using System;

using Org.BouncyCastle.Crypto.Parameters;

namespace Org.BouncyCastle.Crypto.Modes
{
    /**
    * implements a Output-FeedBack (OFB) mode on top of a simple cipher.
    */
    public class OfbBlockCipher
		: IBlockCipher
    {
        private byte[]	IV;
        private byte[]	ofbV;
        private byte[]	ofbOutV;

        private readonly int			blockSize;
        private readonly IBlockCipher	cipher;

        /**
        * Basic constructor.
        *
        * @param cipher the block cipher to be used as the basis of the
        * feedback mode.
        * @param blockSize the block size in bits (note: a multiple of 8)
        */
        public OfbBlockCipher(
            IBlockCipher cipher,
            int         blockSize)
        {
            this.cipher = cipher;
            this.blockSize = blockSize / 8;

            this.IV = new byte[cipher.GetBlockSize()];
            this.ofbV = new byte[cipher.GetBlockSize()];
            this.ofbOutV = new byte[cipher.GetBlockSize()];
        }

        /**
        * return the underlying block cipher that we are wrapping.
        *
        * @return the underlying block cipher that we are wrapping.
        */
        public IBlockCipher GetUnderlyingCipher()
        {
            return cipher;
        }

        /**
        * Initialise the cipher and, possibly, the initialisation vector (IV).
        * If an IV isn't passed as part of the parameter, the IV will be all zeros.
        * An IV which is too short is handled in FIPS compliant fashion.
        *
        * @param forEncryption if true the cipher is initialised for
        *  encryption, if false for decryption.
        * @param param the key and other data required by the cipher.
        * @exception ArgumentException if the parameters argument is
        * inappropriate.
        */
        public void Init(
            bool				forEncryption, //ignored by this OFB mode
            ICipherParameters	parameters)
        {
			if (parameters is ParametersWithIV)
            {
                ParametersWithIV ivParam = (ParametersWithIV)parameters;
                byte[] iv = ivParam.GetIV();

                if (iv.Length < IV.Length)
                {
                    // prepend the supplied IV with zeros (per FIPS PUB 81)
                    Array.Copy(iv, 0, IV, IV.Length - iv.Length, iv.Length);
                    for (int i = 0; i < IV.Length - iv.Length; i++)
                    {
                        IV[i] = 0;
                    }
                }
                else
                {
                    Array.Copy(iv, 0, IV, 0, IV.Length);
                }

				parameters = ivParam.Parameters;
            }

			Reset();

			cipher.Init(true, parameters);
        }

		/**
        * return the algorithm name and mode.
        *
        * @return the name of the underlying algorithm followed by "/OFB"
        * and the block size in bits
        */
        public string AlgorithmName
        {
            get { return cipher.AlgorithmName + "/OFB" + (blockSize * 8); }
        }

		public bool IsPartialBlockOkay
		{
			get { return true; }
		}

		/**
        * return the block size we are operating at (in bytes).
        *
        * @return the block size we are operating at (in bytes).
        */
        public int GetBlockSize()
        {
            return blockSize;
        }

        /**
        * Process one block of input from the array in and write it to
        * the out array.
        *
        * @param in the array containing the input data.
        * @param inOff offset into the in array the data starts at.
        * @param out the array the output data will be copied into.
        * @param outOff the offset into the out array the output will start at.
        * @exception DataLengthException if there isn't enough data in in, or
        * space in out.
        * @exception InvalidOperationException if the cipher isn't initialised.
        * @return the number of bytes processed and produced.
        */
        public int ProcessBlock(
            byte[]	input,
            int		inOff,
            byte[]	output,
            int		outOff)
        {
            if ((inOff + blockSize) > input.Length)
            {
                throw new DataLengthException("input buffer too short");
            }

            if ((outOff + blockSize) > output.Length)
            {
                throw new DataLengthException("output buffer too short");
            }

            cipher.ProcessBlock(ofbV, 0, ofbOutV, 0);

            //
            // XOR the ofbV with the plaintext producing the cipher text (and
            // the next input block).
            //
            for (int i = 0; i < blockSize; i++)
            {
                output[outOff + i] = (byte)(ofbOutV[i] ^ input[inOff + i]);
            }

            //
            // change over the input block.
            //
            Array.Copy(ofbV, blockSize, ofbV, 0, ofbV.Length - blockSize);
            Array.Copy(ofbOutV, 0, ofbV, ofbV.Length - blockSize, blockSize);

            return blockSize;
        }

        /**
        * reset the feedback vector back to the IV and reset the underlying
        * cipher.
        */
        public void Reset()
        {
            Array.Copy(IV, 0, ofbV, 0, IV.Length);

            cipher.Reset();
        }
    }

}
