Question

I’m attempting to analyze a short encryption program and figure out which mechanism it’s using.

#include <stdio.h>
#include <stdlib.h>

int main( int argc, char * argv[] ) {
    long int key;
    char * endptr;

    key = strtol( argv[1], &endptr, 10 );

    srandom( key );
    { /* now copy input to output through crypt transformation */
            char ch;
            while (!feof( stdin )) {
                    putc( (getc(stdin) ^ random())&0xFF, stdout );
            }
            fclose( stdout );
    }
}

I can follow this simply, but I’m having trouble trying to weed out which mechanism it’s using..

I’m looking at the following:

http://en.wikipedia.org/wiki/Public-key_cryptography

http://en.wikipedia.org/wiki/Block_cipher

http://en.wikipedia.org/wiki/Stream_cipher

http://en.wikipedia.org/wiki/Diffie-Hellman

I’m leaning towards iterated block cyphers but I really have no idea at this point.

Answer 1

You need to clearly distinguish in your mind the ciphers in categories. There are:

• Block ciphers, which operate in fixed-size blocks of input
• Stream ciphers, which operate on data streams (i.e. one byte at a time)

The above only distinguishes ciphers by the size of the input they accept; it has nothing to do with the mechanism they use to produce the encrypted text.

Regarding this mechanism, we have:

• Substitution ciphers
• Transposition ciphers
• And many other types which are basically combinations of the above, possibly with many iterations

So try to answer this question first:

Is your example a stream cipher or a block cipher? Remember, this has nothing to do with how it encrypts!