[C++] Diffie-Hellman Key Exchange (Client API)

DHKeyCrypt.h

#if !defined(dhkeycrypt_include)

#define dhkeycrypt_include

#include
#include
#include 

/*
 * Example:
 *
 * // initial steps
 * const char *p, *g, *public_key;
 * char *buf;
 * int buf_size;
 * DHKeyCrypt *key = new DHKeyCrypt(256); // mean generate 256 bits prime
 *
 * // p, g, public_key will point to the hex representation string
 * key->GenerateKey(&p, &g, &public_key);
 *
 * // use the 3rd party public key to compute session key
 * key->ComputeKey(other_pubkey);
 *
 * // usage
 * key->Encode(buf, buf_size); // encode buf
 *
 */
class DHKeyCrypt
{
private:
    char *m_g[2];
    DH *m_dh;
    char *m_p;
    char *m_pubkey;
    unsigned char *m_session_key;

    int m_key_len;
    int m_gindex;

public:
    DHKeyCrypt(int size);
    void GenerateKey(const char **p, const char **g, const char **public_key);
    int ComputeKey(const char *pubkey_3rd);
    void Encode(char *buf, int buf_len);
    ~DHKeyCrypt();
};

#endif

DHKeyCrypt.cpp

#include "DHKeyCrypt.h"
#include
#include 

/*
 * generate size bits prime
 */
DHKeyCrypt::DHKeyCrypt(int size) : m_session_key(NULL)
{
    m_g[0] = strdup("2");
    m_g[1] = strdup("5");
    m_gindex = time(NULL) & 1;
    m_dh = DH_generate_parameters(size, m_g[m_gindex][0] - '0', NULL, NULL);
    DH_generate_key(m_dh);
    m_p = BN_bn2hex(m_dh->p);
    m_pubkey = BN_bn2hex(m_dh->pub_key);
}

void DHKeyCrypt::GenerateKey(const char **p, const char **g, const char **public_key)
{
    *p = m_p;
    *g = m_g[m_gindex];
    *public_key = m_pubkey;
}

int DHKeyCrypt::ComputeKey(const char *pubkey_3rd)
{
    BIGNUM *bn = NULL;

    if(!BN_hex2bn(&bn, pubkey_3rd))
        return -1;
    m_session_key = new (std::nothrow) unsigned char [DH_size(m_dh)];
    if(!m_session_key) return -1;
    m_key_len = DH_compute_key(m_session_key, bn, m_dh);
    if(m_key_len == -1) return -1;
    /*
    printf("SessionKey: ");
    for(int i=0;  i < m_key_len; i++)
       printf("%02X", m_session_key[i]);
    printf("\n");
    */
    return 0;
}

void DHKeyCrypt::Encode(char *buf, int buf_len)
{
    for(int i=0; i < buf_len; i++)
        buf[i] = buf[i] ^ m_session_key[i % m_key_len];
}

DHKeyCrypt::~DHKeyCrypt()
{
    OPENSSL_free(m_p);
    OPENSSL_free(m_pubkey);
    free(m_g[0]);
    free(m_g[1]);
    delete [] m_session_key;
    DH_free(m_dh);
}