tarequeh / des

Implementation of Data Encryption Standard (DES) in C

DES in C

C implementation of Data Encryption Standard algorithm.

Overview

The Data Encryption Standard (DES) is a block cipher (a form of shared secret encryption) that was selected by the National Bureau of Standards as an official Federal Information Processing Standard (FIPS) for the United States in 1976 and which has subsequently enjoyed widespread use internationally. It is based on a symmetric-key algorithm that uses a 56-bit key.

This implementation of DES is not optimized in any way. The code has been written to provide readability and easy understanding of the algorithm. Padding scheme used in this implementation is [PKCS5]

Compilation & Installation

This implementation has only been tested on Unix platform. But you may be able to compile/ run it on Windows.

1. Make sure des.c, des.h and run_des.c are in the same directory
2. Compile using: gcc -O3 des.c run_des.c -o run_des.o

Usage

Say we want to encrypt/ decrypt a file named /home/user/sample.txt

1. Generate a keyfile using:

   run_des.o -g /tmp/keyfile.key
   

2. Encrypt sample.txt using:

   run_des.o -e /tmp/keyfile.key /home/user/sample.txt /home/user/sample.enc
   

3. Decrypt sample.txt using:

   run_des.o -d /tmp/keyfile.key /home/user/sample.enc /home/user/sample_decrypted.txt
   

Don't lose the key file! you won't be able to decrypt an encrypted if you lose the keyfile.

More

DES is provided for educational purposes only. Do not use for any other reason. It has been implemented after J. Orlin Grabbe's DES Algorithm Illustrated

It is possible to use this implementation to facilitate TripleDES encryption process:

1. Generate keys using:

   run_des.o -g /tmp/keyfile1.key
   run_des.o -g /tmp/keyfile2.key
   run_des.o -g /tmp/keyfile3.key
   

2. Encrypt using:

   run_des.o -e /tmp/keyfile1.key /home/user/sample.txt /home/user/sample.enc1
   run_des.o -e /tmp/keyfile2.key /home/user/sample.enc1 /home/user/sample.enc2
   run_des.o -e /tmp/keyfile3.key /home/user/sample.enc2 /home/user/sample.enc3
   

3. Decrypt using:

   run_des.o -d /tmp/keyfile3.key /home/user/sample.enc3 /home/user/sample.dec3
   run_des.o -d /tmp/keyfile2.key /home/user/sample.dec3 /home/user/sample.dec2
   run_des.o -d /tmp/keyfile1.key /home/user/sample.dec2 /home/user/sample_decrypted.txt
   

The primary repository for DES is located at: http://github.com/tarequeh/DES/ The blog post discussing the implementation can be found at: CodeXN This implementation of DES was written by Tareque Hossain

des's Issues

input user's kwy

can the user input the key instead of been generated by the program?

cbc/cfb/ofb/?

wich of these type of des is?

A bug in function generate_sub_keys

Hi, the code is well-written but I find a bug in the function generate_sub_keys. You don't initialize the key_sets before using it neither before calling the function. Thus the generated keys go wrong in my test.
My suggestion is as follows. Just add memset(key_sets, 0, 17 * sizeof(key_set); in the top of the function.

How to change des model to NOPadding?

How to change des model to NOPadding?
output length limit to 8

generate_sub_keys depends on initial value of key_sets

There's a bug somewhere in generate_sub_keys. I haven't dug into it. I just noticed that if write a value into key sets before generating the sub keys, the output ciphertext changes.

$ xxd outx
00000000: 8ca6 4de9 c1b1 23a7 7e42 2822 7736 66c0  ..M...#.~B("w6f.
$ vim run_des.c # Add a memset before generate_sub_keys
$ gcc -o mydes des.c run_des.c
$ ./mydes -e key inp outy
Encrypting..
Finished processing inp. Time taken: 0.000106 seconds.
$ xxd outy
00000000: 038b 53ff 9828 9c81 1bb7 97cd 35ab 14ad  ..S..(......5...

Query regarding working of the code

Does this code encrypt and decrypt any file (eg: jpeg,video,etc.)or does this work only for text file .

test failed

#include <malloc.h>
#include <string.h>
#include <cstdio>
#include <stdlib.h>
#include <cassert>
#include <cstdlib>
#include "des.h"

void c(const char* const src, char* const dst, const int size, const key_set* const key_sets, int m) {
    char buf_src[8], buf_dst[8];
    int block = size / 8;
    int mod = size % 8;
    memcpy(dst + block * 8, src + block * 8, mod);

    for (int i = 0; i < block; ++i) {
        memcpy(buf_src, src + i * 8, 8);
        process_message((unsigned char *) buf_src, (unsigned char *) buf_dst, (key_set *) key_sets, m);
        memcpy(dst + i * 8, buf_dst, 8);
    }
}

