Intro
In the previous article, I described how to extract a QR code from the following message:
{
"n0": 7,
"x": 21,
"c": "AcO9w7fDuCDCoMOdKXbDqkvCt11dwoLCqg/DtV/DgCwBF1/CksKVwqfCiwvDrALDtcKfeQcAw6vDv8O0E8Kgwp/Dg3Vyw6vCpMKmw503Mghkw7/Dk8Kl"
}
The Origin
This message is generated by the service qrshare.io, which serves well in the following cases:
- Transferring a file from a desktop to a mobile.
- Sharing a file with multiple recipients (e.g., presentations, lecture materials).
- Sharing a file during an online meeting, especially on Google Meet calls, using this Chrome Extension.
Explanation of the Format
But why is the QR code data transferred in such a strange format in the first place? This is the logic behind using such a format:
- A QR code is a square matrix of zeroes and ones, which can be represented as a one-dimensional array.
- The array of 0s and 1s can be split into groups of 8 elements, each representing one byte. This significantly reduces the amount of data to be sent over the network.
- The array should be padded with leading zeros so that its length is divisible by 8.
- Since JSON is a text-based format and the string obtained in the previous steps is binary, it has to be base64 encoded.
The Code
The code below takes a string as input and produces that magically ciphered QR code, where:
-
n0- the number of leading zeroes that should be removed before restoring the matrix. -
x- the side length of the square matrix. -
c- the base64-encoded binary string representing the sequence of 0s and 1s.
import com.google.zxing.EncodeHintType;
import com.google.zxing.qrcode.decoder.ErrorCorrectionLevel;
import com.google.zxing.qrcode.encoder.Encoder;
import lombok.SneakyThrows;
import java.nio.charset.StandardCharsets;
import java.util.Base64;
import java.util.Map;
public class QRCodeGenerator {
public static int[] toBinary(int c) {
if (c < 0 || c > 255) {
throw new IllegalArgumentException("Input should be withing the range [0, 255], got: " + c);
}
int[] bits = new int[8];
int i = 7;
while (c > 0) {
bits[i--] = c & 0x1;
c = c >> 1;
}
return bits;
}
private static char oneCharFromBinary(int[] eightBits) {
int code = 0;
for (int i = 7; i >= 0; i--) {
code += eightBits[i] * (1 << (7 - i));
}
return (char) code;
}
private static String stringFromBinary(int[] bits) {
if (bits.length % 8 != 0) {
throw new IllegalArgumentException("Number of bits should be divisible by 8");
}
StringBuilder str = new StringBuilder();
for (int i = 0; i < bits.length; i += 8) {
int[] charBits = new int[8];
System.arraycopy(bits, i, charBits, 0, 8);
str.append(oneCharFromBinary(charBits));
}
return str.toString();
}
@SneakyThrows
public static QR encode(String data) {
if (data == null || data.length() == 0) {
throw new IllegalArgumentException("Data is empty");
}
var hints = Map.of(EncodeHintType.CHARACTER_SET, "UTF-8");
var qrCode = Encoder.encode(data, ErrorCorrectionLevel.M, hints);
var m = qrCode.getMatrix();
var size = m.getWidth() * m.getHeight();
var zeroesCount = size % 8 == 0 ? 0 : (8 - (size % 8));
var bits = new int[size + zeroesCount];
int i = 0;
for (int y = 0; y < m.getHeight(); y++) {
for (int x = 0; x < m.getWidth(); x++) {
bits[(i++) + zeroesCount] = m.get(x, y);
}
}
var rawCode = stringFromBinary(bits);
var code = Base64.getEncoder().encodeToString(rawCode.getBytes(StandardCharsets.UTF_8));
return new QR(zeroesCount, m.getWidth(), code);
}
// used for testing encoded
public static int[][] decode(QR qr) {
var decoded = new String(Base64.getDecoder().decode(qr.code), StandardCharsets.UTF_8);
var data = decoded.toCharArray();
var bits = new int[data.length * 8];
for (int i = 0; i < data.length; i++) {
var byteBits = toBinary(data[i]);
System.arraycopy(byteBits, 0, bits, i * 8, 8);
}
var matrix = new int[qr.size][qr.size];
int i = qr.zeroesCount;
for (int y = 0; y < qr.size; y++) {
for (int x = 0; x < qr.size; x++) {
matrix[y][x] = bits[i++];
}
}
return matrix;
}
public record QR(int zeroesCount, int size, String code) {
}
}
The code uses the zxing library for generating QR codes.
Thanks for reading this far!
Try sharing a file with qrshare.io or use the Chrome Extension!
Next time, we'll discuss how to create that beautiful animation when the code is generated. Stay tuned!
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