Problem - Kevin Tafuro

long i;
unsigned char c;
for (i = 0; i < inlen; i++) {
do {
c = (*keystream_byte)( );
} while(c > s->maxvalid);
*out++ = s->cset[(s->reverse[*in++] + c) % s->csetlen];
}
}
The function setup_charset_map( ) must be called once to set up a table that maps
ASCII values into an index of the valid subset of characters. The data type that stores
the mapping data is ENCMAP. The other two arguments are charset, a list of all characters
in the valid subset, and csetlen, which specifies the number of characters in that
set.
Once the character map is set up, you can call encrypt_within_charset( ) to encrypt
or decrypt data, while staying within the specified character set. This function has
the following arguments:
s
Pointer to the ENCMAP object.
in
Buffer containing the data to be encrypted or decrypted.
inlen
Length in bytes of the input buffer.
out
Buffer into which the encrypted or decrypted data is placed.
keystream_byte
Pointer to a callback function that should return a single byte of cryptographically
strong keystream.
This code needs to know how to get more bytes of keystream on demand, because
some bytes of keystream will be thrown away if they could potentially be leveraged
in a statistical attack. Therefore, the amount of keystream necessary is theoretically
unbounded (though in practice it should never be significantly more than twice the
length of the input). As a result, we need to know how to invoke a function that gives
us new keystream instead of just passing in a buffer of static keystream.
It would be easy (and preferable) to extend this code example to use a cipher context
object (keyed and in a streaming mode) as a parameter instead of the function
pointer. Then you could get the next byte of keystream directly from the passed context
object. If your crypto library does not allow you direct access to keystream,
encrypting all zeros returns the original keystream.
148 | Chapter 4: Symmetric Cryptography Fundamentals
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.