OFB.cpp

/*
 * Copyright (C) 2015 Southern Storm Software, Pty Ltd.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 */

#include "OFB.h"
#include "Crypto.h"
#include <string.h>

/**
 * \class OFBCommon OFB.h <OFB.h>
 * \brief Concrete base class to assist with implementing OFB for
 * 128-bit block ciphers.
 *
 * Reference: http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
 *
 * \sa OFB
 */

/**
 * \brief Constructs a new cipher in OFB mode.
 *
 * This constructor should be followed by a call to setBlockCipher().
 */
OFBCommon::OFBCommon()
    : blockCipher(0)
    , posn(16)
{
}

/**
 * \brief Destroys this cipher object after clearing sensitive information.
 */
OFBCommon::~OFBCommon()
{
    clean(iv);
}

size_t OFBCommon::keySize() const
{
    return blockCipher->keySize();
}

size_t OFBCommon::ivSize() const
{
    return 16;
}

bool OFBCommon::setKey(const uint8_t *key, size_t len)
{
    // Verify the cipher's block size, just in case.
    if (blockCipher->blockSize() != 16)
        return false;

    // Set the key on the underlying block cipher.
    return blockCipher->setKey(key, len);
}

bool OFBCommon::setIV(const uint8_t *iv, size_t len)
{
    if (len != 16)
        return false;
    memcpy(this->iv, iv, 16);
    posn = 16;
    return true;
}

void OFBCommon::encrypt(uint8_t *output, const uint8_t *input, size_t len)
{
    uint8_t size;
    while (len > 0) {
        // If we have exhausted the current keystream block, then encrypt
        // the IV/ciphertext to get another keystream block.
        if (posn >= 16) {
            blockCipher->encryptBlock(iv, iv);
            posn = 0;
        }

        // XOR the plaintext with the encrypted IV to get the new ciphertext.
        size = 16 - posn;
        if (size > len)
            size = len;
        len -= size;
        while (size > 0) {
            *output++ = *input++ ^ iv[posn++];
            --size;
        }
    }
}

void OFBCommon::decrypt(uint8_t *output, const uint8_t *input, size_t len)
{
    encrypt(output, input, len);
}

void OFBCommon::clear()
{
    blockCipher->clear();
    clean(iv);
    posn = 16;
}

/**
 * \fn void OFBCommon::setBlockCipher(BlockCipher *cipher)
 * \brief Sets the block cipher to use for this OFB object.
 *
 * \param cipher The block cipher to use to implement OFB mode,
 * which must have a block size of 16 bytes (128 bits).
 */

/**
 * \class OFB OFB.h <OFB.h>
 * \brief Implementation of the Output Feedback (OFB) mode for
 * 128-bit block ciphers.
 *
 * The template parameter T must be a concrete subclass of BlockCipher
 * indicating the specific block cipher to use.  T must have a block size
 * of 16 bytes (128 bits).
 *
 * For example, the following creates a OFB object using AES192 as the
 * underlying cipher:
 *
 * \code
 * OFB<AES192> ofb;
 * ofb.setKey(key, 24);
 * ofb.setIV(iv, 16);
 * ofb.encrypt(output, input, len);
 * \endcode
 *
 * Decryption is identical to encryption for OFB mode.
 *
 * The size of the ciphertext will always be the same as the size of
 * the plaintext.
 *
 * Reference: http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
 *
 * \sa CTR, CFB, CBC
 */

/**
 * \fn OFB::OFB()
 * \brief Constructs a new OFB object for the block cipher T.
 */