NBitcoin.BouncyCastle.Asn1.Asn1InputStream.ReadTagNumber C# (CSharp) Method

		public static int ReadTagNumber(
			Stream s,
			int tag)
		{
			int tagNo = tag & 0x1f;

			//
			// with tagged object tag number is bottom 5 bits, or stored at the start of the content
			//
			if(tagNo == 0x1f)
			{
				tagNo = 0;

				int b = s.ReadByte();

				// X.690-0207 8.1.2.4.2
				// "c) bits 7 to 1 of the first subsequent octet shall not all be zero."
				if((b & 0x7f) == 0) // Note: -1 will pass
				{
					throw new IOException("Corrupted stream - invalid high tag number found");
				}

				while((b >= 0) && ((b & 0x80) != 0))
				{
					tagNo |= (b & 0x7f);
					tagNo <<= 7;
					b = s.ReadByte();
				}

				if(b < 0)
					throw new EndOfStreamException("EOF found inside tag value.");

				tagNo |= (b & 0x7f);
			}

			return tagNo;
		}

        public virtual IAsn1Convertible ReadObject()
        {
            int tag = _in.ReadByte();

            if (tag == -1)
            {
                return(null);
            }

            // turn of looking for "00" while we resolve the tag
            Set00Check(false);

            //
            // calculate tag number
            //
            int tagNo = Asn1InputStream.ReadTagNumber(_in, tag);

            bool isConstructed = (tag & Asn1Tags.Constructed) != 0;

            //
            // calculate length
            //
            int length = Asn1InputStream.ReadLength(_in, _limit);

            if (length < 0)             // indefinite length method
            {
                if (!isConstructed)
                {
                    throw new IOException("indefinite length primitive encoding encountered");
                }

                IndefiniteLengthInputStream indIn = new IndefiniteLengthInputStream(_in, _limit);
                Asn1StreamParser            sp    = new Asn1StreamParser(indIn, _limit);

                if ((tag & Asn1Tags.Application) != 0)
                {
                    return(new BerApplicationSpecificParser(tagNo, sp));
                }

                if ((tag & Asn1Tags.Tagged) != 0)
                {
                    return(new BerTaggedObjectParser(true, tagNo, sp));
                }

                return(sp.ReadIndef(tagNo));
            }
            else
            {
                DefiniteLengthInputStream defIn = new DefiniteLengthInputStream(_in, length);

                if ((tag & Asn1Tags.Application) != 0)
                {
                    return(new DerApplicationSpecific(isConstructed, tagNo, defIn.ToArray()));
                }

                if ((tag & Asn1Tags.Tagged) != 0)
                {
                    return(new BerTaggedObjectParser(isConstructed, tagNo, new Asn1StreamParser(defIn)));
                }

                if (isConstructed)
                {
                    // TODO There are other tags that may be constructed (e.g. BitString)
                    switch (tagNo)
                    {
                    case Asn1Tags.OctetString:
                        //
                        // yes, people actually do this...
                        //
                        return(new BerOctetStringParser(new Asn1StreamParser(defIn)));

                    case Asn1Tags.Sequence:
                        return(new DerSequenceParser(new Asn1StreamParser(defIn)));

                    case Asn1Tags.Set:
                        return(new DerSetParser(new Asn1StreamParser(defIn)));

                    case Asn1Tags.External:
                        return(new DerExternalParser(new Asn1StreamParser(defIn)));

                    default:
                        throw new IOException("unknown tag " + tagNo + " encountered");
                    }
                }

                // Some primitive encodings can be handled by parsers too...
                switch (tagNo)
                {
                case Asn1Tags.OctetString:
                    return(new DerOctetStringParser(defIn));
                }

                try
                {
                    return(Asn1InputStream.CreatePrimitiveDerObject(tagNo, defIn, tmpBuffers));
                }
                catch (ArgumentException e)
                {
                    throw new Asn1Exception("corrupted stream detected", e);
                }
            }
        }