Question

How one would encrypt a string using asymmetric encryption on WinRT Metro in C# and decrypt this in classic Windows, let’s say in a webservice?

I want to use RsaPkcs1 algorithm and I can see that CryptographicKey class and RSAParameters class are not even near compatible.

How this could be accomplished?

Answer 1

I have found the solution on how to export the keys from the CryptographicKey in a format that .net 4 can successfully use.

CryptographicKey has 2 functions 1. Export and 2. ExportPublicKey. I used the 1st function that exports the private key. This function has 2 parameters 1st is the type of export and 2nd the byte[] that will fill.

If CryptographicPrivateKeyBlobType.Capi1PrivateKey is used, the blob that Export will produce is compatible with the WinCAPI that RSACryptoServiceProvider of .net 4 can import.

WinRT

String str = "String To Encrypt";
IBuffer buf = CryptographicBuffer.ConvertStringToBinary(str,BinaryStringEncoding.Utf16BE);
String AsymmetricAlgName = Windows.Security.Cryptography.Core.AsymmetricAlgorithmNames.RsaPkcs1;
AsymmetricKeyAlgorithmProvider asym = AsymmetricKeyAlgorithmProvider.OpenAlgorithm(AsymmetricAlgName);
CryptographicKey key = asym.CreateKeyPair(512);
IBuffer enc = CryptographicEngine.Encrypt(key, buf, null);
byte[] encryptedbyteArr;
CryptographicBuffer.CopyToByteArray(enc, out encryptedbyteArr);
String encryptedBase64Str = Convert.ToBase64String(encryptedbyteArr);


//Export the private Key in WinCapi format

byte[] privatekeyBytes;
CryptographicBuffer.CopyToByteArray(key.Export(CryptographicPrivateKeyBlobType.Capi1PrivateKey), out privatekeyBytes);
String privatekeyBase64 = Convert.ToBase64String(privatekeyBytes);

encryptedBase64Str now contains the encrypted string in Base64. And lets say that encryptedBase64Str="BwIAAACkAABSU0EyAAIAAAEAAQCFrMTqMU3T14zSUM5..."

Web Service

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
String privateKey64 = "BwIAAACkAABSU0EyAAIAAAEAAQCFrMTqMU3T14zSUM5...";
String EncryptedString = "PbauDOjqMLD2P6WSmEw==";
byte[] EncryptedDataBlob = Convert.FromBase64String(EncryptedString);
byte[] privateKeyBlob = Convert.FromBase64String(privateKey64);
byte[] decryptedBytes;
rsa.ImportCspBlob(privateKeyBlob);
decryptedBytes = rsa.Decrypt(EncryptedDataBlob, false);
String decryptedString =System.Text.Encoding.BigEndianUnicode.GetString(decryptedBytes);

decryptedString now contains the decrypted string that WinRT had.