Obtain the Plain Text Session Key using CryptoAPI

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Introduction

Sometimes, extracting raw session keys is necessary, particularly when implementing cryptographic protocols. However, Microsoft Cryptographic Providers (Base and Enhanced) do not 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 more straightforward.

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

History

• 11^th December, 2001: Initial version
• 22^nd December, 2022: Update - Corrected some problems in the text and the license