Reading PEM RSA Public Key Only using Bouncy Castle

Question

I am trying to use C# to read in a .pem file that contains only a RSA public key. I do not have access to the private key information, nor does my application require it. The file myprivatekey.pem file begins with

-----BEGIN PUBLIC KEY----- and ends with -----END PUBLIC KEY-----.

My current code is as follows:

    Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair keyPair;

    using (var reader = File.OpenText(@"c:\keys\myprivatekey.pem"))
        keyPair = (Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair)new Org.BouncyCastle.OpenSsl.PemReader(reader).ReadObject();

However the code throws an InvalidCastException with the message

Unable to cast object of type 'Org.BouncyCastle.Crypto.Parameters.DsaPublicKeyParameters' to type 'Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair'.

How can I use Bouncy Castle's PemReader to read only a public key, when no private key information is available?

Answer 1

The following code will read a public key from a given filename. The exception handling should be changed for any production code. This method returns an AsymetricKeyParameter:

public Org.BouncyCastle.Crypto.AsymmetricKeyParameter ReadAsymmetricKeyParameter(string pemFilename)
{
    var fileStream = System.IO.File.OpenText(pemFilename);
    var pemReader = new Org.BouncyCastle.OpenSsl.PemReader(fileStream);
    var KeyParameter = (Org.BouncyCastle.Crypto.AsymmetricKeyParameter)pemReader.ReadObject();
    return KeyParameter;
}

Answer 2

Here's a possible solution that reads both public and private PEM files into RSACryptoServiceProvider:

public class PemReaderB
{
    public static RSACryptoServiceProvider GetRSAProviderFromPem(String pemstr)
    {
        CspParameters cspParameters = new CspParameters();
        cspParameters.KeyContainerName = "MyKeyContainer";
        RSACryptoServiceProvider rsaKey = new RSACryptoServiceProvider(cspParameters);

        Func<RSACryptoServiceProvider, RsaKeyParameters, RSACryptoServiceProvider> MakePublicRCSP = (RSACryptoServiceProvider rcsp, RsaKeyParameters rkp) =>
        {
            RSAParameters rsaParameters = DotNetUtilities.ToRSAParameters(rkp);
            rcsp.ImportParameters(rsaParameters);
            return rsaKey;
        };

        Func<RSACryptoServiceProvider, RsaPrivateCrtKeyParameters, RSACryptoServiceProvider> MakePrivateRCSP = (RSACryptoServiceProvider rcsp, RsaPrivateCrtKeyParameters rkp) =>
        {
            RSAParameters rsaParameters = DotNetUtilities.ToRSAParameters(rkp);
            rcsp.ImportParameters(rsaParameters);
            return rsaKey;
        };

        PemReader reader = new PemReader(new StringReader(pemstr));
        object kp = reader.ReadObject();

        // If object has Private/Public property, we have a Private PEM
        return (kp.GetType().GetProperty("Private") != null) ? MakePrivateRCSP(rsaKey, (RsaPrivateCrtKeyParameters)(((AsymmetricCipherKeyPair)kp).Private)) : MakePublicRCSP(rsaKey, (RsaKeyParameters)kp);
    }

    public static RSACryptoServiceProvider GetRSAProviderFromPemFile(String pemfile)
    {
        return GetRSAProviderFromPem(File.ReadAllText(pemfile).Trim());
    }
}

Hope this helps someone.