void c(const char* const src, char* const dst, const int size, int m) {
    char key1[] = {-5, 11, 24, -51, 34, 46, 98, -104};
    char key2[] = {-51, 101, 44, -21, 54, 47, 91, -114};
    char key3[] = {-57, 111, 27, -81, 38, 66, 58, -14};

    key_set* key_sets1 = (key_set*)malloc(17 * sizeof(key_set));
    key_set* key_sets2 = (key_set*)malloc(17 * sizeof(key_set));
    key_set* key_sets3 = (key_set*)malloc(17 * sizeof(key_set));

    generate_sub_keys((unsigned char *) key1, key_sets1);
    generate_sub_keys((unsigned char *) key2, key_sets2);
    generate_sub_keys((unsigned char *) key3, key_sets3);

    if (m == ENCRYPTION_MODE) {
        c(src, dst, size, key_sets1, m);
        c(dst, dst, size, key_sets2, m);
        c(dst, dst, size, key_sets3, m);
    } else {
        c(src, dst, size, key_sets3, m);
        c(dst, dst, size, key_sets2, m);
        c(dst, dst, size, key_sets1, m);
    }

    free(key_sets1);
    free(key_sets2);
    free(key_sets3);
}

bool test(char* src, int size) {
    char* dst = new char[size];
    char* dst2 = new char[size];

    c(src, dst, size, ENCRYPTION_MODE);
    c(dst, dst2, size, DECRYPTION_MODE);

    for (int i = 0; i < size; ++i) {
        if (dst2[i] != src[i]) {
            delete [] dst;
            delete [] dst2;
            return false;
        }
    }

    delete [] dst;
    delete [] dst2;
    return true;
}

int main(int argc, char** argv) {
    for (int i = 0; i < 1000; ++i) {
        int size = rand() % 9999;
        unsigned char* src = new unsigned char[size];
        for (int s = 0; s < size; ++s) {
            src[s] = rand() % 256;
        }
        if (!test((char*)src, size)) {
            printf("bug:%d,count:%d\n", i, size);
        }
        delete [] src;
    }
    return 0;
}

result
bug:201,count:3066
os
windows 10
compiler
E:\workplace\sample\jni-classloader\src\main\java>g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=C:/Program\ Files/mingw-w64/x86_64-6.2.0-posix-seh-rt_v5- rev1/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/6.2.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../../../src/gcc-6.2.0/configure --host=x86_64-w64-mingw32 --build=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --prefix=/mingw64 --with-sysroot=/c/mingw620/x86_64-620-posix-seh-rt_v5-rev1/mingw64 --enable-shared --enable-static --disable-multilib --enable-languages=c,c++,fortran,lto --enable-libstdcxx-time=yes --enable-threads=posix --enable-libgomp --enable-libatomic --enable-lto --enable-graphite --enable-checking=release --enable-fully-dynamic-string --enable-version-specific-runtime-libs --enable-libstdcxx-filesystem-ts=yes --disable-isl-version-check --disable-libstdcxx-pch --disable-libstdcxx-debug --enable-bootstrap --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-gnu-as --with-gnu-ld --with-arch=nocona --with-tune=core2 --with-libiconv --with-system-zlib --with-gmp=/c/mingw620/prerequisites/x86_64-w64-mingw32-static --with-mpfr=/c/mingw620/prerequisites/x86_64-w64-mingw32-static --with-mpc=/c/mingw620/prerequisites/x86_64-w64-mingw32-static --with-isl=/c/mingw620/prerequisites/x86_64-w64-mingw32-static --with-pkgversion='x86_64-posix-seh-rev1, Built by MinGW-W64 project' --with-bugurl=http://sourceforge.net/projects/mingw-w64 CFLAGS='-O2 -pipe -I/c/mingw620/x86_64-620-posix-seh-rt_v5-rev1/mingw64/opt/include -I/c/mingw620/prerequisites/x86_64-zlib-static/include -I/c/mingw620/prerequisites/x86_64-w64-mingw32-static/include' CXXFLAGS='-O2 -pipe -I/c/mingw620/x86_64-620-posix-seh-rt_v5-rev1/mingw64/opt/include -I/c/mingw620/prerequisites/x86_64-zlib-static/include -I/c/mingw620/prerequisites/x86_64-w64-mingw32-static/include' CPPFLAGS= LDFLAGS='-pipe -L/c/mingw620/x86_64-620-posix-seh-rt_v5-rev1/mingw64/opt/lib -L/c/mingw620/prerequisites/x86_64-zlib-static/lib -L/c/mingw620/prerequisites/x86_64-w64-mingw32- static/lib '
Thread model: posix
gcc version 6.2.0 (x86_64-posix-seh-rev1, Built by MinGW-W64 project)

3DES

I think that 3DES works by first encrypting with k1, then decrypt with k2 and encrypt a second time with k3.