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Хостинг:
bdes - encrypt/decrypt using the Data Encryption Standard (DES)
All modes but the electronic code book (ECB) mode require an initialization vector; if none is supplied, the zero vector is used. If no key is specified on the command line, the user is prompted for one (see getpass(3) for more details).
The options are as follows:
The key and initialization vector are taken as sequences of ASCII characters which are then mapped into their bit representations. If either begins with ``0X '' or ``0x '' that one is taken as a sequence of hexadecimal digits indicating the bit pattern; if either begins with ``0B '' or ``0b '' that one is taken as a sequence of binary digits indicating the bit pattern. In either case, only the leading 64 bits of the key or initialization vector are used, and if fewer than 64 bits are provided, enough 0 bits are appended to pad the key to 64 bits.
According to the DES standard, the low-order bit of each character in the key string is deleted. Since most ASCII representations set the high-order bit to 0, simply deleting the low-order bit effectively reduces the size of the key space from 2^56 to 2^48 keys. To prevent this, the high-order bit must be a function depending in part upon the low-order bit; so, the high-order bit is set to whatever value gives odd parity. This preserves the key space size. Note this resetting of the parity bit is not done if the key is given in binary or hex, and can be disabled for ASCII keys as well.
The DES is considered a very strong cryptosystem, and other than table lookup attacks, key search attacks, and Hellman's time-memory tradeoff (all of which are very expensive and time-consuming), no cryptanalytic methods for breaking the DES are known in the open literature. No doubt the choice of keys and key security are the most vulnerable aspect of .
In the ECB and CBC modes, plaintext is encrypted in units of 64 bits (8 bytes, also called a block). To ensure that the plaintext file is encrypted correctly, will (internally) append from 1 to 8 bytes, the last byte containing an integer stating how many bytes of that final block are from the plaintext file, and encrypt the resulting block. Hence, when decrypting, the last block may contain from 0 to 7 characters present in the plaintext file, and the last byte tells how many. Note that if during decryption the last byte of the file does not contain an integer between 0 and 7, either the file has been corrupted or an incorrect key has been given. A similar mechanism is used for the OFB and CFB modes, except that those simply require the length of the input to be a multiple of the mode size, and the final byte contains an integer between 0 and one less than the number of bytes being used as the mode. (This was another reason that the mode size must be a multiple of 8 for those modes.)
Unlike Sun's implementation, unused bytes of that last block are not filled with random data, but instead contain what was in those byte positions in the preceding block. This is quicker and more portable, and does not weaken the encryption significantly.
If the key is entered in ASCII the parity bits of the key characters are set so that each key character is of odd parity. Unlike Sun's implementation, it is possible to enter binary or hexadecimal keys on the command line, and if this is done, the parity bits are not reset. This allows testing using arbitrary bit patterns as keys.
The Sun implementation always uses an initialization vector of 0 (that is, all zeroes). By default, does too, but this may be changed from the command line.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
As the key or key schedule is stored in memory, the encryption can be compromised if memory is readable. Additionally, programs which display programs' arguments may compromise the key and initialization vector, if they are specified on the command line. To avoid this overwrites its arguments, however, the obvious race cannot currently be avoided.
Certain specific keys should be avoided because they introduce potential weaknesses; these keys, called the weak and semiweak keys, are (in hex notation, where p is either 0 or 1, and P is either `e' or `f' ) :
This is inherent in the
DES
algorithm;
see
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