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();
}
}
}
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