Integer to char* and char* to Integer conversion - c++

Question

I'm using the Crypto++ library, and I created this simple class:
class EncoderRSA
{
public:
EncoderRSA();
void keyGeneration();
void substitutePublicKey(Integer e, Integer n);
Integer encode(std::string plainText);
std::string decode(Integer cypher);
private:
AutoSeededRandomPool prng; // Pseudo Random Number Generator
RSA::PublicKey publicKey; // For encrypt plain text
RSA::PrivateKey privateKey; // For decrypt plain text
};
But I have some problem: when I encoded my message using EncoderRSA::encode(message), I want to convert it from Integer to char* (for send by sockets) and from char* to Integer (after receive on the other side). How can I do it?

Answer 1

Integer to char* and char* to Integer conversion
I'm going to answer the question in the title, and side step the body's question because that appear to encrypt, and not encode.
Integer to char*
Its easy to get a char* because the Integer class (header, implementation) overloads operator<<.
So, you would do something like:
#include <iostream>
#include <sstream>
#include <string>
#include "integer.h"
...
using std::cout;
using std::endl;
using std::string;
using CryptoPP::Integer;
// Perhaps this is received from the EncoderRSA::encode() function
Integer n = ...;
ostringstream oss;
oss << n;
// Create this temporary; otherwise, you might get yourself into trouble
string str(oss.str());
cout << "string: " << str << endl;
cout << "char*: " << str.c_str() << endl;
char* to Integer
This is just as easy because the Integer class has a constructor for it. From integer.h:
// convert from string (requires NULL terminator)
Integer (const char *str)
Here's the program:
#include <string>
#include "integer.h"
...
using std::cout;
using std::endl;
using std::string;
using CryptoPP::Integer;
// Read from the wire
const char data[] = "12345678901234567890123456789012345678901234567890123456789012345678901234567890.";
// Perhaps this is passed to the EncoderRSA::decode() function
Integer n(data, strlen(data));
If you want the byte array version, then...
Integer to byte*
#include <vector>
#include "integer.h"
...
using std::vector;
using CryptoPP::Integer;
// Perhaps this is received from the EncoderRSA::encode() function
Integer n = ...;
size_t sz = n.MinEncodedSize();
vector v;
v.resize(sz);
n.Encode(&v[0], v.size());
byte* to Integer
This is just as easy because the Integer class has a constructor for it. From integer.h:
// convert from string (does NOT require NULL terminator)
Decode (const byte *input, size_t inputLen, Signedness=UNSIGNED)
Here's the program:
#include "integer.h"
...
using CryptoPP::Integer;
// Read from the wire; pretend they are binary
const char byte[] = "12345678901234567890123456789012345678901234567890123456789012345678901234567890";
// Perhaps this is passed to the EncoderRSA::decode() function
Integer n;
n.decode(data, sizeof(data));
You can get even fancier with the encoding and decoding. For example, here's a program that HexEncodes encodes them:
#include <iostream>
#include <string>
#include "integer.h"
...
using std::cout;
using std::endl;
using std::string;
using CryptoPP::Integer;
// Perhaps this is received from the EncoderRSA::encode() function
Integer n("0xfedcba0987654321fedcba0987654321fedcba0987654321fedcba0987654321");
string encoded;
size_t req = n.MinEncodedSize();
encoded.reserve(req);
HexEncoder encoder(new StringSink(encoded));
n.Encode(encoder, req);
cout << encoded << endl;
It produces:
$ ./cryptopp-test.exe
FEDCBA0987654321FEDCBA0987654321FEDCBA0987654321FEDCBA0987654321
And you can switch to a Base64 encoder with:
Base64Encoder encoder(new StringSink(encoded));
n.Encode(encoder, req);
cout << encoded << endl;
It produces:
$ ./cryptopp-test.exe
/ty6CYdlQyH+3LoJh2VDIf7cugmHZUMh/ty6CYdl

Answer 2

Resolve of this problem are next two methods:
Integer EncryptorRSA::stringToInteger(std::string str) {
Integer integer(str.c_str());
return integer;
}
std::string EncryptorRSA::integerToString(Integer integer) {
std::stringstream stream;
stream << std::hex << integer;
return stream.str();
}