From 6be59fc8679f65f1fdcdf1f799906efc40859d53 Mon Sep 17 00:00:00 2001
From: Nicolas Constant <github@nicolas-constant.com>
Date: Sat, 21 Mar 2020 04:33:51 -0400
Subject: [PATCH] starting working on RSA

---
 src/BirdsiteLive.Cryptography/Class1.cs       |  8 --
 src/BirdsiteLive.Cryptography/RsaGenerator.cs | 99 +++++++++++++++++++
 2 files changed, 99 insertions(+), 8 deletions(-)
 delete mode 100644 src/BirdsiteLive.Cryptography/Class1.cs
 create mode 100644 src/BirdsiteLive.Cryptography/RsaGenerator.cs

diff --git a/src/BirdsiteLive.Cryptography/Class1.cs b/src/BirdsiteLive.Cryptography/Class1.cs
deleted file mode 100644
index 5b1ef77..0000000
--- a/src/BirdsiteLive.Cryptography/Class1.cs
+++ /dev/null
@@ -1,8 +0,0 @@
-using System;
-
-namespace BirdsiteLive.Cryptography
-{
-    public class Class1
-    {
-    }
-}
diff --git a/src/BirdsiteLive.Cryptography/RsaGenerator.cs b/src/BirdsiteLive.Cryptography/RsaGenerator.cs
new file mode 100644
index 0000000..1be77ce
--- /dev/null
+++ b/src/BirdsiteLive.Cryptography/RsaGenerator.cs
@@ -0,0 +1,99 @@
+using System;
+using System.IO;
+using System.Security.Cryptography;
+
+namespace BirdsiteLive.Cryptography
+{
+    //https://gist.github.com/ststeiger/f4b29a140b1e3fd618679f89b7f3ff4a
+    //https://gist.github.com/valep27/4a720c25b35fff83fbf872516f847863
+    //https://gist.github.com/therightstuff/aa65356e95f8d0aae888e9f61aa29414
+    //https://stackoverflow.com/questions/52468125/export-rsa-public-key-in-der-format-and-decrypt-data
+    public class RsaGenerator
+    {
+        public string GetRsa()
+        {
+            var rsa = RSA.Create();
+            
+            var outputStream = new StringWriter();
+            var parameters = rsa.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);
+                    innerWriter.Write((byte)0x30); // SEQUENCE
+                    EncodeLength(innerWriter, 13);
+                    innerWriter.Write((byte)0x06); // OBJECT IDENTIFIER
+                    var rsaEncryptionOid = new byte[] { 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x01 };
+                    EncodeLength(innerWriter, rsaEncryptionOid.Length);
+                    innerWriter.Write(rsaEncryptionOid);
+                    innerWriter.Write((byte)0x05); // NULL
+                    EncodeLength(innerWriter, 0);
+                    innerWriter.Write((byte)0x03); // BIT STRING
+                    using (var bitStringStream = new MemoryStream())
+                    {
+                        var bitStringWriter = new BinaryWriter(bitStringStream);
+                        bitStringWriter.Write((byte)0x00); // # of unused bits
+                        bitStringWriter.Write((byte)0x30); // SEQUENCE
+                        using (var paramsStream = new MemoryStream())
+                        {
+                            var paramsWriter = new BinaryWriter(paramsStream);
+                            EncodeIntegerBigEndian(paramsWriter, parameters.Modulus); // Modulus
+                            EncodeIntegerBigEndian(paramsWriter, parameters.Exponent); // Exponent
+                            var paramsLength = (int)paramsStream.Length;
+                            EncodeLength(bitStringWriter, paramsLength);
+                            bitStringWriter.Write(paramsStream.GetBuffer(), 0, paramsLength);
+                        }
+                        var bitStringLength = (int)bitStringStream.Length;
+                        EncodeLength(innerWriter, bitStringLength);
+                        innerWriter.Write(bitStringStream.GetBuffer(), 0, bitStringLength);
+                    }
+                    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();
+                // WriteLine terminates with \r\n, we want only \n
+                outputStream.Write("-----BEGIN PUBLIC KEY-----\n");
+                for (var i = 0; i < base64.Length; i += 64)
+                {
+                    outputStream.Write(base64, i, Math.Min(64, base64.Length - i));
+                    outputStream.Write("\n");
+                }
+                outputStream.Write("-----END PUBLIC KEY-----");
+            }
+
+            return outputStream.ToString();
+        }
+
+        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));
+                }
+            }
+        }
+
+    }
+}