Compact implementation of Base64 encoding in C

base64_encode.c

/**
  BSD 3-Clause License

  Copyright (c) 2019 Odzhan. All rights reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are met:

  * Redistributions of source code must retain the above copyright notice, this
    list of conditions and the following disclaimer.

  * Redistributions in binary form must reproduce the above copyright notice,
    this list of conditions and the following disclaimer in the documentation
    and/or other materials provided with the distribution.

  * Neither the name of the copyright holder nor the names of its
    contributors may be used to endorse or promote products derived from
    this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <inttypes.h>
#include <sys/stat.h>
#include <fcntl.h>
#ifndef _MSC_VER
#include <unistd.h>
#endif

#define ROTL32(v,n)(((v)<<(n))|((v)>>(32-(n))))

#ifdef _MSC_VER
  #define rev32(x) _byteswap_ulong(x)
#else
  #define rev32(x) __builtin_bswap32(x)
#endif

// calculate length of buffer required for base64 encoding
#define B64_LEN(N) (((4 * (N / 3)) + 4) & -4)

static const char b64_tbl[] =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  
// openssl base64 -A -e -in <binary> -out binary.base64

// Compact implementation of base64 encoding. Probably slow as hell.
// The main encoding loop is inspired by Qkumba AKA Peter Ferrie, except this
// uses a lookup table.
//
static int b64_encode(
  const void *src, uint64_t inlen, 
  void *dst, uint64_t *outlen, int last) 
{
    uint64_t len;
    uint32_t x;
    int      i = 0;
    uint8_t  *in = (uint8_t*)src, *out = (uint8_t*)dst;
    
    // check arguments
    if(outlen == NULL) return 0;
    
    // calculate length of buffer required for encoded string
    len = B64_LEN(inlen);
    
    // return the length?
    if(out == NULL) {
      *outlen = len;
      return 1;
    }
    
    // can buffer contain string?
    if(len > *outlen) return 0;
    
    // main encoding loop
    while(inlen != 0) {
      // load 3 bytes
      for(x=i=0; i<3; i++) {
        x |= ((i < inlen) ? *in++ : 0); 
        x <<= 8;
      }
      // increase by 1
      inlen++;
      // encode 3 bytes
      for(i=4; inlen && i>0; i--) {
        x = ROTL32(x, 6);
        *out++ = b64_tbl[x % 64];
        --inlen;
      }
    }
    // if required, add padding
    if(last) {
      while(i!=0) { *out++ = '='; i--; }
      // add null terminator
      *out = 0;
    }
    // calculate output length by subtracting 2 pointers
    *outlen = (out - (uint8_t*)dst);
    return 1;
}

int main(int argc, char *argv[]) {
    char     *input;
    uint64_t inlen, outlen;
    void     *output;
    FILE     *in, *out;
    uint8_t  buf[255], str[B64_LEN(sizeof(buf))];
    
    if(argc < 2) {
      printf("usage: base64 <string> | <infile> <outfile>\n");
      return 0;
    }
    
    if(argc == 2) {
      input = argv[1];
      inlen = strlen(argv[1]);
      // calculate length
      if(b64_encode(NULL, inlen, NULL, &outlen, 1)) {
        // allocate memory
        output = malloc(outlen);
        if(output != NULL) {
          // encode input
          if(b64_encode(input, inlen, output, &outlen, 1)) {
            printf("%s\n", (char*)output);
          }
          // release memory
          free(output);
        }
      }
    } else 
    if(argc == 3) {
      in = fopen(argv[1], "rb");
      if(in == NULL) {
        printf("unable to open %s\n", argv[1]);
        return -1;
      }
      out = fopen(argv[2], "wb");
      if(out != NULL) {
        for(;;) {
          // read block
          inlen = fread(buf, 1, sizeof(buf), in);
          // nothing read? break
          if(inlen == 0) break;
          // encode
          outlen = sizeof(str);
          if(!b64_encode(buf, inlen, str, &outlen, inlen<sizeof(buf))) {
            printf("encoding failed.\n");
            break;
          }
          // save to file
          fwrite(str, 1, outlen, out);
        }
        fclose(out);
      } else printf("unable to open %s\n", argv[2]);
      fclose(in);
    }
    return 0;
}