See Base64 encode data.

Now write a program that takes the output of the Base64 encode data task as input and regenerate the original file.

When working on the VBA implementation I found several 'solutions' on the net, including one from the software maker himself, that showed output with incorrect padding. Obviously with incorrect padding in the output you can not decode correctly to the original file again.

#include <stdio.h>
#include <stdlib.h>
 
typedef unsigned char ubyte;
const ubyte BASE64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
int findIndex(const ubyte val) {
    if ('A' <= val && val <= 'Z') {
        return val - 'A';
    }
    if ('a' <= val && val <= 'z') {
        return val - 'a' + 26;
    }
    if ('0' <= val && val <= '9') {
        return val - '0' + 52;
    }
    if (val == '+') {
        return 62;
    }
    if (val == '/') {
        return 63;
    }
    return -1;
}
 
int decode(const ubyte source[], ubyte sink[]) {
    const size_t length = strlen(source);
    const ubyte *it = source;
    const ubyte *end = source + length;
    int acc;
 
    if (length % 4 != 0) {
        return 1;
    }
 
    while (it != end) {
        const ubyte b1 = *it++;
        const ubyte b2 = *it++;
        const ubyte b3 = *it++;         // might be the first padding byte
        const ubyte b4 = *it++;         // might be the first or second padding byte
 
        const int i1 = findIndex(b1);
        const int i2 = findIndex(b2);
 
        acc = i1 << 2;                  // six bits came from the first byte
        acc |= i2 >> 4;                 // two bits came from the first byte
        *sink++ = acc;                  // output the first byte
 
        if (b3 != '=') {
            const int i3 = findIndex(b3);
 
            acc = (i2 & 0xF) << 4;      // four bits came from the second byte
            acc += i3 >> 2;             // four bits came from the second byte
            *sink++ = acc;              // output the second byte
 
            if (b4 != '=') {
                const int i4 = findIndex(b4);
 
                acc = (i3 & 0x3) << 6;  // two bits came from the third byte
                acc |= i4;              // six bits came from the third byte
                *sink++ = acc;          // output the third byte
            }
        }
    }
 
    *sink = '\0';   // add the sigil for end of string
    return 0;
}
 
int main() {
    ubyte data[] = "VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVwIHlvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLVBhdWwgUi5FaHJsaWNo";
    ubyte decoded[1024];
 
    printf("%s\n\n", data);
    decode(data, decoded);
    printf("%s\n\n", decoded);
 
    return 0;
}