attempt to decode a value not in base64 char set

Question

I am using the following code snippet to base64 encode and decode a string using Boost C++ library.

//Base64 Encode Implementation using Boost C++ library
const std::string base64_padding[] = {"", "=", "=="};

std::string X_Privet_Token_Generator::base64_encode(const std::string & s)
{
  namespace bai = boost::archive::iterators;

  std::stringstream os;

  // convert binary values to base64 characters
  typedef bai::base64_from_binary

  // retrieve 6 bit integers from a sequence of 8 bit bytes
  <bai::transform_width<const char *, 6, 8> > base64_enc; // compose all the above operations in to a new iterator

  std::copy(base64_enc(s.c_str()), base64_enc(s.c_str() + s.size()), std::ostream_iterator<char>(os));

  os << base64_padding[s.size() % 3];
  return os.str();
}

std::string X_Privet_Token_Generator::base64_decode(std::string & s)
{
  namespace bai = boost::archive::iterators;

  std::stringstream os;

  // convert binary values to base64 characters
  typedef bai::binary_from_base64

  <bai::transform_width<const char *, 8, 6> > base64_dec;

  unsigned int size = s.size();

  // Remove the padding characters, cf.
  if (size && s[size - 1] == '=')
  {
      --size;
      if (size && s[size - 1] == '=')
          --size;
  }

  if (size == 0)
      return std::string();

  LOGINFO("Hash decoded token : %s", s.c_str());
  std::copy(base64_dec(s.data()), base64_dec(s.data() + size), std::ostream_iterator<char>(os));
  std::cout<< os.str();
  return os.str();
}

Encoding works well, however, while decoding I get the following error:

terminate called after throwing an instance of boost::archive::iterators::dataflow_exception

what(): attempt to decode a value not in base64 char set

Is it one of the padded characters that is causing this issue? Am I missing something here?

Answer 1

The padding characters '=' are part of the b64 encoded data and should not be removed before decoding. b64 is encoded in blocks of 4 character, I suspect that while decoding it reads a '\0' instead of an expected '=' at the end of the string.

Answer 2

Changing the

std::copy(base64_dec(s.data()), base64_dec(s.data() + size), std::ostream_iterator<char>(os))

to

return std::string( base64_dec(s.c_str()), base64_dec(s.c_str() + size))

resolved the issue.

Answer 3

A more efficient solution for base64 encoding and decoding is as follows:

#include <boost/archive/iterators/base64_from_binary.hpp>
#include <boost/archive/iterators/binary_from_base64.hpp>
#include <boost/archive/iterators/insert_linebreaks.hpp>
#include <boost/archive/iterators/remove_whitespace.hpp>
#include <boost/archive/iterators/transform_width.hpp>
#include <boost/archive/iterators/ostream_iterator.hpp>
#include <boost/algorithm/string.hpp>
#include <bits/stl_algo.h>

std::string X_Privet_Token_Generator::base64_encode(std::string s)
{
    namespace bai = boost::archive::iterators;

    std::stringstream os;

    // convert binary values to base64 characters
    typedef bai::base64_from_binary

    // retrieve 6 bit integers from a sequence of 8 bit bytes
    <bai::transform_width<char *, 6, 8> > base64_enc; // compose all the above operations in to a new iterator

    std::copy(base64_enc(s.c_str()), base64_enc(s.c_str() + s.size()), std::ostream_iterator<char>(os));

    os << base64_padding[s.size() % 3];
    return os.str();
}

std::string X_Privet_Token_Generator::base64_decode(std::string s)
{
    namespace bai = boost::archive::iterators;

    std::stringstream os;

    typedef bai::transform_width<bai::binary_from_base64<char * >, 8, 6>
      base64_dec;

    unsigned int size = s.size();

    // Remove the padding characters.
    if(size && s[size - 1] == '=') {
        --size;
    if(size && s[size - 1] == '=')
        --size;
    }

    if(size == 0) return std::string();

    unsigned int paddChars = count(s.begin(), s.end(), '=');
    std::replace(s.begin(),s.end(), '=', 'A');
    std::string decoded_token(base64_dec(s.c_str()), base64_dec(s.c_str() + size));
    decoded_token.erase(decoded_token.end()-paddChars,decoded_token.end());
    return decoded_token;

}