How to import RSA public keys using modulus and exponent in C++ using CNG?

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1. Initialize CNG Provider: Initialize the CNG provider by calling the BCryptOpenAlgorithmProvider function with the desired algorithm name.

BCRYPT_ALG_HANDLE hAlg; // Declare handle for CNG provider
LPCWSTR pszAlgId = BCRYPT_RSA_ALGORITHM; // Algorithm ID for RSA
DWORD dwFlags = 0; // Flags for provider mode
HRESULT hr = BCryptOpenAlgorithmProvider(&hAlg, pszAlgId, NULL, dwFlags);
if(FAILED(hr))
{
    printf("Error in initializing the CNG provider: 0x%x\n", hr);
    return hr;
}

1. Create BCrypt Key Blob from Modulus and Exponent:

The RSA public key can be represented in the form of the modulus and exponent values. To import these values, you need to create a BCRYPTRSAKEYBLOB structure.

BCRYPT_RSAKEY_BLOB *pBlob = NULL; // Declare pointer to RSA key blob
DWORD cbBlob; // Size of the key blob
DWORD cbModulus = nModulusSizeInBytes; // Length of the modulus in bytes
PSZ pszExponent = "010001"; // Exponent value in hex format
cbBlob = sizeof(BCRYPT_RSAKEY_BLOB) + cbModulus + strlen(pszExponent)/2;
pBlob = (BCRYPT_RSAKEY_BLOB*) new BYTE[cbBlob];
pBlob->Magic = BCRYPT_RSAPUBLIC_MAGIC; // Magic value for RSA public key
pBlob->BitLength = nModulusSizeInBits; // Length of modulus in bits
pBlob->cbPublicExp = strlen(pszExponent)/2; // Length of exponent in bytes
pBlob->cbModulus = cbModulus; // Length of modulus in bytes
memcpy((PBYTE)pBlob+sizeof(BCRYPT_RSAKEY_BLOB), pModulus, cbModulus); // Copy modulus data
// Convert exponent to binary
PBYTE pbExponent = new BYTE[pBlob->cbPublicExp];
pszExponent = "010001"; // Exponent value in hex format
StringToBinary(pszExponent, pbExponent, pBlob->cbPublicExp);
memcpy((PBYTE)pBlob+sizeof(BCRYPT_RSAKEY_BLOB)+cbModulus, pbExponent, pBlob->cbPublicExp); // Copy exponent data

1. Import BCrypt Key Blob as CNG Key:

Now that you have created the key blob from the modulus and exponent values, you can import the key blob as a CNG key using the BCryptImportKey function.

BCRYPT_KEY_HANDLE hKey; // Handle to the imported key
BCRYPT_RSAKEY_BLOB *pBlob = (BCRYPT_RSAKEY_BLOB*) pbKeyData; // Pointer to the key blob from previous step
DWORD cbKeyData = dwKeyDataSize; // Size of the key data blob
LPCWSTR pszBlobType = BCRYPT_RSAPUBLIC_BLOB; // Blob type for RSA public key
HRESULT hr = BCryptImportKey(hAlg, NULL, pszBlobType, &hKey, NULL, 0, (PBYTE)pBlob, cbKeyData, BCRYPT_NO_KEY_VALIDATION);
if(FAILED(hr))
{
    printf("Error in importing the RSA public key: 0x%x\n", hr);
    return hr;
}

1. Clean Up:

Finally, clean up by closing the CNG provider and releasing the memory used by the key blob.

// Closing the CNG provider
BCryptCloseAlgorithmProvider(hAlg, 0);
// Releasing memory used by the key blob
delete[] pbKeyData;
delete[] pbExponent;
delete[] pBlob;

Note: This code provides only an outline of the process. You may need to make some modifications based on your specific scenario, architecture, and requirements.