IESETL/CMS/Extension/Jwt/RSAUtils.cs

using System;
using System.IO;
using System.Security.Cryptography;

namespace HiTeachCE.Extension
{
    public static class RSAUtils
    {
        private const string PrivateKeyHeader = "-----BEGIN RSA PRIVATE KEY-----";
        private const string PrivateKeyFooter = "-----END RSA PRIVATE KEY-----";

        public static RSA? FromPrivateKey(string key)
        {
            key = key.Trim();

            if (!key.StartsWith(PrivateKeyHeader) || !key.EndsWith(PrivateKeyFooter))
                throw new ArgumentException("Expect PKCS#1 PEM format key");

            key = key.Substring(PrivateKeyHeader.Length, key.Length - PrivateKeyHeader.Length - PrivateKeyFooter.Length);

            // Convert.FromBase64String ignores whitespace
            byte[] keyBytes = Convert.FromBase64String(key);
            return DecodeRSAPrivateKey(keyBytes);
        }

        //------- Parses binary ans.1 RSA private key; returns RSA ---
        static RSA? DecodeRSAPrivateKey(byte[] privateKey)
        {

            // ---------  Set up stream to decode the asn.1 encoded RSA private key  ------
            using var mem = new MemoryStream(privateKey);
            using var binr = new BinaryReader(mem);
            byte bt = 0;
            ushort twobytes = 0;
            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;

            // ------- create RSA from private key RSAParameters -----
            var rsaParameters = new RSAParameters
            {
                Modulus = binr.ReadBytes(GetIntegerSize(binr)),
                Exponent = binr.ReadBytes(GetIntegerSize(binr)),
                D = binr.ReadBytes(GetIntegerSize(binr)),
                P = binr.ReadBytes(GetIntegerSize(binr)),
                Q = binr.ReadBytes(GetIntegerSize(binr)),
                DP = binr.ReadBytes(GetIntegerSize(binr)),
                DQ = binr.ReadBytes(GetIntegerSize(binr)),
                InverseQ = binr.ReadBytes(GetIntegerSize(binr)),
            };
            return RSA.Create(rsaParameters);
        }

        private const string PublicKeyHeader = "-----BEGIN PUBLIC KEY-----";
        private const string PublicKeyFooter = "-----END PUBLIC KEY-----";

        public static RSA? FromPublicKey(string key)
        {
            key = key.Trim();

            if (!key.StartsWith(PublicKeyHeader) || !key.EndsWith(PublicKeyFooter))
                throw new ArgumentException("Expect PKCS#1 PEM format key");

            key = key.Substring(PublicKeyHeader.Length, key.Length - PublicKeyHeader.Length - PublicKeyFooter.Length);

            // Convert.FromBase64String ignores whitespace
            byte[] keyBytes = Convert.FromBase64String(key);
            return DecodeX509PublicKey(keyBytes);
        }

        static RSA? DecodeX509PublicKey(byte[] x509Key)
        {
            // encoded OID sequence for  PKCS #1 rsaEncryption szOID_RSA_RSA = "1.2.840.113549.1.1.1"
            byte[] seqOid = { 0x30, 0x0D, 0x06, 0x09, 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01, 0x05, 0x00 };
            // ---------  Set up stream to read the asn.1 encoded SubjectPublicKeyInfo blob  ------
            using var mem = new MemoryStream(x509Key);
            using var binr = new BinaryReader(mem);
            ushort twobytes = binr.ReadUInt16();
            switch (twobytes)
            {
                case 0x8130:
                    binr.ReadByte();    //advance 1 byte
                    break;
                case 0x8230:
                    binr.ReadInt16();   //advance 2 bytes
                    break;
                default:
                    return null;
            }

            byte[] seq = binr.ReadBytes(15);
            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;

            byte 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);

            byte firstbyte = binr.ReadByte();
            binr.BaseStream.Seek(-1, SeekOrigin.Current);

            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 = binr.ReadByte();        // should only need one byte for actual exponent data (for all useful values)
            byte[] exponent = binr.ReadBytes(expbytes);

            // ------- create RSA from public key RSAParameters -----
            var rsaParameters = new RSAParameters
            {
                Modulus = modulus,
                Exponent = exponent
            };
            return RSA.Create(rsaParameters);
        }

        private static int GetIntegerSize(BinaryReader binr)
        {
            byte bt = binr.ReadByte();
            if (bt != 0x02)     //expect integer
                return 0;
            bt = binr.ReadByte();

            int count;
            if (bt == 0x81)
                count = binr.ReadByte();    // data size in next byte
            else
                if (bt == 0x82)
            {
                byte highbyte = binr.ReadByte();
                byte lowbyte = binr.ReadByte();
                byte[] modint = { lowbyte, highbyte, 0x00, 0x00 };
                count = BitConverter.ToInt32(modint, 0);
            }
            else
            {
                count = bt;     // we already have the data size
            }

            while (binr.ReadByte() == 0x00)
            {   //remove high order zeros in data
                count -= 1;
            }
            binr.BaseStream.Seek(-1, SeekOrigin.Current);     //last ReadByte wasn't a removed zero, so back up a byte
            return count;
        }

        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;
        }
    }
}