C# Export Private/Public RSA key from RSACryptoServiceProvider to PEM string

Please note: The code below is for exporting a private key. If you are looking to export the public key, please refer to my answer given here.

The PEM format is simply the ASN.1 DER encoding of the key (per PKCS#1) converted to Base64. Given the limited number of fields needed to represent the key, it’s pretty straightforward to create quick-and-dirty DER encoder to output the appropriate format then Base64 encode it. As such, the code that follows is not particularly elegant, but does the job:

private static void ExportPrivateKey(RSACryptoServiceProvider csp, TextWriter outputStream)
{
    if (csp.PublicOnly) throw new ArgumentException("CSP does not contain a private key", "csp");
    var parameters = csp.ExportParameters(true);
    using (var stream = new MemoryStream())
    {
        var writer = new BinaryWriter(stream);
        writer.Write((byte)0x30); // SEQUENCE
        using (var innerStream = new MemoryStream())
        {
            var innerWriter = new BinaryWriter(innerStream);
            EncodeIntegerBigEndian(innerWriter, new byte[] { 0x00 }); // Version
            EncodeIntegerBigEndian(innerWriter, parameters.Modulus);
            EncodeIntegerBigEndian(innerWriter, parameters.Exponent);
            EncodeIntegerBigEndian(innerWriter, parameters.D);
            EncodeIntegerBigEndian(innerWriter, parameters.P);
            EncodeIntegerBigEndian(innerWriter, parameters.Q);
            EncodeIntegerBigEndian(innerWriter, parameters.DP);
            EncodeIntegerBigEndian(innerWriter, parameters.DQ);
            EncodeIntegerBigEndian(innerWriter, parameters.InverseQ);
            var length = (int)innerStream.Length;
            EncodeLength(writer, length);
            writer.Write(innerStream.GetBuffer(), 0, length);
        }

        var base64 = Convert.ToBase64String(stream.GetBuffer(), 0, (int)stream.Length).ToCharArray();
        outputStream.WriteLine("-----BEGIN RSA PRIVATE KEY-----");
        // Output as Base64 with lines chopped at 64 characters
        for (var i = 0; i < base64.Length; i += 64)
        {
            outputStream.WriteLine(base64, i, Math.Min(64, base64.Length - i));
        }
        outputStream.WriteLine("-----END RSA PRIVATE KEY-----");
    }
}

private static void EncodeLength(BinaryWriter stream, int length)
{
    if (length < 0) throw new ArgumentOutOfRangeException("length", "Length must be non-negative");
    if (length < 0x80)
    {
        // Short form
        stream.Write((byte)length);
    }
    else
    {
        // Long form
        var temp = length;
        var bytesRequired = 0;
        while (temp > 0)
        {
            temp >>= 8;
            bytesRequired++;
        }
        stream.Write((byte)(bytesRequired | 0x80));
        for (var i = bytesRequired - 1; i >= 0; i--)
        {
            stream.Write((byte)(length >> (8 * i) & 0xff));
        }
    }
}

private static void EncodeIntegerBigEndian(BinaryWriter stream, byte[] value, bool forceUnsigned = true)
{
    stream.Write((byte)0x02); // INTEGER
    var prefixZeros = 0;
    for (var i = 0; i < value.Length; i++)
    {
        if (value[i] != 0) break;
        prefixZeros++;
    }
    if (value.Length - prefixZeros == 0)
    {
        EncodeLength(stream, 1);
        stream.Write((byte)0);
    }
    else
    {
        if (forceUnsigned && value[prefixZeros] > 0x7f)
        {
            // Add a prefix zero to force unsigned if the MSB is 1
            EncodeLength(stream, value.Length - prefixZeros + 1);
            stream.Write((byte)0);
        }
        else
        {
            EncodeLength(stream, value.Length - prefixZeros);
        }
        for (var i = prefixZeros; i < value.Length; i++)
        {
            stream.Write(value[i]);
        }
    }
}