Obtain the plain text session key using CryptoAPI

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Introduction

Sometimes, extracting raw session keys is necessary, particularly when you need to implement cryptographic protocols. However, Microsoft Cryptographic Providers (Base and Enhanced) don't support this feature. CryptExportKey() and CryptImportKey() require a valid key handle to encrypt and decrypt the session key, respectively. MSDN shows a way of doing this using a exponent-of-one private key. This article shows a better way to perform the same process. This way is faster and easier.

It's ready to go, but you must set the following parameters at Project -> Settings (Visual Studio 6.0 ) :

• Add in C++/Preprocessor definitions: _WIN32_WINNT=0x0500, _CRYPT32_(WIN2K) or _WIN32_WINNT=0x0400, _CRYPT32_(NT4)
• And Link this library  crypt32.lib

Code Listing

#include <windows.h>
#include <wincrypt.h>


#define KEY_PAIR_SIZE     dwSize - 12
#define SESSION_KEY_SIZE  dwKeyMaterial

void main()
{

    HCRYPTPROV hProv = 0;
    HCRYPTKEY hExchangeKeyPair = 0;
    HCRYPTKEY hSessionKey = 0;
    BYTE *pbKeyMaterial  = NULL;
    DWORD dwKeyMaterial ;   
    BYTE *pbExportedKeyBlob = NULL;
    BYTE *pbEncryptedKey    = NULL;
    DWORD dwSize;
    unsigned int c;

    __try
    {

        if (!CryptAcquireContext( &hProv, "Container Name", 
            MS_ENHANCED_PROV , PROV_RSA_FULL, CRYPT_MACHINE_KEYSET ))
        {
            __leave;
        }

        //---------------------------------------------------
        //Creating a session key. In this sample we'll use a 
        //3DES key with 168 bits

        if (!CryptGenKey(hProv,CALG_3DES,CRYPT_EXPORTABLE,
            &hSessionKey))
        {
            __leave;
        }

        //---------------------------------------------------
        //Getting a handle to the Exchange Key pair

        if (!CryptGetUserKey( hProv, AT_KEYEXCHANGE, 
            &hExchangeKeyPair))
        {
            __leave;
        }

        //--------------------------------------------------------
        //Encrypting the session key with the public key part
        //of the keypair. The first call gets the size necessary to 
        //hold the encrypted session key and the second exports it.

        if (!CryptExportKey( hSessionKey, hExchangeKeyPair, 
            SIMPLEBLOB, 0, NULL, &dwSize))
        {
            __leave;
        }

        pbExportedKeyBlob = new BYTE[dwSize];

        if (!CryptExportKey( hSessionKey, hExchangeKeyPair, SIMPLEBLOB, 
            0, pbExportedKeyBlob,  &dwSize))
        {
            __leave;
        }

        //--------------------------------------------------------
        //Let's  remove the first 12 bytes of Blob information 

        pbEncryptedKey  =  new BYTE [KEY_PAIR_SIZE];  

        for ( c = 0 ; c < KEY_PAIR_SIZE ; c++ )
        {
            pbEncryptedKey[c] =  pbExportedKeyBlob[c+12]; 
        }

        //--------------------------------------------------------
        //Now it's time to get the value of the session key, we'll
        //use the private part of the key pair. 

        if (!CryptDecrypt( hExchangeKeyPair,0, TRUE, 0,  
            pbEncryptedKey, &dwKeyMaterial))
        {
            __leave;
        }

        //-------------------------------------------------------
        //The pbKeyMaterial is the value of the key

        pbKeyMaterial = new BYTE[ SESSION_KEY_SIZE ];

        for ( c = 0; c <  SESSION_KEY_SIZE ; c++ )
        {
            pbKeyMaterial[c] = pbEncryptedKey[c];    
        }

        //-------------------------------------------------------
        //The key is in big-endian format, because that's the way
        //the encryption block is built.

    }
    __finally
    {
        if (pbKeyMaterial ) LocalFree(pbKeyMaterial );
        if (hSessionKey) CryptDestroyKey(hSessionKey);
        if (hExchangeKeyPair) CryptDestroyKey(hExchangeKeyPair);
        if (hProv) 
        {  
            CryptReleaseContext(hProv, 0);
        }

    }

} // End Main