Problem - Kevin Tafuro

The function spc_cfb_encrypt_update( ), which is shown later in this section, has the
following signature:
int spc_cfb_encrypt_update(CFB_CTX *ctx, unsigned char *in, size_t il,
unsigned char *out);
This function has the following arguments:
ctx
Pointer to the SPC_CFB_CTX object associated with the current message.
in
Pointer to the plaintext data to be encrypted.
il
Number of bytes of plaintext to be encrypted.
out
Pointer to the output buffer, which needs to be exactly as long as the input plaintext
data.
Our implementation of this function always returns 1, but a hardwarebased
implementation might have an unexpected failure, so it’s important
to check the return value!
This API is in the spirit of PKCS #11, which provides a standard cryptographic interface
to hardware. We do this so that the above functions can have the bulk of their
implementations replaced with calls to PKCS #11–compliant hardware. PKCS #11
APIs generally pass out data explicitly indicating the length of data outputted, while
we ignore that because it will always be zero on failure or the size of the input buffer
on success. Also note that PKCS #11–based calls tend to order their arguments differently
from the way we do, and they will not generally wipe key material, as we do
in our initialization and finalization routines.
Because this API is developed with PKCS #11 in mind, it’s somewhat
more low-level than it needs to be and therefore is a bit difficult to use
properly. First, you need to be sure the output buffer is big enough to
hold the input; otherwise, you will have a buffer overflow. Second,
you need to make sure the out argument always points to the first
unused byte in the output buffer. Otherwise, you will keep overwriting
the same data every time spc_cfb_encrypt_update( ) outputs.
Here’s our implementation of spc_cfb_encrypt_update( ):
int spc_cfb_encrypt_update(SPC_CFB_CTX *ctx, unsigned char *in, size_t il,
unsigned char *out) {
int i;
if (ctx->ix) {
while (ctx->ix) {
190 | Chapter 5: Symmetric Encryption
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.