#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
 
// Simple Base64 decoding table
static const unsigned char b64_table[256] = {
    ['A']=0,  ['B']=1,  ['C']=2,  ['D']=3,  ['E']=4,  ['F']=5,  ['G']=6,  ['H']=7,
    ['I']=8,  ['J']=9,  ['K']=10, ['L']=11, ['M']=12, ['N']=13, ['O']=14, ['P']=15,
    ['Q']=16, ['R']=17, ['S']=18, ['T']=19, ['U']=20, ['V']=21, ['W']=22, ['X']=23,
    ['Y']=24, ['Z']=25,
    ['a']=26, ['b']=27, ['c']=28, ['d']=29, ['e']=30, ['f']=31, ['g']=32, ['h']=33,
    ['i']=34, ['j']=35, ['k']=36, ['l']=37, ['m']=38, ['n']=39, ['o']=40, ['p']=41,
    ['q']=42, ['r']=43, ['s']=44, ['t']=45, ['u']=46, ['v']=47, ['w']=48, ['x']=49,
    ['y']=50, ['z']=51,
    ['0']=52, ['1']=53, ['2']=54, ['3']=55, ['4']=56, ['5']=57, ['6']=58, ['7']=59,
    ['8']=60, ['9']=61, ['+']=62, ['/']=63
};
 
// Decodes Base64 string into output buffer, returns output length
int base64_decode(const char *in, unsigned char *out) {
    int len = strlen(in);
    int i, j = 0;
    unsigned int val = 0;
    int valb = -8;
    for (i = 0; i < len; i++) {
        unsigned char c = in[i];
        if (isspace(c) || c == '=') continue;
        if (c > 127 || b64_table[c] == 0 && c != 'A') continue; // skip invalid
        val = (val << 6) + b64_table[c];
        valb += 6;
        if (valb >= 0) {
            out[j++] = (val >> valb) & 0xFF;
            valb -= 8;
        }
    }
    return j;
}
 
int main() {
    // Base64 string containing "US4Dq"
    const char *b64 = "US4Dq9Rdm05Rx/Z79RzHj6RqGHdO+INI/sVJfspO9jJUJmHKPlQH0mEOlSvsU";
    unsigned char decoded[128];
    int out_len, i;
 
    out_len = base64_decode(b64, decoded);
 
    printf("Decoded %d bytes:\n", out_len);
    for (i = 0; i < out_len; ++i) {
        printf("Byte %3d: 0x%02X", i, decoded[i]);
        if (decoded[i] == 0x00 || decoded[i] == 0x04 || decoded[i] == 0x1A) {
            printf("  <-- problematic byte");
        }
        printf("\n");
    }
    return 0;
}
 

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