How to convert base64 to Integer in Crypto++? - c++

Question

I use Crypto++ library. I have a base64 string saved as CString. I want to convert my string to Integer. actually this base64 built from an Integer and now i want to convert to Integer again.but two Integer not equal.in the other words second Integer not equal with original Integer.
Base64Decoder bd;
CT2CA s(c);
std::string strStd(s);
bd.Put((byte*)strStd.data(), strStd.size());
bd.MessageEnd();
word64 size = bd.MaxRetrievable();
vector<byte> cypherVector(size);
string decoded;
if (size && size <= SIZE_MAX)
{
decoded.resize(size);
bd.Get((byte*)decoded.data(), decoded.size());
}
Integer cipherMessage((byte*)decoded.data(), decoded.size());

Answer 1

string decoded;
if (size && size <= SIZE_MAX)
{
decoded.resize(size);
bd.Get((byte*)decoded.data(), decoded.size());
}
You have a string called decoded, but you never actually decode the data by running it through a Base64Decoder.
Use something like the following. I don't have a MFC project handy to test, so I'm going to assume you converted the CString to a std::string.
// Converted from Unicode CString
std::string str;
StringSource source(str, true, new Base64Decoder);
Integer value(val, source.MaxRetrievable());
std::cout << std::hex << value << std::endl;
The StringSource is a BufferedTransformation. The Integer constructor you are using is:
Integer (BufferedTransformation &bt, size_t byteCount, Signedness sign=UNSIGNED, ByteOrder order=BIG_ENDIAN_ORDER)
In between the StringSource and the Integer is the Base64Decoder. its a filter that decodes the string on the fly. So data flows from the source (StringSource) to the sink (Integer constructor).
Also see Pipelines on the Crypto++ wiki.

Answer 2

Here is my solution to achieve this. It uses some Qt classes but it should be simple to replace them:
#include <QByteArray>
#include <QScopedArrayPointer>
#include <crypto++/base64.h>
#include <crypto++/rsa.h>
using namespace CryptoPP;
Integer convertBase64ToCryptoPpInt(const QByteArray &base64)
{
Base64Decoder decoder;
decoder.Put(reinterpret_cast<const byte*>(base64.data()), base64.size());
decoder.MessageEnd();
const word64 size = decoder.MaxRetrievable();
QScopedArrayPointer<byte> decoded{new byte[size]};
decoder.Get(decoded.data(), size);
return {decoded.data(), size};
}
QByteArray convertCryptoPpIntToBase64(const Integer &i)
{
// Copy content of i into byte array
const unsigned iLen = i.ByteCount();
QScopedArrayPointer<byte> idata{new byte[iLen]};
i.Encode(idata.data(), iLen);
// Encode data
Base64Encoder encoder;
encoder.Put(idata.data(), iLen);
encoder.MessageEnd();
const int encodedSize = encoder.MaxRetrievable();
QScopedArrayPointer<byte> encoded{new byte[encodedSize]};
encoder.Get(encoded.data(), encodedSize);
return {reinterpret_cast<char*>(encoded.data()), encodedSize};
}
It may be much more compact using CryptoPP's pipelining but i didn't find out how to stream from and to a CryptoPP::Integer.