Org.BouncyCastle.Crypto.Encodings.ISO9796d1Encoding.DecodeBlock C# (CSharp) Method

		private byte[] DecodeBlock(
			byte[]	input,
			int		inOff,
			int		inLen)
		{
			byte[]  block = engine.ProcessBlock(input, inOff, inLen);
			int     r = 1;
			int     t = (bitSize + 13) / 16;

			BigInteger iS = new BigInteger(1, block);
			BigInteger iR;
			if (iS.Mod(Sixteen).Equals(Six))
			{
				iR = iS;
			}
			else
			{
				iR = modulus.Subtract(iS);

				if (!iR.Mod(Sixteen).Equals(Six))
					throw new InvalidCipherTextException("resulting integer iS or (modulus - iS) is not congruent to 6 mod 16");
			}

			block = iR.ToByteArrayUnsigned();

			if ((block[block.Length - 1] & 0x0f) != 0x6)
				throw new InvalidCipherTextException("invalid forcing byte in block");

			block[block.Length - 1] =
				(byte)(((ushort)(block[block.Length - 1] & 0xff) >> 4)
				| ((inverse[(block[block.Length - 2] & 0xff) >> 4]) << 4));

			block[0] = (byte)((shadows[(uint) (block[1] & 0xff) >> 4] << 4)
				| shadows[block[1] & 0x0f]);

			bool boundaryFound = false;
			int boundary = 0;

			for (int i = block.Length - 1; i >= block.Length - 2 * t; i -= 2)
			{
				int val = ((shadows[(uint) (block[i] & 0xff) >> 4] << 4)
					| shadows[block[i] & 0x0f]);

				if (((block[i - 1] ^ val) & 0xff) != 0)
				{
					if (!boundaryFound)
					{
						boundaryFound = true;
						r = (block[i - 1] ^ val) & 0xff;
						boundary = i - 1;
					}
					else
					{
						throw new InvalidCipherTextException("invalid tsums in block");
					}
				}
			}

			block[boundary] = 0;

			byte[] nblock = new byte[(block.Length - boundary) / 2];

			for (int i = 0; i < nblock.Length; i++)
			{
				nblock[i] = block[2 * i + boundary + 1];
			}

			padBits = r - 1;

			return nblock;
		}
	}