Converting an RSA Public Key into a RFC 4716 Public Key with Bouncy Castle

Question

I am looking to convert an RSA public key into something that I could use as an SSH public key.

Currently I have Bouncy Castle producing me a public key that looks l

Answer 1

Note: I work at Microsoft but this is not a Microsoft Answer, just Personal.

Adding to Pavels Answer,

I've found that for some reason when generating a 3072 Bit RSA key, PuttyGen would generate a different RSA public key than mine.

After researching, I found that it seems in the source code of the Putty Gen Program it would do .Length + 1 to the Byte array, adding a leading 0.

For the BouncyCastle, you would change this line.

                    ms.Write(ToBytes(n.Length), 0, 4);
                    ms.Write(n, 0, n.Length);

to

                    ms.Write(ToBytes(n.Length+1), 0, 4);//Add +1 to Emulate PuttyGen
                    ms.Write(new byte[] { 0 }, 0, 1); //Add a 0 to Emulate PuttyGen
                    ms.Write(n, 0, n.Length);

For Microsoft .net RSACryptoServiceProvider it would look like this

            RSACryptoServiceProvider RSA = new RSACryptoServiceProvider(3072);
            byte[] sshrsa_bytes = Encoding.Default.GetBytes("ssh-rsa");
            byte[] n = RSA.ExportParameters(false).Modulus;
            byte[] e = RSA.ExportParameters(false).Exponent;
            string buffer64;
            using (MemoryStream ms = new MemoryStream())
            {
                ms.Write(ToBytes(sshrsa_bytes.Length), 0, 4);
                ms.Write(sshrsa_bytes, 0, sshrsa_bytes.Length);
                ms.Write(ToBytes(e.Length), 0, 4);
                ms.Write(e, 0, e.Length);
                ms.Write(ToBytes(n.Length+1), 0, 4); //Remove the +1 if not Emulating Putty Gen
                ms.Write(new byte[] { 0 }, 0, 1); //Add a 0 to Emulate PuttyGen
                ms.Write(n, 0, n.Length);
                ms.Flush();
                buffer64 = Convert.ToBase64String(ms.ToArray());
            }

            string pubssh = string.Format("ssh-rsa {0} generated-key", buffer64);

You can see my Private Key I used for testing & the putty gen source code link https://www.cameronmoten.com/2017/12/21/rsacryptoserviceprovider-create-a-ssh-rsa-public-key/