Secure Web-Based Licensing

Listing 2: Encryption support functions

#include "stdafx.h"
#include <windows.h>
#include <wincrypt.h>
#include <malloc.h>
#include <iostream.h>

// Compute cyclic redundancy checksum - from Binstock & Rex
unsigned short BinstockRexCRC(unsigned short crc, short data)
{
    int i = 0;
    data <<= 8;
    for(i=8; i>0; i--)
    {
        if((data ^ crc) &0x8000)
            crc = (crc << 1) ^ 0x1021;
        else
            crc <<= 1;
        data <<= 1;
    }
    return(crc);
}

unsigned short ComputeCRC(long data)
{
    unsigned short crc = 0;
    char *d = (char*)&data;
    for(int i=0; i<sizeof(long); i++) 
        crc = BinstockRexCRC(crc, d[i]);

    return crc;
}

// Encrypt data with key
bool Encrypt(const char *data,const char *key,char *ret,long size)
{
    HCRYPTPROV hProv = 0;
    HCRYPTHASH hHash = 0;
    HCRYPTKEY hKey = 0;
    BYTE *pbData = NULL;
    DWORD dwDataLen;

    if (!data || !key || !ret)
        return false;

    // Initialize CryptoAPI
    CryptAcquireContext(&hProv, NULL, MS_DEF_PROV,
        PROV_RSA_FULL, CRYPT_VERIFYCONTEXT);
    CryptCreateHash(hProv, CALG_MD5, 0, 0, &hHash); 
    CryptHashData(hHash, (BYTE*)key, strlen(key), 0);
    CryptDeriveKey(hProv, CALG_RC2, hHash, 0, &hKey);
    
    // How big a buffer do I need?
    dwDataLen = strlen(data)+1;
    CryptEncrypt(hKey,NULL,TRUE,0,NULL,&dwDataLen,0);
    
    // If the size is greater than half our buffer length, 
    // we don't have enough room
    if (dwDataLen > (unsigned long) size/2)
    {
        if(pbData !=NULL) free(pbData);
        if(hHash) CryptDestroyHash(hHash);
        if(hKey) CryptDestroyKey(hKey);
        if(hProv) CryptReleaseContext(hProv,0);
        return false;
    }

    pbData = (BYTE*)malloc(dwDataLen);
    strcpy ((char*)pbData,data);
    DWORD inputBufferSize = dwDataLen;
    dwDataLen = strlen(data)+1;

    // Encrypt the data
    CryptEncrypt(hKey,NULL,TRUE,0,pbData,&dwDataLen,
        inputBufferSize);
    
    // Copy the encrypted data to a hex string
    char szBuf[3];
    ret[0] = 0;
    for (int j=0; j< (int)dwDataLen; j++)
    {
        sprintf (szBuf,"%02X",pbData[j]);
        strcat(ret,szBuf);
    }

    if(pbData !=NULL) free(pbData);
    if(hHash) CryptDestroyHash(hHash);
    if(hKey) CryptDestroyKey(hKey);
    if(hProv) CryptReleaseContext(hProv,0);
    return true;
}

// Decrypt data with key
bool Decrypt(const char *data,const char *key,char *ret,long size)
{
    HCRYPTPROV hProv = 0;
    HCRYPTHASH hHash = 0;
    HCRYPTKEY hKey = 0;
    BYTE *pbData = NULL;
    DWORD dwDataLen;

    if (!data || !key ||!ret || strlen(data)%2)
        return false;

    // Convert the string from hex to bin
    pbData = (BYTE*)malloc(strlen(data)/2+10);
    
    char szBuf[3];
    for (int j=0; j<(int)strlen(data); j+=2)
    {
        szBuf[0] = data[j];
        szBuf[1] = data[j+1];
        szBuf[2] = 0;

        sscanf (szBuf,"%02X",pbData+j/2);
    }

    // Initialize the CryptoAPI
    CryptAcquireContext(&hProv, NULL, MS_DEF_PROV,
        PROV_RSA_FULL, CRYPT_VERIFYCONTEXT);
    CryptCreateHash(hProv, CALG_MD5, 0, 0, &hHash);
    CryptHashData(hHash, (BYTE*)key, strlen(key), 0);
    CryptDeriveKey(hProv, CALG_RC2, hHash, 0, &hKey);
    
    dwDataLen = strlen(data)/2;

    // Decrypt the data
    CryptDecrypt(hKey,0,TRUE,0,pbData,&dwDataLen);
    
    // Is the return buffer big enough?
    if (dwDataLen > (unsigned long)size/2)
    {
        if(pbData !=NULL) free(pbData);
        if(hHash) CryptDestroyHash(hHash);
        if(hKey) CryptDestroyKey(hKey);
        if(hProv) CryptReleaseContext(hProv,0);
        return false;
    }

    strcpy(ret,(const char*)pbData);

    if(pbData !=NULL) free(pbData);
    if(hHash) CryptDestroyHash(hHash);
    if(hKey) CryptDestroyKey(hKey);
    if(hProv) CryptReleaseContext(hProv,0);

    return true;
}