Decrypting with private key from .pem file in c# with .NET crypto library

I know this is a similar question to this one but before I head down the Bouncey Castle route, does anyone know if its possible to load an RSA KeyPair from a .pem file, e.g.:

directly with the .NET 3.5 crypto library without having to go to a 3rd party or roll my own?

Solution

http://www.jensign.com/opensslkey/index.html

with source at http://www.jensign.com/opensslkey/opensslkey.cs Update: Source code is no longer available at this url. It can be found at https://gist.github.com/stormwild/7887264 or https://web.archive.org/web/20170731015547/http://www.jensign.com/opensslkey/opensslkey.cs now.

edit: excerpted relevant code:

first, extract the text between the ---- BEGIN ---- and ---- END ---- sections, and base64-decode it into a byte array (see link above for details), then pass it to:

//------- Parses binary ans.1 RSA private key; returns RSACryptoServiceProvider  ---
public static RSACryptoServiceProvider DecodeRSAPrivateKey(byte[] privkey)
{
    byte[] MODULUS, E, D, P, Q, DP, DQ, IQ ;

// ---------  Set up stream to decode the asn.1 encoded RSA private key  ------
    MemoryStream  mem = new MemoryStream(privkey) ;
    BinaryReader binr = new BinaryReader(mem) ;    //wrap Memory Stream with BinaryReader for easy reading
    byte bt = 0;
    ushort twobytes = 0;
    int elems = 0;
    try {
        twobytes = binr.ReadUInt16();
        if (twobytes == 0x8130) //data read as little endian order (actual data order for Sequence is 30 81)
            binr.ReadByte();    //advance 1 byte
        else if (twobytes == 0x8230)
            binr.ReadInt16();   //advance 2 bytes
        else
            return null;

        twobytes = binr.ReadUInt16();
        if (twobytes != 0x0102) //version number
            return null;
        bt = binr.ReadByte();
        if (bt !=0x00)
            return null;


//------  all private key components are Integer sequences ----
        elems = GetIntegerSize(binr);
        MODULUS = binr.ReadBytes(elems);

        elems = GetIntegerSize(binr);
        E = binr.ReadBytes(elems) ;

        elems = GetIntegerSize(binr);
        D = binr.ReadBytes(elems) ;

        elems = GetIntegerSize(binr);
        P = binr.ReadBytes(elems) ;

        elems = GetIntegerSize(binr);
        Q = binr.ReadBytes(elems) ;

        elems = GetIntegerSize(binr);
        DP = binr.ReadBytes(elems) ;

        elems = GetIntegerSize(binr);
        DQ = binr.ReadBytes(elems) ;

        elems = GetIntegerSize(binr);
        IQ = binr.ReadBytes(elems) ;

        Console.WriteLine("showing components ..");
        if (verbose) {
            showBytes("\nModulus", MODULUS) ;
            showBytes("\nExponent", E);
            showBytes("\nD", D);
            showBytes("\nP", P);
            showBytes("\nQ", Q);
            showBytes("\nDP", DP);
            showBytes("\nDQ", DQ);
            showBytes("\nIQ", IQ);
        }

// ------- create RSACryptoServiceProvider instance and initialize with public key -----
        RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
        RSAParameters RSAparams = new RSAParameters();
        RSAparams.Modulus =MODULUS;
        RSAparams.Exponent = E;
        RSAparams.D = D;
        RSAparams.P = P;
        RSAparams.Q = Q;
        RSAparams.DP = DP;
        RSAparams.DQ = DQ;
        RSAparams.InverseQ = IQ;
        RSA.ImportParameters(RSAparams);
        return RSA;
    }
    catch (Exception) {
        return null;
    }
    finally {
        binr.Close();
    }
}