I'm using Wincrypt for Diffie-Hellman— can I export the shared secret in plain text?

Question

OK-- thanks to Mike, I was able to get Wincrypt to generate a Diffie-Hellman keypair. I figured out out to export the public key, and how to import the other party's public key. According to the docs, upon import of the other party's public key, the shared secret has been computed. Great.

I now need to get ahold of that shared secret, but I don't think its possible. Simply calling CryptExportKey with a type of PLAINTEXTKEYBLOB fails unless I call CryptSetKeyParam to change the algorithm id from CALG_AGREEDKEY_ANY to something... else. But I don't want something else, I want the shared secret. The API, however, seems designed to discourage this.

Any ideas out there? I should note that the problem here is that I'm only writing one side of an implementation of WiFi Protected Setup. So the protocol is defined for me, and the other party is not giving me HCRYPTKEYs.

Answer 1

This looks like what you need... from: http://msdn.microsoft.com/en-us/library/aa381969(VS.85).aspx

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To import a Diffie-Hellman public key and calculate the secret session key

1. Call the CryptAcquireContext function to get a handle to the Microsoft Diffie-Hellman Cryptographic Provider.
2. Create a Diffie-Hellman key by calling the CryptGenKey function to create a new key, or by calling the CryptGetUserKey function to retrieve an existing key.
3. To import the Diffie-Hellman public key into the CSP, call the CryptImportKey function, passing a pointer to the public key BLOB in the pbData parameter, the length of the BLOB in the dwDataLen parameter, and the handle to the Diffie-Hellman key in the hPubKey parameter. This causes the calculation, (Y^X) mod P, to be performed, thus creating the shared, secret key and completing the key exchange. This function call returns a handle to the new, secret, session key in the hKey parameter.
4. At this point, the imported Diffie-Hellman is of type CALG_AGREEDKEY_ANY. Before the key can be used, it must be converted into a session key type. This is accomplished by calling the CryptSetKeyParam function with dwParam set to KP_ALGID and with pbData set to a pointer to a ALG_ID value that represents a session key, such as CALG_RC4. The key must be converted before using the shared key in the CryptEncrypt or CryptDecrypt function. Calls made to either of these functions prior to converting the key type will fail.
5. The secret session key is now ready to be used for encryption or decryption.
6. When the key is no longer needed, destroy the key handle by calling the CryptDestroyKey function.