Base64 encoding and decoding with OpenSSL

Personally, I find the OpenSSL API to be so incredibly painful to use, I avoid it unless the cost of avoiding it is extremely high. I find it quite upsetting that it has become the standard API in the crypto world.

I was feeling bored, and I wrote you one in C++. This one should even handle the edge cases that can cause security problems, like, for example, encoding a string that results in integer overflow because it's too large.

I have done some unit testing on it, so it should work.

#include <string>
#include <cassert>
#include <limits>
#include <stdexcept>
#include <cctype>

static const char b64_table[65] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

static const char reverse_table[128] = {
   64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
   64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
   64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63,
   52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64,
   64,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
   15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 64, 64, 64, 64, 64,
   64, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
   41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 64, 64, 64, 64, 64
};

::std::string base64_encode(const ::std::string &bindata)
{
   using ::std::string;
   using ::std::numeric_limits;

   if (bindata.size() > (numeric_limits<string::size_type>::max() / 4u) * 3u) {
      throw ::std::length_error("Converting too large a string to base64.");
   }

   const ::std::size_t binlen = bindata.size();
   // Use = signs so the end is properly padded.
   string retval((((binlen + 2) / 3) * 4), '=');
   ::std::size_t outpos = 0;
   int bits_collected = 0;
   unsigned int accumulator = 0;
   const string::const_iterator binend = bindata.end();

   for (string::const_iterator i = bindata.begin(); i != binend; ++i) {
      accumulator = (accumulator << 8) | (*i & 0xffu);
      bits_collected += 8;
      while (bits_collected >= 6) {
         bits_collected -= 6;
         retval[outpos++] = b64_table[(accumulator >> bits_collected) & 0x3fu];
      }
   }
   if (bits_collected > 0) { // Any trailing bits that are missing.
      assert(bits_collected < 6);
      accumulator <<= 6 - bits_collected;
      retval[outpos++] = b64_table[accumulator & 0x3fu];
   }
   assert(outpos >= (retval.size() - 2));
   assert(outpos <= retval.size());
   return retval;
}

::std::string base64_decode(const ::std::string &ascdata)
{
   using ::std::string;
   string retval;
   const string::const_iterator last = ascdata.end();
   int bits_collected = 0;
   unsigned int accumulator = 0;

   for (string::const_iterator i = ascdata.begin(); i != last; ++i) {
      const int c = *i;
      if (::std::isspace(c) || c == '=') {
         // Skip whitespace and padding. Be liberal in what you accept.
         continue;
      }
      if ((c > 127) || (c < 0) || (reverse_table[c] > 63)) {
         throw ::std::invalid_argument("This contains characters not legal in a base64 encoded string.");
      }
      accumulator = (accumulator << 6) | reverse_table[c];
      bits_collected += 6;
      if (bits_collected >= 8) {
         bits_collected -= 8;
         retval += static_cast<char>((accumulator >> bits_collected) & 0xffu);
      }
   }
   return retval;
}

Here is an example of OpenSSL base64 encode/decode I wrote:

Notice, I have some macros/classes in the code that I wrote, but none of them is important for the example. It is simply some C++ wrappers I wrote:

buffer base64::encode( const buffer& data )
{
    // bio is simply a class that wraps BIO* and it free the BIO in the destructor.

    bio b64(BIO_f_base64()); // create BIO to perform base64
    BIO_set_flags(b64,BIO_FLAGS_BASE64_NO_NL);

    bio mem(BIO_s_mem()); // create BIO that holds the result

    // chain base64 with mem, so writing to b64 will encode base64 and write to mem.
    BIO_push(b64, mem);

    // write data
    bool done = false;
    int res = 0;
    while(!done)
    {
        res = BIO_write(b64, data.data, (int)data.size);

        if(res <= 0) // if failed
        {
            if(BIO_should_retry(b64)){
                continue;
            }
            else // encoding failed
            {
                /* Handle Error!!! */
            }
        }
        else // success!
            done = true;
    }

    BIO_flush(b64);

    // get a pointer to mem's data
    char* dt;
    long len = BIO_get_mem_data(mem, &dt);

    // assign data to output
    std::string s(dt, len);

    return buffer(s.length()+sizeof(char), (byte*)s.c_str());
}

Rather than using the BIO_ interface it's much easier to use the EVP_ interface. For instance:

#include <iostream>
#include <stdlib.h>
#include <openssl/evp.h>

char *base64(const unsigned char *input, int length) {
  const auto pl = 4*((length+2)/3);
  auto output = reinterpret_cast<char *>(calloc(pl+1, 1)); //+1 for the terminating null that EVP_EncodeBlock adds on
  const auto ol = EVP_EncodeBlock(reinterpret_cast<unsigned char *>(output), input, length);
  if (pl != ol) { std::cerr << "Whoops, encode predicted " << pl << " but we got " << ol << "\n"; }
  return output;
}

unsigned char *decode64(const char *input, int length) {
  const auto pl = 3*length/4;
  auto output = reinterpret_cast<unsigned char *>(calloc(pl+1, 1));
  const auto ol = EVP_DecodeBlock(output, reinterpret_cast<const unsigned char *>(input), length);
  if (pl != ol) { std::cerr << "Whoops, decode predicted " << pl << " but we got " << ol << "\n"; }
  return output;
}

The EVP functions include a streaming interface too, see the man page.

This works for me, and verified no memory leaks with valgrind.

#include <openssl/bio.h>
#include <openssl/evp.h>
#include <cstring>
#include <memory>
#include <string>
#include <vector>

#include <iostream>

namespace {
struct BIOFreeAll { void operator()(BIO* p) { BIO_free_all(p); } };
}

std::string Base64Encode(const std::vector<unsigned char>& binary)
{
    std::unique_ptr<BIO,BIOFreeAll> b64(BIO_new(BIO_f_base64()));
    BIO_set_flags(b64.get(), BIO_FLAGS_BASE64_NO_NL);
    BIO* sink = BIO_new(BIO_s_mem());
    BIO_push(b64.get(), sink);
    BIO_write(b64.get(), binary.data(), binary.size());
    BIO_flush(b64.get());
    const char* encoded;
    const long len = BIO_get_mem_data(sink, &encoded);
    return std::string(encoded, len);
}

// Assumes no newlines or extra characters in encoded string
std::vector<unsigned char> Base64Decode(const char* encoded)
{
    std::unique_ptr<BIO,BIOFreeAll> b64(BIO_new(BIO_f_base64()));
    BIO_set_flags(b64.get(), BIO_FLAGS_BASE64_NO_NL);
    BIO* source = BIO_new_mem_buf(encoded, -1); // read-only source
    BIO_push(b64.get(), source);
    const int maxlen = strlen(encoded) / 4 * 3 + 1;
    std::vector<unsigned char> decoded(maxlen);
    const int len = BIO_read(b64.get(), decoded.data(), maxlen);
    decoded.resize(len);
    return decoded;
}

int main()
{
    const char* msg = "hello";
    const std::vector<unsigned char> binary(msg, msg+strlen(msg));
    const std::string encoded = Base64Encode(binary);
    std::cout << "encoded = " << encoded << std::endl;
    const std::vector<unsigned char> decoded = Base64Decode(encoded.c_str());
    std::cout << "decoded = ";
    for (unsigned char c : decoded) std::cout << c;
    std::cout << std::endl;
    return 0;
}

Compile:

g++ -lcrypto main.cc

Output:

encoded = aGVsbG8=
decoded = hello