base64 encoding file directly with OpenSSL, chaining BIO file input to based64 decoder

Question

I would like to encode very small files (smaller than buffer) with base64. Yes I know, many base64 source codes available, but I would like to use OpenSSL.

My working example:

BIO *bin = BIO_new_file("smallfile.bin", "r");
BIO *b64 = BIO_new(BIO_f_base64());
BIO *bmem = BIO_new(BIO_s_mem());
BIO_push(b64, bmem);
BIO_set_flags(b64, BIO_FLAGS_BASE64_NO_NL);

char buff[8192];
int n=BIO_read(bin, buff, sizeof(buff));
BIO_write(b64, buff, n);
BIO_flush(b64);

n = BIO_read(bmem, buff, sizeof(buff));
cout << string(buff,n) << endl;

BIO_free_all(bin);
BIO_free_all(b64);

How can I make chain directly instead read/write BIOs, like: BIO_f_new_file -> BIO_f_base64 -> buffer or BIO_s_mem

It's possible?

Does pushing the file BIO onto the base64 one work? (And reading from the base64; no mem bio) — Shawn
– Shawn, Commented Mar 5, 2022 at 21:46
Sorry, misunderstand your question 1st. I have tired to test this, but not working. Because I can't know, how can I initiate the BIO_f_base64 to start the file reading... :D — sz9
– sz9, Commented Mar 5, 2022 at 22:03

Shawn · Accepted Answer · 2022-03-06 02:04:07Z

It doesn't seem possible to combine a source (BIO_s_file) and sink (BIO_s_mem) in one chain of BIOs. It would likely require an OpenSSL event loop since it doesn't know how many reads are required to consume all the data from the source.

However, a simpler way of using OpenSSL functions to Base64 encode data is to use the EVP interface:

#include <stdio.h>
#include <stdlib.h>
#include <openssl/bio.h>
#include <openssl/evp.h>

int main(int argc, char **argv) {
  if (argc != 2) {
    fprintf(stderr, "Usage: %s filename\n", argv[0]);
    return EXIT_FAILURE;
  }

  FILE *fin = fopen(argv[1], "r");
  if (!fin) {
    fprintf(stderr, "Unable to open %s\n", argv[1]);
    return EXIT_FAILURE;
  }

  unsigned char inbuff[8192];
  int insize = fread(inbuff, 1, sizeof inbuff, fin);
  fclose(fin);

  // For every 3 bytes of input provided 4 bytes of output data will be produced. 
  // (Plus a trailing nul)
  int outsize = 4 * (insize/3 + 1) + 1;
  unsigned char outbuff[outsize];
  outsize = EVP_EncodeBlock(outbuff, inbuff, insize);
  if (outsize < 0) {
      fputs("Encoding error!\n", stderr);
      return EXIT_FAILURE;
  }
  fwrite(outbuff, 1, outsize, stdout);
  putchar('\n');
  return 0;
}

WhozCraig · Accepted Answer · 2022-03-08 01:13:47Z

To answer your question, not it isn't possible with BIOs. Sooner or later a BIO_read/write is going to be required on your part to pull/push data through the BIO chain. In short, what you have is pretty much spot on with how it is generally done.

That being said, one thing worth mentioning: you don't have to pull that data back out of the memory BIO via a BIO_read operation. It is already sitting there, just waiting for the taking, using BIO_get_mem_data. An example speaks to this better than I can just say it, so...:

// like you did before...
char buff[8192];
int n=BIO_read(bin, buff, sizeof buff);
BIO_write(b64, buff, n);
BIO_flush(b64);

// ...but do this to get the final data
char *ptr = NULL;
long len = BIO_get_mem_data(bmem, &ptr);

Note no read from the memory bio back int buff is done. From there, if you want the data as a std::string it is as simple as:

std::string s(ptr, ptr+len);

If you want to just dump it to std:cout (or any std::ostream&) you can just use the stream write method, without the intermediate string object:

std::cout.write(ptr, len);

Fwiw, this is frequently how I transpose binary data into C++ std::string objects.

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