store arbritrary data in unicode emoji, https://paulbutler.org/2025/smuggling-arbitrary-data-through-an-emoji/

emoji.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <wchar.h>
#include <locale.h>

wchar_t byte_to_variant(char byte) {
        if(byte < 16) {
                return 0xfe00 + (uint32_t)byte;
        } else {
                return 0xe0100 + (uint32_t)(byte - 16);
        }
}

char variant_to_byte(wchar_t wc) {
        uint32_t variant = (uint32_t)wc;
        if(variant >= 0xfe00 && variant <= 0xfe0f) {
                return (char)(variant - 0xfe00);
        } else if(variant >= 0xe0100 && variant <= 0xe01ef) {
                return (char)(variant - 0xe0100 + 16);
        } else {
                return 'X';
        }
}

wchar_t *encode(wchar_t base, char *bytes) {
        unsigned int length = strlen(bytes);
        wchar_t *result = (wchar_t *)malloc(sizeof(wchar_t) * (length + 1)); // already have \0 from end of bytes

        result[0] = base;
        for(unsigned int i = 1; i < length + 1; i++) {
                result[i] = byte_to_variant(bytes[i - 1]);
        }

        return result;
}

char *decode(wchar_t *emoji) {
        unsigned int length = wcslen(emoji);
        char *result = (char *)malloc(sizeof(char) * (length - 1));

        for(unsigned int i = 1; i < length; i++) {
                result[i - 1] = variant_to_byte(emoji[i]);
        }
        result[length - 1] = '\0';

        return result;
}

void help_and_exit(char *pname) {
        fprintf(stderr, "usage: %s encode blahblahblah\n       %s decode *\n", pname, pname);
        exit(1);
}

int main(int argc, char *argv[]) {
        setlocale(LC_CTYPE, "");
        if(argc < 2) help_and_exit(argv[0]);

        if(strcmp(argv[1], "encode") == 0) {
                if(argc < 3) help_and_exit(argv[0]);

                wchar_t smiley = L'😊';
                wchar_t *encoded = encode(smiley, argv[2]);
                wprintf(L"%ls\n", encoded);

                free(encoded);
                return 0;
        } else if(strcmp(argv[1], "decode") == 0) {
                if(argc < 3) help_and_exit(argv[0]);

                size_t mbslen = mbstowcs(NULL, argv[2], 0);
                wchar_t *encoded = malloc(sizeof(wchar_t) * (mbslen + 1));
                mbstowcs(encoded, argv[2], mbslen + 1);

                char *decoded = decode(encoded);
                printf("%s\n", decoded);

                free(encoded);
                free(decoded);
                return 0;
        } else {
                help_and_exit(argv[0]);
        }

        return 1;
}