Hi,
I was looking for an algorithm which cryptographically secure permutes integers
in [0 .. n] (or maps a integer from [0..n] to another integer [0..n]).
This is quite easy when n is of the form n=2**(2v)-1 with v being an positive
integer (in this case one could use a simple blockcipher with the blocksize
being 2v).

The strongest criterion for being cryptographically secure I thought of is that
an attacker knowing all mappings of x to x' except a->a' and b->b' (and not
knowing the secret key of course) can not decide with a probability larger than
50%% if a maps to a' or b'.

A possible solution for this problem would be to store the integers from 0 to n
in an array ant then permute the entrys in the array in a pseudo random way.
Due to the relative high memory usage of this method a implementation is not
feasible for me (I'm developing for a microcontroller system which has only 2KiB
RAM).

So I designed an algorithm by myself and I'm very interested in your opinions,
comments, critics and analysis.

void decodebio( unsigned char *encbuf, unsigned char * decbuf, int
destbuf ) {

/* Read Base64 encoded data from standard input and write the
decoded data to standard output: */

BIO *b64, *bio ;
long i ;
char buffer[512] ;
memset(buffer, 0, 512) ;

b64 = BIO_new(BIO_f_base64() ) ;
bio = BIO_new(BIO_s_mem()) ;

i=BIO_write(bio, encbuf, strlen(encbuf)) ;

bio = BIO_push(b64, bio) ;
//i = BIO_ctrl_pending(bio);
i = BIO_read(bio, decbuf, 1024) ;
printf( "the old buffer size is %%d %%d\n\r", i, strlen(decbuf)) ;

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