Base64 decoder

base64.c

/*
 * Copyright (c) 2009 Ricardo Martins <ricardo@scarybox.net>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 */

#include <stdlib.h>
#include <string.h>

static const char *base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

char *decode64(const char *orig)
{
    int i, j;
    size_t length = strlen(orig);
    char *in, *out, *head;
    union x {
        char data[3];
        int as_integer;
    } alpha;

    in = malloc(length - (length%4) + 4 + 1);
    memset(in, '=', length - (length%4) + 4);
    strcpy(in, orig);

    out = calloc(length+1, sizeof(char));
    head = out;

    for (i = 0; i < length; i++) {
        for (j = 0; in[i] != '=' && in[i] != base64[j]; j++);
        in[i] = j;
    }

    for (i = 0; i < length; i += 4) {
        alpha.as_integer = in[i] << 18;
        alpha.as_integer |= in[i+1] << 12;
        alpha.as_integer |= in[i+2] << 6;
        alpha.as_integer |= in[i+3];
        *(head++) = alpha.data[2];
        *(head++) = alpha.data[1];
        *(head++) = alpha.data[0];
    }
    *head = 0;

    free(in);

    return out;
}

int main(void)
{
    char *text =
    "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aG"
    "lzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qg"
    "b2YgdGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY2"
    "9udGludWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNl"
    "ZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=";

    char *out = decode64(text);
    printf("%s\n", out);

    free(out);
    return 0;
}

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