PEMKeyLoader.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Security.Cryptography;
using System.IO;

namespace TestJWT
{
    public class PEMKeyLoader
    {

        // encoded OID sequence for  PKCS #1 rsaEncryption szOID_RSA_RSA = "1.2.840.113549.1.1.1"
        static byte[] SeqOID = { 0x30, 0x0D, 0x06, 0x09, 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01, 0x05, 0x00 };

        private static bool CompareBytearrays(byte[] a, byte[] b)
        {
            if (a.Length != b.Length)
                return false;
            int i = 0;
            foreach (byte c in a)
            {
                if (c != b[i])
                    return false;
                i++;
            }
            return true;
        }

        public static RSACryptoServiceProvider CryptoServiceProviderFromPublicKeyInfo(byte[] x509key)
        {
            byte[] seq = new byte[15];
            int x509size;

            if (x509key == null || x509key.Length == 0)
                return null;

            x509size = x509key.Length;

            // ---------  Set up stream to read the asn.1 encoded SubjectPublicKeyInfo blob  ------
            MemoryStream mem = new MemoryStream(x509key);
            BinaryReader binr = new BinaryReader(mem);    //wrap Memory Stream with BinaryReader for easy reading
            byte bt = 0;
            ushort twobytes = 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;

                seq = binr.ReadBytes(15);		//read the Sequence OID
                if (!CompareBytearrays(seq, SeqOID))	//make sure Sequence for OID is correct
                    return null;

                twobytes = binr.ReadUInt16();
                if (twobytes == 0x8103)	//data read as little endian order (actual data order for Bit String is 03 81)
                    binr.ReadByte();	//advance 1 byte
                else if (twobytes == 0x8203)
                    binr.ReadInt16();	//advance 2 bytes
                else
                    return null;

                bt = binr.ReadByte();
                if (bt != 0x00)		//expect null byte next
                    return null;

                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();
                byte lowbyte = 0x00;
                byte highbyte = 0x00;

                if (twobytes == 0x8102)	//data read as little endian order (actual data order for Integer is 02 81)
                    lowbyte = binr.ReadByte();	// read next bytes which is bytes in modulus
                else if (twobytes == 0x8202)
                {
                    highbyte = binr.ReadByte();	//advance 2 bytes
                    lowbyte = binr.ReadByte();
                }
                else
                    return null;
                byte[] modint = { lowbyte, highbyte, 0x00, 0x00 };   //reverse byte order since asn.1 key uses big endian order
                int modsize = BitConverter.ToInt32(modint, 0);

                int firstbyte = binr.PeekChar();
                if (firstbyte == 0x00)
                {	//if first byte (highest order) of modulus is zero, don't include it
                    binr.ReadByte();	//skip this null byte
                    modsize -= 1;	//reduce modulus buffer size by 1
                }

                byte[] modulus = binr.ReadBytes(modsize);	//read the modulus bytes

                if (binr.ReadByte() != 0x02)			//expect an Integer for the exponent data
                    return null;
                int expbytes = (int)binr.ReadByte();		// should only need one byte for actual exponent data (for all useful values)
                byte[] exponent = binr.ReadBytes(expbytes);


                // ------- create RSACryptoServiceProvider instance and initialize with public key -----
                RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
                RSAParameters RSAKeyInfo = new RSAParameters();
                RSAKeyInfo.Modulus = modulus;
                RSAKeyInfo.Exponent = exponent;
                RSA.ImportParameters(RSAKeyInfo);

                return RSA;
            }
            finally
            {
                binr.Close();
            }
        }

        public static RSACryptoServiceProvider CryptoServiceProviderFromPublicKeyInfo(String base64EncodedKey)
        {
            try
            {
                //see if the file is a valid Base64 encoded cert
                return CryptoServiceProviderFromPublicKeyInfo(Convert.FromBase64String(base64EncodedKey));
            }
            catch (System.FormatException)
            {
            }

            return null;
        }

        public static byte[] X509KeyFromFile(String filename)
        {
            byte[] x509key;

            if (String.IsNullOrWhiteSpace(filename) || !File.Exists(filename))
                return null;

            StreamReader sr = File.OpenText(filename);
            String filestr = sr.ReadToEnd();
            sr.Close();
            StringBuilder sb = new StringBuilder(filestr);
            sb.Replace("-----BEGIN PUBLIC KEY-----", "");  //remove headers/footers, if present
            sb.Replace("-----END PUBLIC KEY-----", "");

            try
            {
                //see if the file is a valid Base64 encoded cert
                x509key = Convert.FromBase64String(sb.ToString());
            }
            catch (System.FormatException)
            {
                //if not a b64-encoded publiccert, assume it's binary
                Stream stream = new FileStream(filename, FileMode.Open);
                int datalen = (int)stream.Length;
                x509key = new byte[datalen];
                stream.Read(x509key, 0, datalen);
                stream.Close();
            }

            return x509key;
        }

    }
}