Java中的编码/解码字节如何工作？

Question

Little background: I'm doing cryptopals challenges and I finished https://cryptopals.com/sets/1/challenges/1 but realized I didn't learn what I guess is meant to be learned (or coded).

I'm using the Apache Commons Codec library for Hex and Base64 encoding/decoding. The goal is to decode the hex string and re-encode it to Base64. The "hint" at the bottom of the page says

Here's my answer...

private static Hex forHex = new Hex();
private static Base64 forBase64 = new Base64();

public static  byte[] hexDecode(String hex) throws DecoderException {
    byte[] rawBytes = forHex.decode(hex.getBytes());
    return rawBytes;
}
public static byte[] encodeB64(byte[] bytes) {
    byte[] base64Bytes = forBase64.encode(bytes);
    return base64Bytes;
}

public static void main(String[] args) throws DecoderException {

String hex = "49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d";


//decode hex String to byte[]
byte[] myHexDecoded = hexDecode(hex);
String myHexDecodedString = new String(myHexDecoded);

//Lyrics from Queen's "Under Pressure"
System.out.println(myHexDecodedString);

//encode myHexDecoded to Base64 encoded byte[]
byte[] myHexEncoded = encodeB64(myHexDecoded);
String myB64String = new String(myHexEncoded);

//"pretty printing" of base64
System.out.println(myB64String);

}

...but I feel like I cheated. I didn't learn how to decode bytes that were encoded as hex, and I didn't learn how to encode "pure" bytes to Base64, I just learned how to use a library to do something for me.

If I were to take a String in Java then get its bytes, how would I encode those bytes into hex? For example, the following code snip turns "Hello" (which is readable English) to the byte value of each character:

String s = "Hello";
char[] sChar = s.toCharArray();
byte[] sByte = new byte[sChar.length]
for(int i = 0; i < sChar.length; i++) {
    sByte[i] = (byte) sChar[i];
    System.out.println("sByte[" + i + "] = " +sByte[i]);
}

which yields sByte[0] = 72, sByte[1] = 101, sByte[2] = 108, sByte[3] = 108, sByte[4] = 111

Lets use 'o' as an example - I am guessing its decimal version is 111 - do I just take its decimal version and change that to its hex version?

If so, to decode, do I just take the the characters in the hex String 2 at a time, decompose them to decimal values, then convert to ASCII? Will it always be ASCII?

Answer 1

to decode, do I just take the the characters in the hex String 2 at a time, decompose them to decimal values, then convert to ASCII? Will it always be ASCII?

No. You take the characters 2 at a time, transform the character '0' to the numeric value 0, the character '1' to the numeric value 1, ..., the character 'a' (or 'A', depending on which encoding you want to support) to the numeric value 10, ..., the character 'f' or 'F' to the numeric value 15.

Then you multiply the first numeric value by 16, and you add it to the second numeric value to get the unsigned integer value of your byte. Then you transform that unsigned integer value to a signed byte.

ASCII has nothing to do with this algorithm.

To see how it's done in practice, since commons-codec is open-source, you can just look at its implementation.