Programming Assignment 2

Programming Assignment 2

Two days of grace period with daily penalty of 10% (until October 22)

Use the openssl library (www.openssl.org) to write the following
two functions for encryption and decryption in a file fscrypt.cc.
You should use block cipher method
blowfish for encryption. Blowfish uses 64-bit blocks and typically
128-bit keys.

// put the following lines in fscrypt.h

#include "openssl/blowfish.h"

// encrypt plaintext of length bufsize. Use keystr as the key.
const int BLOCKSIZE = 8; // Block size for blowfish
void *fs_encrypt(void *plaintext, int bufsize, char *keystr,
int *resultlen);

// decrypt ciphertext of length bufsize. Use keystr as the key.
void *fs_decrypt(void *ciphertext, int bufsize, char *keystr,
int *resultlen);

Both functions allocate the result buffer of at least the required
size (using new()) and return a pointer to it. Both functions
also return the number of valid bytes in the result buffer in resultlen.
The application code is responsible for deleting the buffer.

Use CBC mode of encryption. For padding, pad with length of the pad
in all the padded characters.

Assume that the initialization vector contains NULL characters
(all 0's).

Description of blowfish functions can be found at:
https://www.openssl.org/docs/man1.0.2/crypto/blowfish.html

Use the following functions to faciliate your work:

BF_set_key: use all characters of the keystr, excluding NULL
terminator. Valid keystr is assumed to be a string.

BF_cbc_encrypt and BF_ecb_encrypt

You should use BF_ecb_encrypt to implement the CBC mode on your own.

You will need to include "openssl/blowfish.h" from the
openssl package) and link with the "crypto" library.

Below is a small test code (main.cc).

You can compile it with your code in fscrypt.cc using
gcc (or g++) main.cc fscrypt.cc -lcrypto

Submit your fscrypt.cc, which uses only BF_ecb_encrypt.

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#include <assert.h
#include <stdio.h
#include <string.h
#include "fscrypt.h"

int main()
{
char s[] = "hello world";
char *outbuf, *recvbuf;
char pass[] = "top secret";
int len = 0;
int recvlen = 0;

outbuf = (char *) fs_encrypt((void *) s, strlen(s)+1, pass, &len);
printf("%s %d\n", "length after encryption = ", len);

int i = 0;
printf("ciphertext = ");
for (i = 0; i < len; i++)
printf("%02x", outbuf[i]);
printf("\n");

recvbuf = (char *) fs_decrypt((void *) outbuf, len, pass, &recvlen);
assert(memcmp(s, recvbuf, recvlen) == 0);
assert(recvlen == (strlen(s) + 1));
printf("plaintext = %s\n", recvbuf);
}

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