use 3rd party library? — sol4me
– sol4me, Commented Dec 15, 2014 at 22:10
I would prefer manually for the purpose of understanding the inner workings — YazanLpizra
– YazanLpizra, Commented Dec 15, 2014 at 22:11
What that means is that you have to convert your hexadecimal string to a byte[], so that a string like 8DAB11 is converted to { (byte)0x8D, (byte)0xAB, (byte)0x11 } - or to a similar collection of bytes. No StringBuilders. — RealSkeptic
– RealSkeptic, Commented Dec 15, 2014 at 22:11
I don't mind, I would just like to understand what is going on. I saw many implementations online with byte arrays, but I don't get whats going on — YazanLpizra
– YazanLpizra, Commented Dec 15, 2014 at 22:15
So you'll need to scratch your program and start fresh. General guidelines: Write methods to convert hex to byte array, byte array to hex, and byte array to string (each byte to a char). Then write a method that converts a byte array to another byte array using the base64 algorithm. Manipulate the bits using bitwise operations like &, |, <<, >> etc. — RealSkeptic
– RealSkeptic, Commented Dec 15, 2014 at 22:18

Community · Accepted Answer · 2017-05-23 10:26:08Z

6

Extracted from this Stack Overflow post, which references Apache Commons Codec

byte[] decodedHex = Hex.decodeHex(hex);
byte[] encodedHexB64 = Base64.codeBase64(decodedHex);

edited May 23, 2017 at 10:26

CommunityBot

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answered Dec 15, 2014 at 22:11

JP Ventura

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3 Comments

sol4me Over a year ago

You forget to mention that Hex is from Commons Codec

YazanLpizra Over a year ago

Thank you for that. But can you explain what the code does? And why does it use byte arrays?

SnakeDoc Over a year ago

@yazan here's the related javadoc's that explain what they do and what they return: commons.apache.org/proper/commons-codec/apidocs/org/apache/… -- and -- commons.apache.org/proper/commons-codec/apidocs/org/apache/…

jso · Accepted Answer · 2023-11-22 10:53:38Z

2

With Java 17+ this can be done without additional dependencies:

import java.util.Base64;
import java.util.HexFormat;

String inputHex = "9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08";

byte[] hexBytes = HexFormat.of().parseHex(inputHex);
byte[] base64Bytes = Base64.getEncoder().encode(hexBytes);

Or, optionally, getting it as String directly:

String base64String = Base64.getEncoder().encodeToString(hexBytes);

References:

https://stackoverflow.com/a/140861/5484072

answered Nov 22, 2023 at 10:53

jso

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Comments

Extreme · Accepted Answer · 2017-06-27 20:08:02Z

1

String hex = "00bc9d2a05ef06c79a6e972f8a36737e";
byte[] decodedHex = org.apache.commons.codec.binary.Hex.decodeHex(hex.toCharArray());
String result = Base64.encodeBase64String(decodedHex);
System.out.println("==> " + result);

answered Jun 27, 2017 at 20:08

Extreme

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