Relay.java

import java.nio.charset.StandardCharsets;

public class Relay {
    private static byte hexVal(char c) {
        if (c >= '0' && c <= '9') return (byte)(c - '0');
        if (c >= 'a' && c <= 'f') return (byte)(c - 'a' + 10);
        return (byte)(c - 'A' + 10);
    }

    private static byte[] decodeHex(String hex) {
        byte[] out = new byte[hex.length() / 2];
        for (int i = 0; i < out.length; ++i) {
            int hi = hexVal(hex.charAt(i * 2)) & 0xF;
            int lo = hexVal(hex.charAt(i * 2 + 1)) & 0xF;
            out[i] = (byte)((hi << 4) | lo);
        }
        return out;
    }

    public static void main(String[] args) throws Exception {
        String payloadHex = "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        String slot = "__HEX_PAYLOAD__";

        String all = new String(decodeHex(payloadHex), StandardCharsets.UTF_8);
        String[] parts = all.split("\u001F", -1);
        String tmpl = parts[5];
        int pos = tmpl.indexOf(slot);

        String out = tmpl.substring(0, pos) + payloadHex + tmpl.substring(pos + slot.length());
        System.out.write(out.getBytes(StandardCharsets.UTF_8));
    }
}